RAW                                                      P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Intended status: Informational                             D. Cavalcanti
Expires: 19 November 2020                                          Intel
                                                           X. Vilajosana
                                         Universitat Oberta de Catalunya
                                                              C. Schmitt
                                         Research Institute CODE, UniBwM
                                                               J. Farkas
                                                             18 May 2020

              Reliable and Available Wireless Technologies


   This document presents a series of recent technologies that are
   capable of time synchronization and scheduling of transmission,
   making them suitable to carry time-sensitive flows with high
   reliability and availbility.

Status of This Memo

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   This Internet-Draft will expire on 19 November 2020.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  On Scheduling . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Benefits of Scheduling on Wires . . . . . . . . . . . . .   4
     3.2.  Benefits of Scheduling on Wireless  . . . . . . . . . . .   5
   4.  IEEE 802.11 . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Provenance and Documents  . . . . . . . . . . . . . . . .   6
     4.2.  802.11ax High Efficiency (HE) . . . . . . . . . . . . . .   8
       4.2.1.  General Characteristics . . . . . . . . . . . . . . .   8
       4.2.2.  Applicability to deterministic flows  . . . . . . . .   9
     4.3.  802.11be Extreme High Throughput (EHT)  . . . . . . . . .  10
       4.3.1.  General Characteristics . . . . . . . . . . . . . . .  10
       4.3.2.  Applicability to deterministic flows  . . . . . . . .  11
     4.4.  802.11ad and 802.11ay (mmWave operation)  . . . . . . . .  12
       4.4.1.  General Characteristics . . . . . . . . . . . . . . .  12
       4.4.2.  Applicability to deterministic flows  . . . . . . . .  13
   5.  IEEE 802.15.4 . . . . . . . . . . . . . . . . . . . . . . . .  13
     5.1.  Provenance and Documents  . . . . . . . . . . . . . . . .  13
     5.2.  TimeSlotted Channel Hopping . . . . . . . . . . . . . . .  15
       5.2.1.  General Characteristics . . . . . . . . . . . . . . .  15
       5.2.2.  Applicability to Deterministic Flows  . . . . . . . .  16
   6.  5G  . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  30
     6.1.  Provenance and Documents  . . . . . . . . . . . . . . . .  30
     6.2.  General Characteristics . . . . . . . . . . . . . . . . .  32
     6.3.  Deployment and Spectrum . . . . . . . . . . . . . . . . .  33
     6.4.  Applicability to Deterministic Flows  . . . . . . . . . .  34
       6.4.1.  System Architecture . . . . . . . . . . . . . . . . .  34
       6.4.2.  Overview of The Radio Protocol Stack  . . . . . . . .  36
       6.4.3.  Radio (PHY) . . . . . . . . . . . . . . . . . . . . .  37
       6.4.4.  Scheduling and QoS (MAC)  . . . . . . . . . . . . . .  39
       6.4.5.  Time-Sensitive Networking (TSN) Integration . . . . .  41
     6.5.  Summary . . . . . . . . . . . . . . . . . . . . . . . . .  44
   7.  L-band Digital Aeronautical Communications System . . . . . .  45
     7.1.  Provenance and Documents  . . . . . . . . . . . . . . . .  45
     7.2.  General Characteristics . . . . . . . . . . . . . . . . .  46
     7.3.  Applicability to Deterministic Flows  . . . . . . . . . .  47
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  48
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  48
   10. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  48
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  49

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   12. Normative References  . . . . . . . . . . . . . . . . . . . .  49
   13. Informative References  . . . . . . . . . . . . . . . . . . .  49
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  56

1.  Introduction

   When used in math or philosophy, the term "deterministic" generally
   refers to a perfection where all aspect are understood and
   predictable.  A perfectly Deterministic Network would ensure that
   every packet reach its destination following a predetermined path
   along a predefined schedule to be delivered at the exact due time.
   In a real and imperfect world, a Deterministic Network must highly
   predictable, which is a combination of reliability and availability.
   On the one hand the network must be reliable, meaning that it will
   perform as expected for all packets and in particular that it will
   always deliver the packet at the destination in due time.  On the
   other hand, the network must be available, meaning that it is
   resilient to any single outage, whether the cause is a software, a
   hardware or a transmission issue.

   RAW (Reliable and Available Wireless) is an effort to provide
   Deterministic Networking on across a path that include a wireless
   physical layer.  Making Wireless Reliable and Available is even more
   challenging than it is with wires, due to the numerous causes of loss
   in transmission that add up to the congestion losses and the delays
   caused by overbooked shared resources.  In order to maintain a
   similar quality of service along a multihop path that is composed of
   wired and wireless hops, additional methods that are specific to
   wireless must be leveraged to combat the sources of loss that are
   also specific to wireless.

   Such wireless-specific methods include per-hop retransmissions (HARQ)
   and P2MP overhearing whereby multiple receivers are scheduled to
   receive the same transmission, which balances the adverse effects of
   the transmission losses that are experienced when a radio is used as
   pure P2P.  Those methods are collectively referred to as PAREO
   functions in the "Reliable and Available Wireless Architecture/
   Framework" [I-D.pthubert-raw-architecture].

2.  Terminology

   This specification uses several terms that are uncommon on protocols
   that ensure bets effort transmissions for stochastics flows, such as
   found in the traditional Internet and other statistically multiplexed
   packet networks.

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   ARQ:  Automatic Repeat Request, enabling an acknowledged transmission
      and retries.  ARQ is a typical model at Layer-2 on a wireless
      medium.  It is typically avoided end-to-end on deterministic flows
      because it introduces excessive indetermination in latency, but a
      limited number of retries within a bounded time may be used over a
      wireless link and yet respect end-to-end constraints.

   Available:  That is exempt of unscheduled outage, the expectation for
      a network being that the flow is maintained in the face of any
      single breakage.

   FEC:  Forward error correction, sending redundant coded data to help
      the receiver recover transmission errors without the delays
      incurred with ARQ.

   HARQ:  Hybrid ARQ, a combination of FEC and ARQ.

   PCE:  Path Computation Element.

   PAREO (functions):  the wireless extension of DetNet PREOF.  PAREO
      functions include scheduled ARQ at selected hops, and expect the
      use of new operations like overhearing where available.

   Reliable:  That consistently performs as expected, the expectation
      for a network being to always deliver a packet in due time.

   Track:  A DODAG oriented to a destination, and that enables Packet
      ARQ, Replication, Elimination, and Ordering Functions.

3.  On Scheduling

   The operations of a Deterministic Network often rely on precisely
   applying a tight schedule, in order to avoid collision loss and
   guarantee the worst-case time of delivery.  To achieve this, there
   must be a shared sense of time throughout the network.  The sense of
   time is usually provided by the lower layer and is not in scope for

3.1.  Benefits of Scheduling on Wires

   A network is reliable when the statistical effects that affect the
   packet transmission are eliminated.  This involves maintaining at all
   time the amount of critical packets within the physical capabilities
   of the hardware and that of the radio medium.  This is achieved by
   controlling the use of time-shared resources such as CPUs and
   buffers, by shaping the flows and by scheduling the time of
   transmission of the packets that compose the flow at every hop.

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   Equipment failure, such as an access point rebooting, a broken radio
   adapter, or a permanent obstacle to the transmission, is a secondary
   source of packet loss.  When a breakage occurs, multiple packets are
   lost in a row before the flows are rerouted or the system may
   recover.  This is not acceptable for critical applications such as
   related to safety.  A typical process control loop will tolerate an
   occasional packet loss, but a loss of several packets in a row will
   cause an emergency stop (e.g., after 4 packets lost, within a period
   of 1 second).

   Network Availability is obtained by making the transmission resilient
   against hardware failures and radio transmission losses due to
   uncontrolled events such as co-channel interferers, multipath fading
   or moving obstacles.  The best results are typically achieved by
   pseudo randomly cumulating all forms of diversity, in the spatial
   domain with replication and elimination, in the time domain with ARQ
   and diverse scheduled transmissions, and in the frequency domain with
   frequency hopping or channel hopping between frames.

3.2.  Benefits of Scheduling on Wireless

   In addition to the benefits listed in Section 3.1, scheduling
   transmissions provides specific value to the wireless medium.

   On the one hand, scheduling avoids collisions between scheduled
   transmissions and can ensure both time and frequency diversity
   between retries in order to defeat co-channel interference from un-
   controlled transmitters as well as multipath fading.  Transmissions
   can be scheduled on multiple channels in parallel, which enables to
   use the full available spectrum while avoiding the hidden terminal
   problem, e.g., when the next packet in a same flow interferes on a
   same channel with the previous one that progressed a few hops

   On the other hand, scheduling optimizes the bandwidth usage: compared
   to classical Collision Avoidance techniques, there is no blank time
   related to inter-frame space (IFS) and exponential back-off in
   scheduled operations.  A minimal Clear Channel Assessment may be
   needed to comply with the local regulations such as ETSI 300-328, but
   that will not detect a collision when the senders are synchronized.
   And because scheduling allows a time-sharing operation, there is no
   limit to the ratio of isolated critical traffic.

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   Finally, scheduling plays a critical role to save energy.  In IOT,
   energy is the foremost concern, and synchronizing sender and listener
   enables to always maintain them in deep sleep when there is no
   scheduled transmission.  This avoids idle listening and long
   preambles and enables long sleep periods between traffic and
   resynchronization, allowing battery-operated nodes to operate in a
   mesh topology for multiple years.

4.  IEEE 802.11

4.1.  Provenance and Documents

   With an active portfolio of nearly 1,300 standards and projects under
   development, IEEE is a leading developer of industry standards in a
   broad range of technologies that drive the functionality,
   capabilities, and interoperability of products and services,
   transforming how people live, work, and communicate.

   The IEEE 802 LAN/MAN Standards Committee (SC) develops and maintains
   networking standards and recommended practices for local,
   metropolitan, and other area networks, using an open and accredited
   process, and advocates them on a global basis.  The most widely used
   standards are for Ethernet, Bridging and Virtual Bridged LANs
   Wireless LAN, Wireless PAN, Wireless MAN, Wireless Coexistence, Media
   Independent Handover Services, and Wireless RAN.  An individual
   Working Group provides the focus for each area.  Standards produced
   by the IEEE 802 SC are freely available from the IEEE GET Program
   after they have been published in PDF for six months.

   The IEEE 802.11 LAN standards define the underlying MAC and PHY
   layers for the Wi-Fi technology.  Wi-Fi/802.11 is one of the most
   successful wireless technologies, supporting many application
   domains.  While previous 802.11 generations, such as 802.11n and
   802.11ac, have focused mainly on improving peak throughput, more
   recent generations are also considering other performance vectors,
   such as efficiency enhancements for dense environments in 802.11ax,
   and latency and support for Time-Sensitive Networking (TSN)
   capabilities in 802.11be.

   IEEE 802.11 already supports some 802.1 TSN standards and it is
   undergoing efforts to support for other 802.1 TSN capabilities
   required to address the use cases that require time synchronization
   and timeliness (bounded latency) guarantees with high reliability and
   availability.  The IEEE 802.11 working group has been working in
   collaboration with the IEEE 802.1 group for several years extending
   802.1 features over 802.11.  As with any wireless media, 802.11
   imposes new constraints and restrictions to TSN-grade QoS, and
   tradeoffs between latency and reliability guarantees must be

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   considered as well as managed deployment requirements.  An overview
   of 802.1 TSN capabilities and their extensions to 802.11 are
   discussed in [Cavalcanti_2019].

   Wi-Fi Alliance (WFA) is the worldwide network of companies that
   drives global Wi-Fi adoption and evolution through thought
   leadership, spectrum advocacy, and industry-wide collaboration.  The
   WFA work helps ensure that Wi-Fi devices and networks provide users
   the interoperability, security, and reliability they have come to

   The following [IEEE Std. 802.11] specifications/certifications are
   relevant in the context of reliable and available wireless services
   and support for time-sensitive networking capabilities:

   Time Synchronization:  IEEE802.11-2016 with IEEE802.1AS; WFA TimeSync

   Congestion Control:  IEEE802.11-2016 Admission Control; WFA Admission

   Security:  WFA Wi-Fi Protected Access, WPA2 and WPA3.

   Interoperating with IEEE802.1Q bridges:  [IEEE Std. 802.11ak].

   Stream Reservation Protocol (part of [IEEE Std. 802.1Qat]):

   Scheduled channel access:  IEEE802.11ad Enhancements for very high
      throughput in the 60 GHz band [IEEE Std. 802.11ad].

   802.11 Real-Time Applications:  Topic Interest Group (TIG) ReportDoc

   In addition, major amendments being developed by the IEEE802.11
   Working Group include capabilities that can be used as the basis for
   providing more reliable and predictable wireless connectivity and
   support time-sensitive applications:

   IEEE 802.11ax D4.0: Enhancements for High Efficiency (HE).  [IEEE
      Std. 802.11ax]

   IEEE 802.11be Extreme High Throughput (EHT).  [IEEE 802.11be WIP]

   IEE 802.11ay Enhanced throughput for operation in license-exempt
   bands above 45 GHz.  [IEEE
      Std. 802.11ay]

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   The main 802.11ax and 802.11be capabilities and their relevance to
   RAW are discussed in the remainder of this document.

4.2.  802.11ax High Efficiency (HE)

4.2.1.  General Characteristics

   The next generation Wi-Fi (Wi-Fi 6) is based on the IEEE802.11ax
   amendment [IEEE Std. 802.11ax], which includes new capabilities to
   increase efficiency, control and reduce latency.  Some of the new
   features include higher order 1024-QAM modulation, support for uplink
   multi-user MIMO, OFDMA, trigger-based access and Target Wake time
   (TWT) for enhanced power savings.  The OFDMA mode and trigger-based
   access enable scheduled operation, which is a key capability required
   to support deterministic latency and reliability for time-sensitive
   flows. 802.11ax can operate in up to 160 MHz channels and it includes
   support for operation in the new 6 GHz band, which is expected to be
   open to unlicensed use by the FCC and other regulatory agencies
   worldwide.  Multi-User OFDMA and Trigger-based Scheduled Access

   802.11ax introduced a new orthogonal frequency-division multiple
   access (OFDMA) mode in which multiple users can be scheduled across
   the frequency domain.  In this mode, the Access Point (AP) can
   initiate multi-user (MU) Uplink (UL) transmissions in the same PHY
   Protocol Data Unit (PPDU) by sending a trigger frame.  This
   centralized scheduling capability gives the AP much more control of
   the channel, and it can remove contention between devices for uplink
   transmissions, therefore reducing the randomness caused by CSMA-based
   access between stations.  The AP can also transmit simultaneously to
   multiple users in the downlink direction by using a Downlink (DL) MU
   OFDMA PPDU.  In order to initiate a contention free Transmission
   Opportunity (TXOP) using the OFDMA mode, the AP still follows the
   typical listen before talk procedure to acquire the medium, which
   ensures interoperability and compliance with unlicensed band access
   rules.  However, 802.11ax also includes a multi-user Enhanced
   Distributed Channel Access (MU-EDCA) capability, which allows the AP
   to get higher channel access priority.  Improved PHY Robustness

   The 802.11ax PHY can operate with 0.8, 1.6 or 3.2 microsecond guard
   interval (GI).  The larger GI options provide better protection
   against multipath, which is expected to be a challenge in industrial
   environments.  The possibility to operate with smaller resource units
   (e.g. 2 MHz) enabled by OFDMA also helps reduce noise power and
   improve SNR, leading to better packet error rate (PER) performance.

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   802.11ax supports beamforming as in 802.11ac, but introduces UL MU
   MIMO, which helps improve reliability.  The UL MU MIMO capability is
   also enabled by the trigger based access operation in 802.11ax.  Support for 6GHz band

   The 802.11ax specification [IEEE Std. 802.11ax] includes support for
   operation in the new 6 GHz band.  Given the amount of new spectrum
   available as well as the fact that no legacy 802.11 device (prior
   802.11ax) will be able to operate in this new band, 802.11ax
   operation in this new band can be even more efficient.

4.2.2.  Applicability to deterministic flows

   TSN capabilities, as defined by the IEEE 802.1 TSN standards, provide
   the underlying mechanism for supporting deterministic flows in a
   Local Area Network (LAN).  The 802.11 working group has already
   incorporated support for several TSN capabilities, so that time-
   sensitive flow can experience precise time synchronization and
   timeliness when operating over 802.11 links.  TSN capabilities
   supported over 802.11 (which also extends to 802.11ax), include:

   1.  802.1AS based Time Synchronization (other time synchronization
       techniques may also be used)

   2.  Interoperating with IEEE802.1Q bridges

   3.  Time-sensitive Traffic Stream identification

   The exiting 802.11 TSN capabilities listed above, and the 802.11ax
   OFDMA and scheduled access provide a new set of tools to better
   server time-sensitive flows.  However, it is important to understand
   the tradeoffs and constraints associated with such capabilities, as
   well as redundancy and diversity mechanisms that can be used to
   provide more predictable and reliable performance.  802.11 Managed network operation and admission control

   Time-sensitive applications and TSN standards are expected to operate
   under a managed network (e.g. industrial/enterprise network).  Thus,
   the Wi-Fi operation must also be carefully managed and integrated
   with the overall TSN management framework, as defined in the
   [IEEE8021Qcc] specification.

   Some of the random-access latency and interference from legacy/
   unmanaged devices can be minimized under a centralized management
   mode as defined in [IEEE8021Qcc], in which admission control
   procedures are enforced.

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   Existing traffic stream identification, configuration and admission
   control procedures defined in [IEEE Std. 802.11] QoS mechanism can be
   re-used.  However, given the high degree of determinism required by
   many time-sensitive applications, additional capabilities to manage
   interference and legacy devices within tight time-constraints need to
   be explored.  Scheduling for bounded latency and diversity

   As discussed earlier, the [IEEE Std. 802.11ax] OFDMA mode introduces
   the possibility of assigning different RUs (frequency resources) to
   users within a PPDU.  Several RU sizes are defined in the
   specification (26, 52, 106, 242, 484, 996 subcarriers).  In addition,
   the AP can also decide on MCS and grouping of users within a given
   OFMDA PPDU.  Such flexibility can be leveraged to support time-
   sensitive applications with bounded latency, especially in a managed
   network where stations can be configured to operate under the control
   of the AP.

   As shown in [Cavalcanti_2019], it is possible to achieve latencies in
   the order of 1msec with high reliability in an interference free
   environment.  Obviously, there are latency, reliability and capacity
   tradeoffs to be considered.  For instance, smaller Resource Units
   (RU)s result in longer transmission durations, which may impact the
   minimal latency that can be achieved, but the contention latency and
   randomness elimination due to multi-user transmission is a major
   benefit of the OFDMA mode.

   The flexibility to dynamically assign RUs to each transmission also
   enables the AP to provide frequency diversity, which can help
   increase reliability.

4.3.  802.11be Extreme High Throughput (EHT)

4.3.1.  General Characteristics

   The [IEEE 802.11be WIP]is the next major 802.11 amendment (after
   [IEEE Std. 802.11ax]) for operation in the 2.4, 5 and 6 GHz bands.
   802.11be is expected to include new PHY and MAC features and it is
   targeting extremely high throughput (at least 30 Gbps), as well as
   enhancements to worst case latency and jitter.  It is also expected
   to improve the integration with 802.1 TSN to support time-sensitive
   applications over Ethernet and Wireless LANs.

   The 802.11be Task Group started its operation in May 2019, therefore,
   detailed information about specific features is not yet available.
   Only high level candidate features have been discussed so far,

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   1.  320MHz bandwidth and more efficient utilization of non-contiguous

   2.  Multi-band/multi-channel aggregation and operation.

   3.  16 spatial streams and related MIMO enhancements.

   4.  Multi-Access Point (AP) Coordination.

   5.  Enhanced link adaptation and retransmission protocol, e.g.
       Hybrid Automatic Repeat Request (HARQ).

   6.  Any required adaptations to regulatory rules for the 6 GHz

4.3.2.  Applicability to deterministic flows

   The 802.11 Real-Time Applications (RTA) Topic Interest Group (TIG)
   provided detailed information on use cases, issues and potential
   solution directions to improve support for time-sensitive
   applications in 802.11.  The RTA TIG report [IEEE_doc_11-18-2009-06]
   was used as input to the 802.11be project scope.

   Improvements for worst-case latency, jitter and reliability were the
   main topics identified in the RTA report, which were motivated by
   applications in gaming, industrial automation, robotics, etc.  The
   RTA report also highlighted the need to support additional TSN
   capabilities, such as time-aware (802.1Qbv) shaping and packet
   replication and elimination as defined in 802.1CB.

   802.11be is expected to build on and enhance 802.11ax capabilities to
   improve worst case latency and jitter.  Some of the enhancement areas
   are discussed next.  Enhanced scheduled operation for bounded latency

   In addition to the throughput enhancements, 802.11be will leverage
   the trigger-based scheduled operation enabled by 802.11ax to provide
   efficient and more predictable medium access. 802.11be is expected to
   include enhancements to reduce overhead and enable more efficient
   operation in managed network deployments [IEEE_doc_11-19-0373-00].

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   Multi-AP coordination is one of the main new candidate features in
   802.11be.  It can provide benefits in throughput and capacity and has
   the potential to address some of the issues that impact worst case
   latency and reliability.  Multi-AP coordination is expected to
   address the contention due to overlapping Basic Service Sets (OBSS),
   which is one of the main sources of random latency variations.
   802.11be can define methods to enable better coordination between
   APs, for instance, in a managed network scenario, in order to reduce
   latency due to unmanaged contention.

   Several multi-AP coordination approaches have been discussed with
   different levels of complexities and benefits, but specific
   coordination methods have not yet been defined.  Multi-band operation

   802.11be will introduce new features to improve operation over
   multiple bands and channels.  By leveraging multiple bands/channels,
   802.11be can isolate time-sensitive traffic from network congestion,
   one of the main causes of large latency variations.  In a managed
   802.11be network, it should be possible to steer traffic to certain
   bands/channels to isolate time-sensitive traffic from other traffic
   and help achieve bounded latency.

4.4.  802.11ad and 802.11ay (mmWave operation)

4.4.1.  General Characteristics

   The IEEE 802.11ad amendment defines PHY and MAC capabilities to
   enable multi-Gbps throughput in the 60 GHz millimeter wave (mmWave)
   band.  The standard addresses the adverse mmWave signal propagation
   characteristics and provides directional communication capabilities
   that take advantage of beamforming to cope with increased
   attenuation.  An overview of the 802.11ad standard can be found in
   [Nitsche_2015] .

   The IEEE 802.11ay is currently developing enhancements to the
   802.11ad standard to enable the next generation mmWave operation
   targeting 100 Gbps throughput.  Some of the main enhancements in
   802.11ay include MIMO, channel bonding, improved channel access and
   beamforming training.  An overview of the 802.11ay capabilities can
   be found in [Ghasempour_2017]

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4.4.2.  Applicability to deterministic flows

   The high data rates achievable with 802.11ad and 802.11ay can
   significantly reduce latency down to microsecond levels.  Limited
   interference from legacy and other unlicensed devices in 60 GHz is
   also a benefit.  However, directionality and short range typical in
   mmWave operation impose new challenges such as the overhead required
   for beam training and blockage issues, which impact both latency and
   reliability.  Therefore, it is important to understand the use case
   and deployment conditions in order to properly apply and configure
   802.11ad/ay networks for time sensitive applications.

   The 802.11ad standard include a scheduled access mode in which
   stations can be allocated contention-free service periods by a
   central controller.  This scheduling capability is also available in
   802.11ay, and it is one of the mechanisms that can be used to provide
   bounded latency to time-sensitive data flows.  An analysis of the
   theoretical latency bounds that can be achieved with 802.11ad service
   periods is provided in [Cavalcanti_2019].

5.  IEEE 802.15.4

5.1.  Provenance and Documents

   The IEEE802.15.4 Task Group has been driving the development of low-
   power low-cost radio technology.  The IEEE802.15.4 physical layer has
   been designed to support demanding low-power scenarios targeting the
   use of unlicensed bands, both the 2.4 GHz and sub GHz Industrial,
   Scientific and Medical (ISM) bands.  This has imposed requirements in
   terms of frame size, data rate and bandwidth to achieve reduced
   collision probability, reduced packet error rate, and acceptable
   range with limited transmission power.  The PHY layer supports frames
   of up to 127 bytes.  The Medium Access Control (MAC) sublayer
   overhead is in the order of 10-20 bytes, leaving about 100 bytes to
   the upper layers.  IEEE802.15.4 uses spread spectrum modulation such
   as the Direct Sequence Spread Spectrum (DSSS).

   The Timeslotted Channel Hopping (TSCH) mode was added to the 2015
   revision of the IEEE802.15.4 standard [IEEE Std. 802.15.4].  TSCH is
   targeted at the embedded and industrial world, where reliability,
   energy consumption and cost drive the application space.

   At the IETF, the 6TiSCH Working Group (WG) [TiSCH] deals with best
   effort operation of IPv6 [RFC8200] over TSCH. 6TiSCH has enabled
   distributed scheduling to exploit the deterministic access
   capabilities provided by TSCH.  The group designed the essential
   mechanisms to enable the management plane operation while ensuring
   IPv6 is supported.  Yet the charter did not focus to providing a

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   solution to establish end to end Tracks while meeting quality of
   service requirements. 6TiSCH, through the RFC8480 [RFC8480] defines
   the 6P protocol which provides a pairwise negotiation mechanism to
   the control plane operation.  The protocol supports agreement on a
   schedule between neighbors, enabling distributed scheduling.  6P goes
   hand-in-hand with a Scheduling Function (SF), the policy that decides
   how to maintain cells and trigger 6P transactions.  The Minimal
   Scheduling Function (MSF) [I-D.ietf-6tisch-msf] is the default SF
   defined by the 6TiSCH WG; other standardized SFs can be defined in
   the future.  MSF extends the minimal schedule configuration, and is
   used to add child-parent links according to the traffic load.

   Time sensitive networking on low power constrained wireless networks
   have been partially addressed by ISA100.11a [ISA100.11a] and
   WirelessHART [WirelessHART].  Both technologies involve a central
   controller that computes redundant paths for industrial process
   control traffic over a TSCH mesh.  Moreover, ISA100.11a introduces
   IPv6 capabilities with a Link-Local Address for the join process and
   a global unicast addres for later exchanges, but the IPv6 traffic
   typically ends at a local application gateway and the full power of
   IPv6 for end-to-end communication is not enabled.  Compared to that
   state of the art, work at the IETF and in particular at RAW could
   provide additional techniques such as optimized P2P routing, PAREO
   functions, and end-to-end secured IPv6/CoAP connectivity.

   The 6TiSCH architecture [I-D.ietf-6tisch-architecture] identifies
   different models to schedule resources along so-called Tracks (see
   Section exploiting the TSCH schedule structure however the
   focus at 6TiSCH is on best effort traffic and the group was never
   chartered to produce standard work related to Tracks.

   Useful References include:

   1.  IEEE Std 802.15.4: "IEEE Std. 802.15.4, Part. 15.4: Wireless
       Medium Access Control (MAC) and Physical Layer (PHY)
       Specifications for Low-Rate Wireless Personal Area Networks"
       [IEEE Std. 802.15.4].  The latest version at the time of this
       writing is dated year 2015.

   2.  Morell, A. , Vilajosana, X. , Vicario, J.  L. and Watteyne, T.
       (2013), Label switching over IEEE802.15.4e networks.  Trans.
       Emerging Tel. Tech., 24: 458-475. doi:10.1002/ett.2650"

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   3.  De Armas, J., Tuset, P., Chang, T., Adelantado, F., Watteyne, T.,
       Vilajosana, X. (2016, September).  Determinism through path
       diversity: Why packet replication makes sense.  In 2016
       International Conference on Intelligent Networking and
       Collaborative Systems (INCoS) (pp. 150-154).  IEEE. [dearmas16].

   4.  X.  Vilajosana, T.  Watteyne, M.  Vucinic, T.  Chang and K.  S.
       J.  Pister, "6TiSCH: Industrial Performance for IPv6 Internet-of-
       Things Networks," in Proceedings of the IEEE, vol. 107, no. 6,
       pp. 1153-1165, June 2019. [vilajosana19].

5.2.  TimeSlotted Channel Hopping

5.2.1.  General Characteristics

   As a core technique in IEEE802.15.4, TSCH splits time in multiple
   time slots that repeat over time.  A set of timeslots constructs a
   Slotframe (see Section  For each timeslot, a set of
   available frequencies can be used, resulting in a matrix-like
   schedule (see Figure 1).

                          timeslot offset
        | 0    1    2    3    4  | 0    1    2    3    4  |    Nodes
        +------------------------+------------------------+   +-----+
        |    |    |    |    |    |    |    |    |    |    |   |  C  |
   CH-1 | EB |    |    |C->B|    | EB |    |    |C->B|    |   |     |
        |    |    |    |    |    |    |    |    |    |    |   +-----+
        +-------------------------------------------------+      |
        |    |    |    |    |    |    |    |    |    |    |      |
   CH-2 |    |    |B->C|    |B->A|    |    |B->C|    |B->A|   +-----+
        |    |    |    |    |    |    |    |    |    |    |   |  B  |
        +-------------------------------------------------+   |     |
    ...                                                       +-----+
        +-------------------------------------------------+      |
        |    |    |    |    |    |    |    |    |    |    |   +-----+
   CH-15|    |A->B|    |    |    |    |A->B|    |    |    |   |  A  |
        |    |    |    |    |    |    |    |    |    |    |   |     |
        +-------------------------------------------------+   +-----+

     Figure 1: Slotframe example with scheduled cells between nodes A,
                                  B and C

   This schedule represents the possible communications of a node with
   its neighbors, and is managed by a Scheduling Function such as the
   Minimal Scheduling Function (MSF) [I-D.ietf-6tisch-msf].  Each cell

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   in the schedule is identified by its slotoffset and channeloffset
   coordinates.  A cell's timeslot offset indicates its position in
   time, relative to the beginning of the slotframe.  A cell's channel
   offset is an index which maps to a frequency at each iteration of the
   slotframe.  Each packet exchanged between neighbors happens within
   one cell.  The size of a cell is a timeslot duration, between 10 to
   15 milliseconds.  An Absolute Slot Number (ASN) indicates the number
   of slots elapsed since the network started.  It increments at every
   slot.  This is a 5 byte counter that can support networks running for
   more than 300 years without wrapping (assuming a 10 ms timeslot).
   Channel hopping provides increased reliability to multi-path fading
   and external interference.  It is handled by TSCH through a channel
   hopping sequence referred as macHopSeq in the IEEE802.15.4

   The Time-Frequency Division Multiple Access provided by TSCH enables
   the orchestration of traffic flows, spreading them in time and
   frequency, and hence enabling an efficient management of the
   bandwidth utilization.  Such efficient bandwidth utilization can be
   combined to OFDM modulations also supported by the IEEE802.15.4
   standard [IEEE Std. 802.15.4] since the 2015 version.

   In the RAW context, low power reliable networks should address non-
   critical control scenarios such as Class 2 and monitoring scenarios
   such as Class 4 defined by the RFC5673 [RFC5673].  As a low power
   technology targeting industrial scenarios radio transducers provide
   low data rates (typically between 50kbps to 250kbps) and robust
   modulations to trade-off performance to reliability.  TSCH networks
   are organized in mesh topologies and connected to a backbone.
   Latency in the mesh network is mainly influenced by propagation
   aspects such as interference.  ARQ methods and redundancy techniques
   such as replication and elimination should be studied to provide the
   needed performance to address deterministic scenarios.

5.2.2.  Applicability to Deterministic Flows

   Nodes in a TSCH network are tightly synchronized.  This enables to
   build the slotted structure and ensure efficient utilization of
   resources thanks to proper scheduling policies.  Scheduling is a key
   to orchestrate the resources that different nodes in a Track or a
   path are using.  Slotframes can be split in resource blocks reserving
   the needed capacity to certain flows.  Periodic and bursty traffic
   can be handled independently in the schedule, using active and
   reactive policies and taking advantage of overprovisionned cells to
   measu reth excursion.  Along a Track, resource blocks can be chained
   so nodes in previous hops transmit their data before the next packet
   comes.  This provides a tight control to latency along a Track.
   Collision loss is avoided for best effort traffic by

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   overprovisionning resources, giving time to the management plane of
   the network to dedicate more resources if needed.  Centralized Path Computation

   In a controlled environment, a 6TiSCH device usually does not place a
   request for bandwidth between itself and another device in the
   network.  Rather, an Operation Control System (OCS) invoked through
   an Human/Machine Interface (HMI) iprovides the Traffic Specification,
   in particular in terms of latency and reliability, and the end nodes,
   to a Path Computation element (PCE).  With this, the PCE computes a
   Track between the end nodes and provisions every hop in the Track
   with per-flow state that describes the per-hop operation for a given
   packet, the corresponding timeSlots, and the flow identification to
   recognize which packet is placed in which Track, sort out duplicates,
   etc.  In Figure 2, an example of Operational Control System and HMI
   is depicted.

   For a static configuration that serves a certain purpose for a long
   period of time, it is expected that a node will be provisioned in one
   shot with a full schedule, which incorporates the aggregation of its
   behavior for multiple Tracks.  The 6TiSCH Architecture expects that
   the programing of the schedule is done over CoAP as discussed in
   "6TiSCH Resource Management and Interaction using CoAP"

   But an Hybrid mode may be required as well whereby a single Track is
   added, modified, or removed, for instance if it appears that a Track
   does not perform as expected for, say, Packet Delivery Ratio (PDR).
   For that case, the expectation is that a protocol that flows along a
   Track (to be), in a fashion similar to classical Traffic Engineering
   (TE) [CCAMP], may be used to update the state in the devices.  6TiSCH
   provides means for a device to negotiate a timeSlot with a neighbor,
   but in general that flow was not designed and no protocol was
   selected and it is expected that DetNet will determine the
   appropriate end-to-end protocols to be used in that case.

   Stream Management Entity

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                         Operational Control System and HMI

      -+-+-+-+-+-+-+ Northbound -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                PCE         PCE              PCE              PCE

      -+-+-+-+-+-+-+ Southbound -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

              --- 6TiSCH------6TiSCH------6TiSCH------6TiSCH--
     6TiSCH /     Device      Device      Device      Device   \
     Device-                                                    - 6TiSCH
            \     6TiSCH      6TiSCH      6TiSCH      6TiSCH   /  Device

                                  Figure 2  Packet Marking and Handling

   Section "Packet Marking and Handling" of
   [I-D.ietf-6tisch-architecture] describes the packet tagging and
   marking that is expected in 6TiSCH networks.  Tagging Packets for Flow Identification

   For packets that are routed by a PCE along a Track, the tuple formed
   by the IPv6 source address and a local RPLInstanceID is tagged in the
   packets to identify uniquely the Track and associated transmit bundle
   of timeSlots.

   It results that the tagging that is used for a DetNet flow outside
   the 6TiSCH LLN MUST be swapped into 6TiSCH formats and back as the
   packet enters and then leaves the 6TiSCH network.

   Note: The method and format used for encoding the RPLInstanceID at
   6lo is generalized to all 6TiSCH topological Instances, which
   includes Tracks.  Replication, Retries and Elimination

   PRE establishes several paths in a network to provide redundancy and
   parallel transmissions to bound the end-to-end delay.  Considering
   the scenario shown in Figure 3, many different paths are possible for
   S to reach R.  A simple way to benefit from this topology could be to
   use the two independent paths via nodes A, C, E and via B, D, F.  But
   more complex paths are possible as well.

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                            (A)   (C)   (E)

              source (S)                       (R) (destination)

                            (B)   (D)   (F)

      Figure 3: A Typical Ladder Shape with Two Parallel Paths Toward
                              the Destination

   By employing a Packet Replication function, each node forwards a copy
   of each data packet over two different branches.  For instance, in
   Figure 4, the source node S transmits the data packet to nodes A and
   B, in two different timeslots within the same TSCH slotframe.

                      ===> (A) => (C) => (E) ===
                    //        \\//   \\//       \\
          source (S)          //\\   //\\         (R) (destination)
                    \\       //  \\ //  \\      //
                      ===> (B) => (D) => (F) ===

       Figure 4: Packet Replication: S transmits twice the same data
                  packet, to its DP (A) and to its AP (B).

   By employing Packet Elimination function once a node receives the
   first copy of a data packet, it discards the subsequent copies.
   Because the first copy that reaches a node is the one that matters,
   it is the only copy that will be forwarded upward.

   Considering that the wireless medium is broadcast by nature, any
   neighbor of a transmitter may overhear a transmission.  By employing
   the Promiscuous Overhearing function, nodes will have multiple
   opportunities to receive a given data packet.  For instance, in
   Figure 4, when the source node S transmits the data packet to node A,
   node B may overhear this transmission.

   6TiSCH expects elimination and replication of packets along a complex
   Track, but has no position about how the sequence numbers would be
   tagged in the packet.

   As it goes, 6TiSCH expects that timeSlots corresponding to copies of
   a same packet along a Track are correlated by configuration, and does
   not need to process the sequence numbers.

   The semantics of the configuration MUST enable correlated timeSlots
   to be grouped for transmit (and respectively receive) with
   a'OR'relations, and then a'AND'relation MUST be configurable between

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   groups.  The semantics is that if the transmit (and respectively
   receive) operation succeeded in one timeSlot in a'OR'group, then all
   the other timeSLots in the group are ignored.  Now, if there are at
   least two groups, the'AND'relation between the groups indicates that
   one operation must succeed in each of the groups.

   On the transmit side, timeSlots provisioned for retries along a same
   branch of a Track are placed a same'OR'group.  The'OR'relation
   indicates that if a transmission is acknowledged, then further
   transmissions SHOULD NOT be attempted for timeSlots in that group.
   There are as many'OR'groups as there are branches of the Track
   departing from this node.  Different'OR'groups are programmed for the
   purpose of replication, each group corresponding to one branch of the
   Track.  The'AND'relation between the groups indicates that
   transmission over any of branches MUST be attempted regardless of
   whether a transmission succeeded in another branch.  It is also
   possible to place cells to different next-hop routers in a
   same'OR'group.  This allows to route along multi-path Tracks, trying
   one next-hop and then another only if sending to the first fails.

   On the receive side, all timeSlots are programmed in a same'OR'group.
   Retries of a same copy as well as converging branches for elimination
   are converged, meaning that the first successful reception is enough
   and that all the other timeSlots can be ignored.  Differentiated Services Per-Hop-Behavior

   Additionally, an IP packet that is sent along a Track uses the
   Differentiated Services Per-Hop-Behavior Group called Deterministic
   Forwarding, as described in
   [I-D.svshah-tsvwg-deterministic-forwarding].  Topology and capabilities

   6TiSCH nodes are usually IoT devices, characterized by very limited
   amount of memory, just enough buffers to store one or a few IPv6
   packets, and limited bandwidth between peers.  It results that a node
   will maintain only a small number of peering information, and will
   not be able to store many packets waiting to be forwarded.  Peers can
   be identified through MAC or IPv6 addresses.

   Neighbors can be discovered over the radio using mechanism such as
   Enhanced Beacons, but, though the neighbor information is available
   in the 6TiSCH interface data model, 6TiSCH does not describe a
   protocol to pro-actively push the neighborhood information to a PCE.
   This protocol should be described and should operate over CoAP.  The
   protocol should be able to carry multiple metrics, in particular the
   same metrics as used for RPL operations [RFC6551].

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   The energy that the device consumes in sleep, transmit and receive
   modes can be evaluated and reported.  So can the amount of energy
   that is stored in the device and the power that it can be scavenged
   from the environment.  The PCE SHOULD be able to compute Tracks that
   will implement policies on how the energy is consumed, for instance
   balance between nodes, ensure that the spent energy does not exceeded
   the scavenged energy over a period of time, etc...  Schedule Management by a PCE

   6TiSCH supports a mixed model of centralized routes and distributed
   routes.  Centralized routes can for example be computed by a entity
   such as a PCE [PCE].  Distributed routes are computed by RPL

   Both methods may inject routes in the Routing Tables of the 6TiSCH
   routers.  In either case, each route is associated with a 6TiSCH
   topology that can be a RPL Instance topology or a Track.  The 6TiSCH
   topology is indexed by a Instance ID, in a format that reuses the
   RPLInstanceID as defined in RPL.

   Both RPL and PCE rely on shared sources such as policies to define
   Global and Local RPLInstanceIDs that can be used by either method.
   It is possible for centralized and distributed routing to share a
   same topology.  Generally they will operate in different slotFrames,
   and centralized routes will be used for scheduled traffic and will
   have precedence over distributed routes in case of conflict between
   the slotFrames.  SlotFrames and Priorities

   A slotFrame is the base object that a PCE needs to manipulate to
   program a schedule into an LLN node.  Elaboration on that concept can
   be fond in section "SlotFrames and Priorities" of

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   IEEE802.15.4 TSCH avoids contention on the medium by formatting time
   and frequencies in cells of transmission of equal duration.  In order
   to describe that formatting of time and frequencies, the 6TiSCH
   architecture defines a global concept that is called a Channel
   Distribution and Usage (CDU) matrix; a CDU matrix is a matrix of
   cells with an height equal to the number of available channels
   (indexed by ChannelOffsets) and a width (in timeSlots) that is the
   period of the network scheduling operation (indexed by slotOffsets)
   for that CDU matrix.  The size of a cell is a timeSlot duration, and
   values of 10 to 15 milliseconds are typical in 802.15.4 TSCH to
   accommodate for the transmission of a frame and an acknowledgement,
   including the security validation on the receive side which may take
   up to a few milliseconds on some device architecture.

   The frequency used by a cell in the matrix rotates in a pseudo-random
   fashion, from an initial position at an epoch time, as the matrix
   iterates over and over.

   A CDU matrix is computed by the PCE, but unallocated timeSlots may be
   used opportunistically by the nodes for classical best effort IP
   traffic.  The PCE has precedence in the allocation in case of a

   In a given network, there might be multiple CDU matrices that operate
   with different width, so they have different durations and represent
   different periodic operations.  It is recommended that all CDU
   matrices in a 6TiSCH domain operate with the same cell duration and
   are aligned, so as to reduce the chances of interferences from
   slotted-aloha operations.  The PCE MUST compute the CDU matrices and
   shared that knowledge with all the nodes.  The matrices are used in
   particular to define slotFrames.

   A slotFrame is a MAC-level abstraction that is common to all nodes
   and contains a series of timeSlots of equal length and precedence.
   It is characterized by a slotFrame_ID, and a slotFrame_size.  A
   slotFrame aligns to a CDU matrix for its parameters, such as number
   and duration of timeSlots.

   Multiple slotFrames can coexist in a node schedule, i.e., a node can
   have multiple activities scheduled in different slotFrames, based on
   the precedence of the 6TiSCH topologies.  The slotFrames may be
   aligned to different CDU matrices and thus have different width.
   There is typically one slotFrame for scheduled traffic that has the
   highest precedence and one or more slotFrame(s) for RPL traffic.  The
   timeSlots in the slotFrame are indexed by the SlotOffset; the first
   cell is at SlotOffset 0.

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   The 6TiSCH architecture introduces the concept of chunks
   ([I-D.ietf-6tisch-architecture]) to operate such spectrum
   distribution for a whole group of cells at a time.  The CDU matrix is
   formatted into a set of chunks, each of them identified uniquely by a
   chunk-ID, see Figure 5.  The PCE MUST compute the partitioning of CDU
   matrices into chunks and shared that knowledge with all the nodes in
   a 6TiSCH network.

                +-----+-----+-----+-----+-----+-----+-----+     +-----+
   chan.Off. 0  |chnkA|chnkP|chnk7|chnkO|chnk2|chnkK|chnk1| ... |chnkZ|
                +-----+-----+-----+-----+-----+-----+-----+     +-----+
   chan.Off. 1  |chnkB|chnkQ|chnkA|chnkP|chnk3|chnkL|chnk2| ... |chnk1|
                +-----+-----+-----+-----+-----+-----+-----+     +-----+
                +-----+-----+-----+-----+-----+-----+-----+     +-----+
   chan.Off. 15 |chnkO|chnk6|chnkN|chnk1|chnkJ|chnkZ|chnkI| ... |chnkG|
                +-----+-----+-----+-----+-----+-----+-----+     +-----+
                   0     1     2     3     4     5     6          M

                Figure 5: CDU matrix Partitioning in Chunks

   The appropriation of a chunk can be requested explicitly by the PCE
   to any node.  After a successful appropriation, the PCE owns the
   cells in that chunk, and may use them as hard cells to set up Tracks.
   Then again, 6TiSCH did not propose a method for chunk definition and
   a protocol for appropriation.  This is to be done at RAW.  6TiSCH Tracks

   A Track at 6TiSCH is the application to wireless of the concept of a
   path in the Detnet architecture [I-D.ietf-detnet-architecture].  A
   Track can follow a simple sequence of relay nodes or can be
   structured as a more complex Destination Oriented Directed Acyclic
   Graph (DODAG) to a unicast destination.  Along a Track, 6TiSCH nodes
   reserve the resources to enable the efficient transmission of packets
   while aiming to optimize certain properties such as reliability and
   ensure small jitter or bounded latency.  The Track structure enables
   Layer-2 forwarding schemes, reducing the overhead of taking routing
   decisions at the Layer-3.

   Serial Tracks can be understood as the concatenation of cells or
   bundles along a routing path from a source towards a destination.
   The serial Track concept is analogous to the circuit concept where
   resources are chained through the multi-hop topology.  For example, A
   bundle of Tx Cells in a particular node is paired to a bundle of Rx
   Cells in the next hop node following a routing path.

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   Whereas scheduling ensures reliable delivery in bounded time along
   any Track, high availability requires the application of PAREO
   functions along a more complex DODAG Track structure.  A DODAG has
   forking and joining nodes where the concepts such as Replication and
   Elimination can be exploited.  Spatial redundancy increases the
   oveall energy consumption in the network but improves significantly
   the availability of the network as well as the packet delivery ratio.
   A Track may also branch off and rejoin, for the purpose of the so-
   called Packet Replication and Elimination (PRE), over non congruent
   branches.  PRE may be used to complement layer-2 Automatic Repeat
   reQuest (ARQ) and receiver-end Ordering to form the PAREO functions.
   PAREO functions enable to meet industrial expectations in PDR within
   bounded delivery time over a Track that includes wireless links, even
   when the Track extends beyond the 6TiSCH network.

                     | IoT |
                     | G/W |
                        ^  <---- Elimination
                       | |
        Track branch   | |
               +-------+ +--------+ Subnet Backbone
               |                  |
            +--|--+            +--|--+
            |  |  | Backbone   |  |  | Backbone
       o    |  |  | router     |  |  | router
            +--/--+            +--|--+
       o     /    o     o---o----/       o
           o    o---o--/   o      o   o  o   o
      o     \  /     o               o   LLN    o
         o   v  <---- Replication

                  Figure 6: End-to-End deterministic Track

   In the example above (see Figure 6), a Track is laid out from a field
   device in a 6TiSCH network to an IoT gateway that is located on a
   IEEE802.1 TSN backbone.

   The Replication function in the field device sends a copy of each
   packet over two different branches, and a PCE schedules each hop of
   both branches so that the two copies arrive in due time at the
   gateway.  In case of a loss on one branch, hopefully the other copy
   of the packet still makes it in due time.  If two copies make it to
   the IoT gateway, the Elimination function in the gateway ignores the
   extra packet and presents only one copy to upper layers.

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   At each 6TiSCH hop along the Track, the PCE may schedule more than
   one timeSlot for a packet, so as to support Layer-2 retries (ARQ).
   It is also possible that the field device only uses the second branch
   if sending over the first branch fails.

   In current deployments, a TSCH Track does not necessarily support PRE
   but is systematically multi-path.  This means that a Track is
   scheduled so as to ensure that each hop has at least two forwarding
   solutions, and the forwarding decision is to try the preferred one
   and use the other in case of Layer-2 transmission failure as detected
   by ARQ.

   Methods to implement complex Tracks are described in
   [I-D.papadopoulos-paw-pre-reqs] and complemented by extensions to the
   RPL routing protocol in [I-D.ietf-roll-nsa-extension] for best effort
   traffic, but a centralized routing technique such as promoted in
   DetNet is still missing.  Track Scheduling Protocol

   Section "Schedule Management Mechanisms" of the 6TiSCH architecture
   describes 4 paradigms to manage the TSCH schedule of the LLN nodes:
   Static Scheduling, neighbor-to-neighbor Scheduling, remote monitoring
   and scheduling management, and Hop-by-hop scheduling.  The Track
   operation for DetNet corresponds to a remote monitoring and
   scheduling management by a PCE.

   Early work at 6TiSCH on a data model and a protocol to program the
   schedule in the 6TiSCH device was never concluded as the group
   focussed on best effort traffic.  This work would be revived by RAW:

      The 6top interface document [RFC8480] (to be reopened at RAW) was
      intended to specify the generic data model that can be used to
      monitor and manage resources of the 6top sublayer.  Abstract
      methods were suggested for use by a management entity in the
      device.  The data model also enables remote control operations on
      the 6top sublayer.

      [I-D.ietf-6tisch-coap] (to be reopened at RAW) was intended to
      define a mapping of the 6top set of commands, which is described
      in RFC 8480, to CoAP resources.  This allows an entity to interact
      with the 6top layer of a node that is multiple hops away in a
      RESTful fashion.

      [I-D.ietf-6tisch-coap] also defined a basic set CoAP resources and
      associated RESTful access methods (GET/PUT/POST/DELETE).  The
      payload (body) of the CoAP messages is encoded using the CBOR
      format.  The PCE commands are expected to be issued directly as

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      CoAP requests or to be mapped back and forth into CoAP by a
      gateway function at the edge of the 6TiSCH network.  For instance,
      it is possible that a mapping entity on the backbone transforms a
      non-CoAP protocol such as PCEP into the RESTful interfaces that
      the 6TiSCH devices support.  Track Forwarding

   By forwarding, this specification means the per-packet operation that
   allows to deliver a packet to a next hop or an upper layer in this
   node.  Forwarding is based on pre-existing state that was installed
   as a result of the routing computation of a Track by a PCE.  The
   6TiSCH architecture supports three different forwarding model, G-MPLS
   Track Forwarding (TF), 6LoWPAN Fragment Forwarding (FF) and IPv6
   Forwarding (6F) which is the classical IP operation
   [I-D.ietf-6tisch-architecture].  The DetNet case relates to the Track
   Forwarding operation under the control of a PCE.

   A Track is a unidirectional path between a source and a destination.
   In a Track cell, the normal operation of IEEE802.15.4 Automatic
   Repeat-reQuest (ARQ) usually happens, though the acknowledgment may
   be omitted in some cases, for instance if there is no scheduled cell
   for a retry.

   Track Forwarding is the simplest and fastest.  A bundle of cells set
   to receive (RX-cells) is uniquely paired to a bundle of cells that
   are set to transmit (TX-cells), representing a layer-2 forwarding
   state that can be used regardless of the network layer protocol.
   This model can effectively be seen as a Generalized Multi-protocol
   Label Switching (G-MPLS) operation in that the information used to
   switch a frame is not an explicit label, but rather related to other
   properties of the way the packet was received, a particular cell in
   the case of 6TiSCH.  As a result, as long as the TSCH MAC (and
   Layer-2 security) accepts a frame, that frame can be switched
   regardless of the protocol, whether this is an IPv6 packet, a 6LoWPAN
   fragment, or a frame from an alternate protocol such as WirelessHART
   or ISA100.11a.

   A data frame that is forwarded along a Track normally has a
   destination MAC address that is set to broadcast - or a multicast
   address depending on MAC support.  This way, the MAC layer in the
   intermediate nodes accepts the incoming frame and 6top switches it
   without incurring a change in the MAC header.  In the case of
   IEEE802.15.4, this means effectively broadcast, so that along the
   Track the short address for the destination of the frame is set to

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   A Track is thus formed end-to-end as a succession of paired bundles,
   a receive bundle from the previous hop and a transmit bundle to the
   next hop along the Track, and a cell in such a bundle belongs to at
   most one Track.  For a given iteration of the device schedule, the
   effective channel of the cell is obtained by adding a pseudo-random
   number to the channelOffset of the cell, which results in a rotation
   of the frequency that used for transmission.  The bundles may be
   computed so as to accommodate both variable rates and
   retransmissions, so they might not be fully used at a given iteration
   of the schedule.  The 6TiSCH architecture provides additional means
   to avoid waste of cells as well as overflows in the transmit bundle,
   as follows:

   In one hand, a TX-cell that is not needed for the current iteration
   may be reused opportunistically on a per-hop basis for routed
   packets.  When all of the frame that were received for a given Track
   are effectively transmitted, any available TX-cell for that Track can
   be reused for upper layer traffic for which the next-hop router
   matches the next hop along the Track.  In that case, the cell that is
   being used is effectively a TX-cell from the Track, but the short
   address for the destination is that of the next-hop router.  It
   results that a frame that is received in a RX-cell of a Track with a
   destination MAC address set to this node as opposed to broadcast must
   be extracted from the Track and delivered to the upper layer (a frame
   with an unrecognized MAC address is dropped at the lower MAC layer
   and thus is not received at the 6top sublayer).

   On the other hand, it might happen that there are not enough TX-cells
   in the transmit bundle to accommodate the Track traffic, for instance
   if more retransmissions are needed than provisioned.  In that case,
   the frame can be placed for transmission in the bundle that is used
   for layer-3 traffic towards the next hop along the Track as long as
   it can be routed by the upper layer, that is, typically, if the frame
   transports an IPv6 packet.  The MAC address should be set to the
   next-hop MAC address to avoid confusion.  It results that a frame
   that is received over a layer-3 bundle may be in fact associated to a
   Track.  In a classical IP link such as an Ethernet, off-Track traffic
   is typically in excess over reservation to be routed along the non-
   reserved path based on its QoS setting.  But with 6TiSCH, since the
   use of the layer-3 bundle may be due to transmission failures, it
   makes sense for the receiver to recognize a frame that should be re-
   Tracked, and to place it back on the appropriate bundle if possible.
   A frame should be re-Tracked if the Per-Hop-Behavior group indicated
   in the Differentiated Services Field in the IPv6 header is set to
   Deterministic Forwarding, as discussed in Section  A frame
   is re-Tracked by scheduling it for transmission over the transmit
   bundle associated to the Track, with the destination MAC address set
   to broadcast.

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   There are 2 modes for a Track, transport mode and tunnel mode.  Transport Mode

   In transport mode, the Protocol Data Unit (PDU) is associated with
   flow-dependant meta-data that refers uniquely to the Track, so the
   6top sublayer can place the frame in the appropriate cell without
   ambiguity.  In the case of IPv6 traffic, this flow identification is
   transported in the Flow Label of the IPv6 header.  Associated with
   the source IPv6 address, the Flow Label forms a globally unique
   identifier for that particular Track that is validated at egress
   before restoring the destination MAC address (DMAC) and punting to
   the upper layer.

                          |                                    ^
      +--------------+    |                                    |
      |     IPv6     |    |                                    |
      +--------------+    |                                    |
      |  6LoWPAN HC  |    |                                    |
      +--------------+  ingress                              egress
      |     6top     |   sets     +----+          +----+     restores
      +--------------+  dmac to   |    |          |    |     dmac to
      |   TSCH MAC   |   brdcst   |    |          |    |      self
      +--------------+    |       |    |          |    |       |
      |   LLN PHY    |    +-------+    +--...-----+    +-------+

                 Figure 7: Track Forwarding, Transport Mode  Tunnel Mode

   In tunnel mode, the frames originate from an arbitrary protocol over
   a compatible MAC that may or may not be synchronized with the 6TiSCH
   network.  An example of this would be a router with a dual radio that
   is capable of receiving and sending WirelessHART or ISA100.11a frames
   with the second radio, by presenting itself as an Access Point or a
   Backbone Router, respectively.

   In that mode, some entity (e.g.  PCE) can coordinate with a
   WirelessHART Network Manager or an ISA100.11a System Manager to
   specify the flows that are to be transported transparently over the

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      |     IPv6     |
      |  6LoWPAN HC  |
      +--------------+             set            restore
      |     6top     |            +dmac+          +dmac+
      +--------------+          to|brdcst       to|nexthop
      |   TSCH MAC   |            |    |          |    |
      +--------------+            |    |          |    |
      |   LLN PHY    |    +-------+    +--...-----+    +-------+
      +--------------+    |   ingress                 egress   |
                          |                                    |
      +--------------+    |                                    |
      |   LLN PHY    |    |                                    |
      +--------------+    |                                    |
      |   TSCH MAC   |    |                                    |
      +--------------+    | dmac =                             | dmac =
      |ISA100/WiHART |    | nexthop                            v nexthop

                  Figure 8: Track Forwarding, Tunnel Mode

   In that case, the flow information that identifies the Track at the
   ingress 6TiSCH router is derived from the RX-cell.  The dmac is set
   to this node but the flow information indicates that the frame must
   be tunneled over a particular Track so the frame is not passed to the
   upper layer.  Instead, the dmac is forced to broadcast and the frame
   is passed to the 6top sublayer for switching.

   At the egress 6TiSCH router, the reverse operation occurs.  Based on
   metadata associated to the Track, the frame is passed to the
   appropriate link layer with the destination MAC restored.  Tunnel Metadata

   Metadata coming with the Track configuration is expected to provide
   the destination MAC address of the egress endpoint as well as the
   tunnel mode and specific data depending on the mode, for instance a
   service access point for frame delivery at egress.  If the tunnel
   egress point does not have a MAC address that matches the
   configuration, the Track installation fails.

   In transport mode, if the final layer-3 destination is the tunnel
   termination, then it is possible that the IPv6 address of the
   destination is compressed at the 6LoWPAN sublayer based on the MAC
   address.  It is thus mandatory at the ingress point to validate that
   the MAC address that was used at the 6LoWPAN sublayer for compression
   matches that of the tunnel egress point.  For that reason, the node

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   that injects a packet on a Track checks that the destination is
   effectively that of the tunnel egress point before it overwrites it
   to broadcast.  The 6top sublayer at the tunnel egress point reverts
   that operation to the MAC address obtained from the tunnel metadata.  OAM

   An Overview of Operations, Administration, and Maintenance (OAM)
   Tools [RFC7276] provides an overwiew of the existing tooling for OAM
   [RFC6291].  Tracks are complex paths and new tooling is necessary to
   manage them, with respect to load control, timing, and the Packet
   Replication and Elimination Functions (PREF).

   An example of such tooling can be found in the context of BIER
   [RFC8279] and more specifically BIER Traffic Engineering
   [I-D.ietf-bier-te-arch] (BIER-TE):
   [I-D.thubert-bier-replication-elimination] leverages BIER-TE to
   control the process of PREF, and to provide traceability of these
   operations, in the deterministic dataplane, along a complex Track.
   For the 6TiSCH type of constrained environment,
   [I-D.thubert-6lo-bier-dispatch] enables an efficient encoding of the
   BIER bitmap within the 6LoRH framework.

6.  5G

6.1.  Provenance and Documents

   The 3rd Generation Partnership Project (3GPP) incorporates many
   companies whose business is related to cellular network operation as
   well as network equipment and device manufacturing.  All generations
   of 3GPP technologies provide scheduled wireless segments, primarily
   in licensed spectrum which is beneficial for reliability and

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   In 2016, the 3GPP started to design New Radio (NR) technology
   belonging to the fifth generation (5G) of cellular networks.  NR has
   been designed from the beginning to not only address enhanced Mobile
   Broadband (eMBB) services for consumer devices such as smart phones
   or tablets but is also tailored for future Internet of Things (IoT)
   communication and connected cyber-physical systems.  In addition to
   eMBB, requirement categories have been defined on Massive Machine-
   Type Communication (M-MTC) for a large number of connected devices/
   sensors, and Ultra-Reliable Low-Latency Communication (URLLC) for
   connected control systems and critical communication as illustrated
   in Figure 9.  It is the URLLC capabilities that make 5G a great
   candidate for reliable low-latency communication.  With these three
   corner stones, NR is a complete solution supporting the connectivity
   needs of consumers, enterprises, and public sector for both wide area
   and local area, e.g. indoor deployments.  A general overview of NR
   can be found in [TS38300].

                            Mobile Broadband
                                  / \
                                 /   \
                                /     \
                               /       \
                              /   5G    \
                             /           \
                            /             \
                           /               \
                       Massive          Ultra-Reliable
                     Machine-Type        Low-Latency
                    Communication       Communication

                       Figure 9: 5G Application Areas

   As a result of releasing the first NR specification in 2018 (Release
   15), it has been proven by many companies that NR is a URLLC-capable
   technology and can deliver data packets at 10^-5 packet error rate
   within 1ms latency budget [TR37910].  Those evaluations were
   consolidated and forwarded to ITU to be included in the [IMT2020]

   In order to understand communication requirements for automation in
   vertical domains, 3GPP studied different use cases [TR22804] and
   released technical specification with reliability, availability and
   latency demands for a variety of applications [TS22104].

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   As an evolution of NR, multiple studies have been conducted in scope
   of 3GPP Release 16 including the following two, focusing on radio

   1.  Study on physical layer enhancements for NR ultra-reliable and
       low latency communication (URLLC) [TR38824].

   2.  Study on NR industrial Internet of Things (I-IoT) [TR38825].

   In addition, several enhancements have been done on system
   architecture level which are reflected in System architecture for the
   5G System (5GS) [TS23501].

6.2.  General Characteristics

   The 5G Radio Access Network (5G RAN) with its NR interface includes
   several features to achieve Quality of Service (QoS), such as a
   guaranteeably low latency or tolerable packet error rates for
   selected data flows.  Determinism is achieved by centralized
   admission control and scheduling of the wireless frequency resources,
   which are typically licensed frequency bands assigned to a network

   NR enables short transmission slots in a radio subframe, which
   benefits low-latency applications.  NR also introduces mini-slots,
   where prioritized transmissions can be started without waiting for
   slot boundaries, further reducing latency.  As part of giving
   priority and faster radio access to URLLC traffic, NR introduces
   preemption where URLLC data transmission can preempt ongoing non-
   URLLC transmissions.  Additionally, NR applies very fast processing,
   enabling retransmissions even within short latency bounds.

   NR defines extra-robust transmission modes for increased reliability
   both for data and control radio channels.  Reliability is further
   improved by various techniques, such as multi-antenna transmission,
   the use of multiple frequency carriers in parallel and packet
   duplication over independent radio links.  NR also provides full
   mobility support, which is an important reliability aspect not only
   for devices that are moving, but also for devices located in a
   changing environment.

   Network slicing is seen as one of the key features for 5G, allowing
   vertical industries to take advantage of 5G networks and services.
   Network slicing is about transforming a Public Land Mobile Network
   (PLMN) from a single network to a network where logical partitions
   are created, with appropriate network isolation, resources, optimized
   topology and specific configuration to serve various service

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   requirements.  An operator can configure and manage the mobile
   network to support various types of services enabled by 5G, for
   example eMBB and URLLC, depending on the different customers' needs.

   Exposure of capabilities of 5G Systems to the network or applications
   outside the 3GPP domain have been added to Release 16 [TS23501].  Via
   exposure interfaces, applications can access 5G capabilities, e.g.,
   communication service monitoring and network maintenance.

   For several generations of mobile networks, 3GPP has considered how
   the communication system should work on a global scale with billions
   of users, taking into account resilience aspects, privacy regulation,
   protection of data, encryption, access and core network security, as
   well as interconnect.  Security requirements evolve as demands on
   trustworthiness increase.  For example, this has led to the
   introduction of enhanced privacy protection features in 5G. 5G also
   employs strong security algorithms, encryption of traffic, protection
   of signaling and protection of interfaces.

   One particular strength of mobile networks is the authentication,
   based on well-proven algorithms and tightly coupled with a global
   identity management infrastructure.  Since 3G, there is also mutual
   authentication, allowing the network to authenticate the device and
   the device to authenticate the network.  Another strength is secure
   solutions for storage and distribution of keys fulfilling regulatory
   requirements and allowing international roaming.  When connecting to
   5G, the user meets the entire communication system, where security is
   the result of standardization, product security, deployment,
   operations and management as well as incident handling capabilities.
   The mobile networks approach the entirety in a rather coordinated
   fashion which is beneficial for security.

6.3.  Deployment and Spectrum

   The 5G system allows deployment in a vast spectrum range, addressing
   use-cases in both wide-area as well as local networks.  Furthermore,
   5G can be configured for public and non-public access.

   When it comes to spectrum, NR allows combining the merits of many
   frequency bands, such as the high bandwidths in millimeter Waves
   (mmW) for extreme capacity locally, as well as the broad coverage
   when using mid- and low frequency bands to address wide-area
   scenarios.  URLLC is achievable in all these bands.  Spectrum can be
   either licensed, which means that the license holder is the only
   authorized user of that spectrum range, or unlicensed, which means
   that anyone who wants to use the spectrum can do so.

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   A prerequisite for critical communication is performance
   predictability, which can be achieved by the full control of the
   access to the spectrum, which 5G provides.  Licensed spectrum
   guarantees control over spectrum usage by the system, making it a
   preferable option for critical communication.  However, unlicensed
   spectrum can provide an additional resource for scaling non-critical
   communications.  While NR is initially developed for usage of
   licensed spectrum, the functionality to access also unlicensed
   spectrum was introduced in 3GPP Release 16.

   Licensed spectrum dedicated to mobile communications has been
   allocated to mobile service providers, i.e. issued as longer-term
   licenses by national administrations around the world.  These
   licenses have often been associated with coverage requirements and
   issued across whole countries, or in large regions.  Besides this,
   configured as a non-public network (NPN) deployment, 5G can provide
   network services also to a non-operator defined organization and its
   premises such as a factory deployment.  By this isolation, quality of
   service requirements, as well as security requirements can be
   achieved.  An integration with a public network, if required, is also
   possible.  The non-public (local) network can thus be interconnected
   with a public network, allowing devices to roam between the networks.

   In an alternative model, some countries are now in the process of
   allocating parts of the 5G spectrum for local use to industries.
   These non-service providers then have a choice of applying for a
   local license themselves and operating their own network or
   cooperating with a public network operator or service provider.

6.4.  Applicability to Deterministic Flows

6.4.1.  System Architecture

   The 5G system [TS23501] consists of the User Equipment (UE) at the
   terminal side, and the Radio Access Network (RAN) with the gNB as
   radio base station node, as well as the Core Network (CN).  The core
   network is based on a service-based architecture with the central
   functions: Access and Mobility Management Function (AMF), Session
   Management Function (SMF) and User Plane Function (UPF) as
   illustrated in Figure 10.

   The gNB's main responsibility is the radio resource management,
   including admission control and scheduling, mobility control and
   radio measurement handling.  The AMF handles the UE's connection
   status and security, while the SMF controls the UE's data sessions.
   The UPF handles the user plane traffic.

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   The SMF can instantiate various Packet Data Unit (PDU) sessions for
   the UE, each associated with a set of QoS flows, i.e., with different
   QoS profiles.  Segregation of those sessions is also possible, e.g.,
   resource isolation in the RAN and in the CN can be defined (slicing).

             +----+  +---+   +---+    +---+    +---+   +---+
             |NSSF|  |NEF|   |NRF|    |PCF|    |UDM|   |AF |
             +--+-+  +-+-+   +-+-+    +-+-+    +-+-+   +-+-+
                |      |       |        |        |       |
           Nnssf|  Nnef|   Nnrf|    Npcf|    Nudm|    Naf|
                |      |       |        |        |       |
                         |       |            |         |
                    Nausf|  Nausf|        Nsmf|         |
                         |       |            |         |
                      +--+-+   +-+-+        +-+-+     +-+-+
                      |AUSF|   |AMF|        |SMF|     |SCP|
                      +----+   +++-+        +-+-+     +---+
                               / |            |
                              /  |            |
                             /   |            |
                            N1   N2           N4
                           /     |            |
                          /      |            |
                         /       |            |
                     +--+-+   +--+--+      +--+---+      +----+
                     | UE +---+(R)AN+--N3--+ UPF  +--N6--+ DN |
                     +----+   +-----+      ++----++      +----+
                                            |    |

                     Figure 10: 5G System Architecture

   To allow UE mobility across cells/gNBs, handover mechanisms are
   supported in NR.  For an established connection, i.e., connected mode
   mobility, a gNB can configure a UE to report measurements of received
   signal strength and quality of its own and neighbouring cells,
   periodically or event-based.  Based on these measurement reports, the
   gNB decides to handover a UE to another target cell/gNB.  Before
   triggering the handover, it is hand-shaked with the target gNB based
   on network signalling.  A handover command is then sent to the UE and
   the UE switches its connection to the target cell/gNB.  The Packet
   Data Convergence Protocol (PDCP) of the UE can be configured to avoid
   data loss in this procedure, i.e., handle retransmissions if needed.
   Data forwarding is possible between source and target gNB as well.
   To improve the mobility performance further, i.e., to avoid
   connection failures, e.g., due to too-late handovers, the mechanism
   of conditional handover is introduced in Release 16 specifications.

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   Therein a conditional handover command, defining a triggering point,
   can be sent to the UE before UE enters a handover situation.  A
   further improvement introduced in Release 16 is the Dual Active
   Protocol Stack (DAPS), where the UE maintains the connection to the
   source cell while connecting to the target cell.  This way, potential
   interruptions in packet delivery can be avoided entirely.

6.4.2.  Overview of The Radio Protocol Stack

   The protocol architecture for NR consists of the L1 Physical layer
   (PHY) and as part of the L2, the sublayers of Medium Access Control
   (MAC), Radio Link Control (RLC), Packet Data Convergence Protocol
   (PDCP), as well as the Service Data Adaption Protocol (SDAP).

   The PHY layer handles signal processing related actions, such as
   encoding/decoding of data and control bits, modulation, antenna
   precoding and mapping.

   The MAC sub-layer handles multiplexing and priority handling of
   logical channels (associated with QoS flows) to transport blocks for
   PHY transmission, as well as scheduling information reporting and
   error correction through Hybrid Automated Repeat Request (HARQ).

   The RLC sublayer handles sequence numbering of higher layer packets,
   retransmissions through Automated Repeat Request (ARQ), if
   configured, as well as segmentation and reassembly and duplicate

   The PDCP sublayer consists of functionalities for ciphering/
   deciphering, integrity protection/verification, re-ordering and in-
   order delivery, duplication and duplicate handling for higher layer
   packets, and acts as the anchor protocol to support handovers.

   The SDAP sublayer provides services to map QoS flows, as established
   by the 5G core network, to data radio bearers (associated with
   logical channels), as used in the 5G RAN.

   Additionally, in RAN, the Radio Resource Control (RRC) protocol,
   handles the access control and configuration signalling for the
   aforementioned protocol layers.  RRC messages are considered L3 and
   thus transmitted also via those radio protocol layers.

   To provide low latency and high reliability for one transmission
   link, i.e., to transport data (or control signaling) of one radio
   bearer via one carrier, several features have been introduced on the
   user plane protocols for PHY and L2, as explained in the following.

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6.4.3.  Radio (PHY)

   NR is designed with native support of antenna arrays utilizing
   benefits from beamforming, transmissions over multiple MIMO layers
   and advanced receiver algorithms allowing effective interference
   cancellation.  Those antenna techniques are the basis for high signal
   quality and effectiveness of spectral usage.  Spatial diversity with
   up to 4 MIMO layers in UL and up to 8 MIMO layers in DL is supported.
   Together with spatial-domain multiplexing, antenna arrays can focus
   power in desired direction to form beams.  NR supports beam
   management mechanisms to find the best suitable beam for UE initially
   and when it is moving.  In addition, gNBs can coordinate their
   respective DL and UL transmissions over the backhaul network keeping
   interference reasonably low, and even make transmissions or
   receptions from multiple points (multi-TRP).  Multi-TRP can be used
   for repetition of data packet in time, in frequency or over multiple
   MIMO layers which can improve reliability even further.

   Any downlink transmission to a UE starts from resource allocation
   signaling over the Physical Downlink Control Channel (PDCCH).  If it
   is successfully received, the UE will know about the scheduled
   transmission and may receive data over the Physical Downlink Shared
   Channel (PDSCH).  If retransmission is required according to the HARQ
   scheme, a signaling of negative acknowledgement (NACK) on the
   Physical Uplink Control Channel (PUCCH) is involved and PDCCH
   together with PDSCH transmissions (possibly with additional
   redundancy bits) are transmitted and soft-combined with previously
   received bits.  Otherwise, if no valid control signaling for
   scheduling data is received, nothing is transmitted on PUCCH
   (discontinuous transmission - DTX),and the base station upon
   detecting DTX will retransmit the initial data.

   An uplink transmission normally starts from a Scheduling Request (SR)
   - a signaling message from the UE to the base station sent via PUCCH.
   Once the scheduler is informed about buffer data in UE, e.g., by SR,
   the UE transmits a data packet on the Physical Uplink Shared Channel
   (PUSCH).  Pre-scheduling not relying on SR is also possible (see
   following section).

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   Since transmission of data packets require usage of control and data
   channels, there are several methods to maintain the needed
   reliability.  NR uses Low Density Parity Check (LDPC) codes for data
   channels, Polar codes for PDCCH, as well as orthogonal sequences and
   Polar codes for PUCCH.  For ultra-reliability of data channels, very
   robust (low spectral efficiency) Modulation and Coding Scheme (MCS)
   tables are introduced containing very low (down to 1/20) LDPC code
   rates using BPSK or QPSK.  Also, PDCCH and PUCCH channels support
   multiple code rates including very low ones for the channel

   A connected UE reports downlink (DL) quality to gNB by sending
   Channel State Information (CSI) reports via PUCCH while uplink (UL)
   quality is measured directly at gNB.  For both uplink and downlink,
   gNB selects the desired MCS number and signals it to the UE by
   Downlink Control Information (DCI) via PDCCH channel.  For URLLC
   services, the UE can assist the gNB by advising that MCS targeting
   10^-5 Block Error Rate (BLER) are used.  Robust link adaptation
   algorithms can maintain the needed level of reliability considering a
   given latency bound.

   Low latency on the physical layer is provided by short transmission
   duration which is possible by using high Subcarrier Spacing (SCS) and
   the allocation of only one or a few Orthogonal Frequency Division
   Multiplexing (OFDM) symbols.  For example, the shortest latency for
   the worst case in DL can be 0.23ms and in UL can be 0.24ms according
   to (section 5.7.1 in [TR37910]).  Moreover, if the initial
   transmission has failed, HARQ feedback can quickly be provided and an
   HARQ retransmission is scheduled.

   Dynamic multiplexing of data associated with different services is
   highly desirable for efficient use of system resources and to
   maximize system capacity.  Assignment of resources for eMBB is
   usually done with regular (longer) transmission slots, which can lead
   to blocking of low latency services.  To overcome the blocking, eMBB
   resources can be pre-empted and re-assigned to URLLC services.  In
   this way, spectrally efficient assignments for eMBB can be ensured
   while providing flexibility required to ensure a bounded latency for
   URLLC services.  In downlink, the gNB can notify the eMBB UE about
   pre-emption after it has happened, while in uplink there are two pre-
   emption mechanisms: special signaling to cancel eMBB transmission and
   URLLC dynamic power boost to suppress eMBB transmission.

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6.4.4.  Scheduling and QoS (MAC)

   One integral part of the 5G system is the Quality of Service (QoS)
   framework [TS23501].  QoS flows are setup by the 5G system for
   certain IP or Ethernet packet flows, so that packets of each flow
   receive the same forwarding treatment, i.e., in scheduling and
   admission control.  QoS flows can for example be associated with
   different priority level, packet delay budgets and tolerable packet
   error rates.  Since radio resources are centrally scheduled in NR,
   the admission control function can ensure that only those QoS flows
   are admitted for which QoS targets can be reached.

   NR transmissions in both UL and DL are scheduled by the gNB
   [TS38300].  This ensures radio resource efficiency, fairness in
   resource usage of the users and enables differentiated treatment of
   the data flows of the users according to the QoS targets of the
   flows.  Those QoS flows are handled as data radio bearers or logical
   channels in NR RAN scheduling.

   The gNB can dynamically assign DL and UL radio resources to users,
   indicating the resources as DL assignments or UL grants via control
   channel to the UE.  Radio resources are defined as blocks of OFDM
   symbols in spectral domain and time domain.  Different lengths are
   supported in time domain, i.e., (multiple) slot or mini-slot lengths.
   Resources of multiple frequency carriers can be aggregated and
   jointly scheduled to the UE.

   Scheduling decisions are based, e.g., on channel quality measured on
   reference signals and reported by the UE (cf. periodical CSI reports
   for DL channel quality).  The transmission reliability can be chosen
   in the scheduling algorithm, i.e., by link adaptation where an
   appropriate transmission format (e.g., robustness of modulation and
   coding scheme, controlled UL power) is selected for the radio channel
   condition of the UE.  Retransmissions, based on HARQ feedback, are
   also controlled by the scheduler.  If needed to avoid HARQ round-trip
   time delays, repeated transmissions can be also scheduled beforehand,
   to the cost of reduced spectral efficiency.

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   In dynamic DL scheduling, transmission can be initiated immediately
   when DL data becomes available in the gNB.  However, for dynamic UL
   scheduling, when data becomes available but no UL resources are
   available yet, the UE indicates the need for UL resources to the gNB
   via a (single bit) scheduling request message in the UL control
   channel.  When thereupon UL resources are scheduled to the UE, the UE
   can transmit its data and may include a buffer status report,
   indicating the exact amount of data per logical channel still left to
   be sent.  More UL resources may be scheduled accordingly.  To avoid
   the latency introduced in the scheduling request loop, UL radio
   resources can also be pre-scheduled.

   In particular for periodical traffic patterns, the pre-scheduling can
   rely on the scheduling features DL Semi-Persistent Scheduling (SPS)
   and UL Configured Grant (CG).  With these features, periodically
   recurring resources can be assigned in DL and UL.  Multiple parallels
   of those configurations are supported, in order to serve multiple
   parallel traffic flows of the same UE.

   To support QoS enforcement in the case of mixed traffic with
   different QoS requirements, several features have recently been
   introduced.  This way, e.g., different periodical critical QoS flows
   can be served together with best effort transmissions, by the same
   UE.  Among others, these features (partly Release 16) are: 1) UL
   logical channel transmission restrictions allowing to map logical
   channels of certain QoS only to intended UL resources of a certain
   frequency carrier, slot-length, or CG configuration, and 2) intra-UE
   pre-emption, allowing critical UL transmissions to pre-empt non-
   critical transmissions.

   When multiple frequency carriers are aggregated, duplicate parallel
   transmissions can be employed (beside repeated transmissions on one
   carrier).  This is possible in the Carrier Aggregation (CA)
   architecture where those carriers originate from the same gNB, or in
   the Dual Connectivity (DC) architecture where the carriers originate
   from different gNBs, i.e., the UE is connected to two gNBs in this
   case.  In both cases, transmission reliability is improved by this
   means of providing frequency diversity.

   In addition to licensed spectrum, a 5G system can also utilize
   unlicensed spectrum to offload non-critical traffic.  This version of
   NR is called NR-U, part of 3GPP Release 16.  The central scheduling
   approach applies also for unlicensed radio resources, but in addition
   also the mandatory channel access mechanisms for unlicensed spectrum,
   e.g., Listen Before Talk (LBT) are supported in NR-U.  This way, by
   using NR, operators have and can control access to both licensed and
   unlicensed frequency resources.

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6.4.5.  Time-Sensitive Networking (TSN) Integration

   The main objective of Time-Sensitive Networking (TSN) is to provide
   guaranteed data delivery within a guaranteed time window, i.e.,
   bounded low latency.  IEEE 802.1 TSN [IEEE802.1TSN] is a set of open
   standards that provide features to enable deterministic communication
   on standard IEEE 802.3 Ethernet [IEEE802.3].  TSN standards can be
   seen as a toolbox for traffic shaping, resource management, time
   synchronization, and reliability.

   A TSN stream is a data flow between one end station (Talker) to
   another end station (Listener).  In the centralized configuration
   model, TSN bridges are configured by the Central Network Controller
   (CNC) [IEEE802.1Qcc] to provide deterministic connectivity for the
   TSN stream through the network.  Time-based traffic shaping provided
   by Scheduled Traffic [IEEE802.1Qbv] may be used to achieve bounded
   low latency.  The TSN tool for time synchronization is the
   generalized Precision Time Protocol (gPTP) [IEEE802.1AS]), which
   provides reliable time synchronization that can be used by end
   stations and by other TSN tools, e.g., Scheduled Traffic
   [IEEE802.1Qbv].  High availability, as a result of ultra-reliability,
   is provided for data flows by the Frame Replication and Elimination
   for Reliability (FRER) [IEEE802.1CB] mechanism.

   3GPP Release 16 includes integration of 5G with TSN, i.e., specifies
   functions for the 5G System (5GS) to deliver TSN streams such that
   the meet their QoS requirements.  A key aspect of the integration is
   the 5GS appears from the rest of the network as a set of TSN bridges,
   in particular, one virtual bridge per User Plane Function (UPF) on
   the user plane.  The 5GS includes TSN Translator (TT) functionality
   for the adaptation of the 5GS to the TSN bridged network and for
   hiding the 5GS internal procedures.  The 5GS provides the following

   1.  interface to TSN controller, as per [IEEE802.1Qcc] for the fully
       centralized configuration model

   2.  time synchronization via reception and transmission of gPTP PDUs

   3.  low latency, hence, can be integrated with Scheduled Traffic

   4.  reliability, hence, can be integrated with FRER [IEEE802.1CB]

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   Figure 10 shows an illustration of 5G-TSN integration where an
   industrial controller (Ind Ctrlr) is connected to industrial Input/
   Output devices (I/O dev) via 5G.  The 5GS can directly transport
   Ethernet frames since Release 15, thus, end-to-end Ethernet
   connectivity is provided.  The 5GS implements the required interfaces
   towards the TSN controller functions such as the CNC, thus adapts to
   the settings of the TSN network.  A 5G user plane virtual bridge
   interconnects TSN bridges or connect end stations, e.g., I/O devices
   to the network.  Note that the introduction of 5G brings flexibility
   in various aspects, e.g., more flexible network topology because a
   wireless hop can replace several wireline hops thus significantly
   reduce the number of hops end-to-end.  [ETR5GTSN] dives more into the
   integration of 5G with TSN.

                    | 5G System                    |
                    |                         +---+|
                    |     +-+ +-+ +-+ +-+ +-+ |TSN||
                    |     | | | | | | | | | | |AF |......+
                    |     +++ +++ +++ +++ +++ +-+-+|     .
                    |      |   |   |   |   |    |  |     .
                    |     -+---+---++--+-+-+--+-+- |     .
                    |          |    |    |    |    |  +--+--+
                    |         +++  +++  +++  +++   |  | TSN |
                    |         | |  | |  | |  | |   |  |Ctrlr+.......+
                    |         +++  +++  +++  +++   |  +--+--+       .
                    |                              |     .          .
                    |                              |     .          .
                    | +..........................+ |     .          .
                    | .      Virtual Bridge      . |     .          .
   +---+            | . +--+--+   +---+ +---+--+ . |  +--+---+      .
   |I/O+----------------+DS|UE+---+RAN+-+UPF|NW+------+ TSN  +----+ .
   |dev|            | . |TT|  |   |   | |   |TT| . |  |bridge|    | .
   +---+            | . +--+--+   +---+ +---+--+ . |  +------+    | .
                    | +..........................+ |     .      +-+-+-+
                    |                              |     .      | Ind |
                    | +..........................+ |     .      |Ctrlr|
                    | .      Virtual Bridge      . |     .      +-+---+
   +---+  +------+  | . +--+--+   +---+ +---+--+ . |  +--+---+    |
   |I/O+--+ TSN  +------+DS|UE+---+RAN+-+UPF|NW+------+ TSN  +----+
   |dev|  |bridge|  | . |TT|  |   |   | |   |TT| . |  |bridge|
   +---+  +------+  | . +--+--+   +---+ +---+--+ . |  +------+
                    | +..........................+ |

       <----------------- end-to-end Ethernet ------------------->

                      Figure 11: 5G - TSN Integration

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   NR supports accurate reference time synchronization in 1us accuracy
   level.  Since NR is a scheduled system, an NR UE and a gNB are
   tightly synchronized to their OFDM symbol structures.  A 5G internal
   reference time can be provided to the UE via broadcast or unicast
   signaling, associating a known OFDM symbol to this reference clock.
   The 5G internal reference time can be shared within the 5G network,
   i.e., radio and core network components.  For the interworking with
   gPTP for multiple time domains, the 5GS acts as a virtual gPTP time-
   aware system and supports the forwarding of gPTP time synchronization
   information between end stations and bridges through the 5G user
   plane TTs.  These account for the residence time of the 5GS in the
   time synchronization procedure.  One special option is when the 5GS
   internal reference time in not only used within the 5GS, but also to
   the rest of the devices in the deployment, including connected TSN
   bridges and end stations.

   Redundancy architectures were specified in order to provide
   reliability against any kind of failure on the radio link or nodes in
   the RAN and the core network, Redundant user plane paths can be
   provided based on the dual connectivity architecture, where the UE
   sets up two PDU sessions towards the same data network, and the 5G
   system makes the paths of the two PDU sessions independent as
   illustrated in Figure 13.  There are two PDU sessions involved in the
   solution: the first spans from the UE via gNB1 to UPF1, acting as the
   first PDU session anchor, while the second spans from the UE via gNB2
   to UPF2, acting as second the PDU session anchor.  The independent
   paths may continue beyond the 3GPP network.  Redundancy Handling
   Functions (RHFs) are deployed outside of the 5GS, i.e., in Host A
   (the device) and in Host B (the network).  RHF can implement
   replication and elimination functions as per [IEEE802.1CB] or the
   Packet Replication, Elimination, and Ordering Functions (PREOF) of
   IETF Deterministic Networking (DetNet) [RFC8655].

              . Device . +------+      +------+      +------+
              .        . + gNB1 +--N3--+ UPF1 |--N6--+      |
              .        ./+------+      +------+      |      |
              . +----+ /                             |      |
              . |    |/.                             |      |
              . | UE + .                             |  DN  |
              . |    |\.                             |      |
              . +----+ \                             |      |
              .        .\+------+      +------+      |      |
              +........+ + gNB2 +--N3--+ UPF2 |--N6--+      |
                         +------+      +------+      +------+

                   Figure 12: Reliability with Single UE

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   An alternative solution is that multiple UEs per device are used for
   user plane redundancy as illustrated in Figure 13.  Each UE sets up a
   PDU session.  The 5GS ensures that those PDU sessions of the
   different UEs are handled independently internal to the 5GS.  There
   is no single point of failure in this solution, which also includes
   RHF outside of the 5G system, e.g., as per FRER or as PREOF

             .  Device .
             .         .
             . +----+  .  +------+      +------+      +------+
             . | UE +-----+ gNB1 +--N3--+ UPF1 |--N6--+      |
             . +----+  .  +------+      +------+      |      |
             .         .                              |  DN  |
             . +----+  .  +------+      +------+      |      |
             . | UE +-----+ gNB2 +--N3--+ UPF2 |--N6--+      |
             . +----+  .  +------+      +------+      +------+
             .         .

                    Figure 13: Reliability with Dual UE

   Note that the abstraction provided by the RHF and the location of the
   RHF being outside of the 5G system make 5G equally supporting
   integration for reliability both with FRER of TSN and PREOF of DetNet
   as they both rely on the same concept.

   Note also that TSN is the primary subnetwork technology for DetNet.
   Thus, the DetNet over TSN work, e.g., [I-D.ietf-detnet-ip-over-tsn],
   can be leveraged via the TSN support built in 5G.

6.5.  Summary

   5G technology enables deterministic communication.  Based on the
   centralized admission control and the scheduling of the wireless
   resources, licensed or unlicensed, quality of service such as latency
   and reliability can be guaranteed. 5G contains several features to
   achieve ultra-reliable and low latency performance, e.g., support for
   different OFDM numerologies and slot-durations, as well as fast
   processing capabilities and redundancy techniques that lead to
   achievable latency numbers of below 1ms with reliability guarantees
   up to 99.999%.

   5G also includes features to support Industrial IoT use cases, e.g.,
   via the integration of 5G with TSN.  This includes 5G capabilities
   for each TSN component, latency, resource management, time
   synchronization, and reliability.  Furthermore, 5G support for TSN

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   can be leveraged when 5G is used as subnet technology for DetNet, in
   combination with or instead of TSN, which is the primary subnet for
   DetNet.  In addition, the support for integration with TSN
   reliability was added to 5G by making DetNet reliability also
   applicable, thus making 5G DetNet ready.  Moreover, providing IP
   service is native to 5G.

   Overall, 5G provides scheduled wireless segments with high
   reliability and availability.  In addition, 5G includes capabilities
   for integration to IP networks.

7.  L-band Digital Aeronautical Communications System

   One of the main pillars of the modern Air Traffic Management (ATM)
   system is the existence of a communication infrastructure that
   enables efficient aircraft guidance and safe separation in all phases
   of flight.  Although current systems are technically mature, they are
   suffering from the VHF band's increasing saturation in high-density
   areas and the limitations posed by analogue radio.  Therefore,
   aviation globally and the European Union (EU) in particular, strives
   for a sustainable modernization of the aeronautical communication

   In the long-term, ATM communication shall transition from analogue
   VHF voice and VDL2 communication to more spectrum efficient digital
   data communication.  The European ATM Master Plan foresees this
   transition to be realized for terrestrial communications by the
   development and implementation of the L-band Digital Aeronautical
   Communications System (LDACS).  LDACS shall enable IPv6 based air-
   ground communication related to the safety and regularity of the
   flight.  The particular challenge is that no new frequencies can be
   made available for terrestrial aeronautical communication.  It was
   thus necessary to develop procedures to enable the operation of LDACS
   in parallel with other services in the same frequency band.

7.1.  Provenance and Documents

   The development of LDACS has already made substantial progress in the
   Single European Sky ATM Research (SESAR) framework, and is currently
   being continued in the follow-up program, SESAR2020 [RIH18].  A key
   objective of the SESAR activities is to develop, implement and
   validate a modern aeronautical data link able to evolve with aviation
   needs over long-term.  To this end, an LDACS specification has been
   produced [GRA19] and is continuously updated; transmitter
   demonstrators were developed to test the spectrum compatibility of
   LDACS with legacy systems operating in the L-band [SAJ14]; and the
   overall system performance was analyzed by computer simulations,
   indicating that LDACS can fulfil the identified requirements [GRA11].

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   LDACS standardization within the framework of the International Civil
   Aviation Organization (ICAO) started in December 2016.  The ICAO
   standardization group has produced an initial Standards and
   Recommended Practices (SARPs) document [ICAO18].  The SARPs document
   defines the general characteristics of LDACS.  The ICAO
   standardization group plans to produce an ICAO technical manual - the
   ICAO equivalent to a technical standard - within the next years.
   Generally, the group is open to input from all sources and develops
   LDACS in the open.

   Up to now the LDACS standardization has been focused on the
   development of the physical layer and the data link layer, only
   recently have higher layers come into the focus of the LDACS
   development activities.  There is currently no "IPv6 over LDACS"
   specification; however, SESAR2020 has started the testing of
   IPv6-based LDACS testbeds.  The IPv6 architecture for the
   aeronautical telecommunication network is called the Future
   Communications Infrastructure (FCI).  FCI shall support quality of
   service, diversity, and mobility under the umbrella of the "multi-
   link concept".  This work is conducted by ICAO working group WG-I.

   In addition to standardization activities several industrial LDACS
   prototypes have been built.  One set of LDACS prototypes has been
   evaluated in flight trials confirming the theoretical results
   predicting the system performance [GRA18][SCH19].

7.2.  General Characteristics

   LDACS will become one of several wireless access networks connecting
   aircraft to the Aeronautical Telecommunications Network (ATN).  The
   LDACS access network contains several ground stations, each of them
   providing one LDACS radio cell.  The LDACS air interface is a
   cellular data link with a star-topology connecting aircraft to
   ground-stations with a full duplex radio link.  Each ground-station
   is the centralized instance controlling all air-ground communications
   within its radio cell.  A ground-station supports up to 512 aircraft.

   The LDACS air interface protocol stack defines two layers, the
   physical layer and the data link layer.

   The physical layer provides the means to transfer data over the radio
   channel.  The LDACS ground-station supports bi-directional links to
   multiple aircraft under its control.  The forward link direction (FL;
   ground-to-air) and the reverse link direction (RL; air-to-ground) are
   separated by frequency division duplex.  Forward link and reverse
   link use a 500 kHz channel each.  The ground-station transmits a
   continuous stream of OFDM symbols on the forward link.  In the
   reverse link different aircraft are separated in time and frequency

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   using a combination of Orthogonal Frequency-Division Multiple-Access
   (OFDMA) and Time-Division Multiple-Access (TDMA).  Aircraft thus
   transmit discontinuously on the reverse link with radio bursts sent
   in precisely defined transmission opportunities allocated by the
   ground-station.  LDACS does not support beam-forming or Multiple
   Input Multiple Output (MIMO).

   The data-link layer provides the necessary protocols to facilitate
   concurrent and reliable data transfer for multiple users.  The LDACS
   data link layer is organized in two sub-layers: The medium access
   sub-layer and the logical link control sub-layer.  The medium access
   sub-layer manages the organization of transmission opportunities in
   slots of time and frequency.  The logical link control sub-layer
   provides acknowledged point-to-point logical channels between the
   aircraft and the ground-station using an automatic repeat request
   protocol.  LDACS supports also unacknowledged point-to-point channels
   and ground-to-air broadcast.

   The user data rate of LDACS is 315 kbit/s to 1428 kbit/s on the
   forward link, and 294 kbit/s to 1390 kbit/s on the reverse link,
   depending on coding and modulation.  Due to strong interference from
   legacy systems in the L-band, the most robust coding and modulation
   should be expected for initial deployment i.e. 315/294 kbit/s on the
   forward/reverse link, respectively.

   Since LDACS has been mainly designed for air traffic management
   communication it supports mutual entity authentication, integrity and
   confidentiality capabilities of user data messages and some control
   channel protection capabilities [MAE19].

7.3.  Applicability to Deterministic Flows

   LDACS has been designed with applications related to the safety and
   regularity of the flight in mind.  It has therefore been designed as
   a deterministic wireless data link (as far as possible).

   LDACS medium access is always under the control of the ground-station
   of a radio cell.  Any medium access for the transmission of user data
   has to be requested with a resource request message stating the
   requested amount of resources and class of service.  The ground-
   station performs resource scheduling on the basis of these requests
   and grants resources with resource allocation messages.  Resource
   request and allocation messages are exchanged over dedicated
   contention-free control channels.

   LDACS has two mechanisms to request resources from the scheduler in
   the ground-station.  Resources can either be requested "on demand"
   with a given class of service.  On the forward link, this is done

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   locally in the ground-station, on the reverse link a dedicated
   contention-free control channel is used (Dedicated Control Channel
   (DCCH); roughly 83 bit every 60 ms).  A resource allocation is always
   announced in the control channel of the forward link (Common Control
   Channel (CCCH); variable sized).  Due to the spacing of the reverse
   link control channels of every 60 ms, a medium access delay in the
   same order of magnitude is to be expected.

   Resources can also be requested "permanently".  The permanent
   resource request mechanism supports requesting recurring resources in
   given time intervals.  A permanent resource request has to be
   canceled by the user (or by the ground-station, which is always in
   control).  User data transmissions over LDACS are therefore always
   scheduled by the ground-station, while control data uses statically
   (i.e. at net entry) allocated recurring resources (DCCH and CCCH).
   The current specification documents specify no scheduling algorithm.
   However performance evaluations so far have used strict priority
   scheduling and round robin for equal priorities for simplicity.  In
   the current prototype implementations LDACS classes of service are
   thus realized as priorities of medium access and not as flows.  Note
   that this can starve out low priority flows.  However, this is not
   seen as a big problem since safety related message always go first in
   any case.  Scheduling of reverse link resources is done in physical
   Protocol Data Units (PDU) of 112 bit (or larger if more aggressive
   coding and modulation is used).  Scheduling on the forward link is
   done Byte-wise since the forward link is transmitted continuously by
   the ground-station.

   In order to support diversity, LDACS supports handovers to other
   ground-stations on different channels.  Handovers may be initiated by
   the aircraft (break-before-make) or by the ground-station (make-
   before-break) if it is connected to an alternative ground-station via
   the same ground-station controller.  Beyond this, FCI diversity shall
   be implemented by the multi-link concept.

8.  IANA Considerations

   This specification does not require IANA action.

9.  Security Considerations

   Most RAW technologies integrate some authentication or encryption
   mechanisms that were defined outside the IETF.

10.  Contributors

   Georgios Z.  Papadopoulos:  Contributed to the TSCH section.

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   Nils M&#228;urer:  Contributed to the LDACS section.

   Thomas Gr&#228;upl:  Contributed to the LDACS section.

   Janos Farkas, Torsten Dudda, Alexey Shapin, and Sara Sandberg:
      Contributed to the 5G section.

11.  Acknowledgments

   Many thanks to the participants of the RAW WG where a lot of the work
   discussed here happened.

12.  Normative References

   [RFC8480]  Wang, Q., Ed., Vilajosana, X., and T. Watteyne, "6TiSCH
              Operation Sublayer (6top) Protocol (6P)", RFC 8480,
              DOI 10.17487/RFC8480, November 2018,

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,

   [RFC5673]  Pister, K., Ed., Thubert, P., Ed., Dwars, S., and T.
              Phinney, "Industrial Routing Requirements in Low-Power and
              Lossy Networks", RFC 5673, DOI 10.17487/RFC5673, October
              2009, <https://www.rfc-editor.org/info/rfc5673>.

              Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", Work in Progress,
              Internet-Draft, draft-ietf-detnet-architecture-13, 6 May
              2019, <https://tools.ietf.org/html/draft-ietf-detnet-

              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", Work in Progress, Internet-Draft,
              draft-ietf-6tisch-architecture-28, 29 October 2019,

13.  Informative References

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   [RFC6550]  Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
              Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
              JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
              Low-Power and Lossy Networks", RFC 6550,
              DOI 10.17487/RFC6550, March 2012,

   [RFC6551]  Vasseur, JP., Ed., Kim, M., Ed., Pister, K., Dejean, N.,
              and D. Barthel, "Routing Metrics Used for Path Calculation
              in Low-Power and Lossy Networks", RFC 6551,
              DOI 10.17487/RFC6551, March 2012,

   [RFC6291]  Andersson, L., van Helvoort, H., Bonica, R., Romascanu,
              D., and S. Mansfield, "Guidelines for the Use of the "OAM"
              Acronym in the IETF", BCP 161, RFC 6291,
              DOI 10.17487/RFC6291, June 2011,

   [RFC7276]  Mizrahi, T., Sprecher, N., Bellagamba, E., and Y.
              Weingarten, "An Overview of Operations, Administration,
              and Maintenance (OAM) Tools", RFC 7276,
              DOI 10.17487/RFC7276, June 2014,

   [RFC8279]  Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
              Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
              Explicit Replication (BIER)", RFC 8279,
              DOI 10.17487/RFC8279, November 2017,

              Chang, T., Vucinic, M., Vilajosana, X., Duquennoy, S., and
              D. Dujovne, "6TiSCH Minimal Scheduling Function (MSF)",
              Work in Progress, Internet-Draft, draft-ietf-6tisch-msf-
              16, 2 April 2020,

              Thubert, P. and G. Papadopoulos, "Reliable and Available
              Wireless Architecture/Framework", Work in Progress,
              Internet-Draft, draft-pthubert-raw-architecture-01, 2
              April 2020, <https://tools.ietf.org/html/draft-pthubert-

              Koutsiamanis, R., Papadopoulos, G., Montavont, N., and P.
              Thubert, "Common Ancestor Objective Function and Parent

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              Set DAG Metric Container Extension", Work in Progress,
              Internet-Draft, draft-ietf-roll-nsa-extension-08, 27 March
              2020, <https://tools.ietf.org/html/draft-ietf-roll-nsa-

              Papadopoulos, G., Koutsiamanis, R., Montavont, N., and P.
              Thubert, "Exploiting Packet Replication and Elimination in
              Complex Tracks in LLNs", Work in Progress, Internet-Draft,
              draft-papadopoulos-paw-pre-reqs-01, 25 March 2019,

              Thubert, P., Eckert, T., Brodard, Z., and H. Jiang, "BIER-
              TE extensions for Packet Replication and Elimination
              Function (PREF) and OAM", Work in Progress, Internet-
              Draft, draft-thubert-bier-replication-elimination-03, 3
              March 2018, <https://tools.ietf.org/html/draft-thubert-

              Thubert, P., Brodard, Z., Jiang, H., and G. Texier, "A
              6loRH for BitStrings", Work in Progress, Internet-Draft,
              draft-thubert-6lo-bier-dispatch-06, 28 January 2019,

              Eckert, T., Cauchie, G., and M. Menth, "Tree Engineering
              for Bit Index Explicit Replication (BIER-TE)", Work in
              Progress, Internet-Draft, draft-ietf-bier-te-arch-07, 9
              March 2020,

              Sudhaakar, R. and P. Zand, "6TiSCH Resource Management and
              Interaction using CoAP", Work in Progress, Internet-Draft,
              draft-ietf-6tisch-coap-03, 9 March 2015,

              Shah, S. and P. Thubert, "Deterministic Forwarding PHB",
              Work in Progress, Internet-Draft, draft-svshah-tsvwg-
              deterministic-forwarding-04, 30 August 2015,

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   [IEEE Std. 802.15.4]
              IEEE standard for Information Technology, "IEEE Std.
              802.15.4, Part. 15.4: Wireless Medium Access Control (MAC)
              and Physical Layer (PHY) Specifications for Low-Rate
              Wireless Personal Area Networks".

   [IEEE Std. 802.11]
              "IEEE Standard 802.11 - IEEE Standard for Information
              Technology - Telecommunications and information exchange
              between systems Local and metropolitan area networks -
              Specific requirements - Part 11: Wireless LAN Medium
              Access Control (MAC) and Physical Layer (PHY)

   [IEEE Std. 802.11ak]
              "802.11ak: Enhancements for Transit Links Within Bridged
              Networks", 2017.

   [IEEE Std. 802.11ax]
              "802.11ax D4.0: Enhancements for High Efficiency WLAN".

   [IEEE Std. 802.11ay]
              "802.11ay: Enhanced throughput for operation in license-
              exempt bands above 45 GHz".

   [IEEE Std. 802.11ad]
              "802.11ad: Enhancements for very high throughput in the 60
              GHz band".

   [IEEE 802.11be WIP]
              "802.11be: Extreme High Throughput".

   [IEEE Std. 802.1Qat]
              "802.1Qat: Stream Reservation Protocol".

              "802.1Qcc: IEEE Standard for Local and Metropolitan Area
              Networks--Bridges and Bridged Networks -- Amendment 31:
              Stream Reservation Protocol (SRP) Enhancements and
              Performance Improvements".

              Dave Cavalcanti et al., "Extending Time Distribution and
              Timeliness Capabilities over the Air to Enable Future
              Wireless Industrial Automation Systems, the Proceedings of
              IEEE", June 2019.

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              Thomas Nitsche et al., "IEEE 802.11ad: directional 60 GHz
              communication for multi-Gigabit-per-second Wi-Fi",
              December 2014.

              Yasaman Ghasempour et al., "802.11ay: Next-Generation 60
              GHz Communications for 100 Gb/s Wi-Fi", December 2017.

              "802.11 Real-Time Applications (RTA) Topic Interest Group
              (TIG) Report", November 2018.

              Kevin Stanton et Al., "Time-Sensitive Applications Support
              in EHT", March 2019.

   [morell13] Antoni Morell et al., "Label switching over IEEE802.15.4e
              networks", April 2013.

              Jesica de Armas et al., "Determinism through path
              diversity: Why packet replication makes sense", September

              Xavier Vilajosana et al., "6TiSCH: Industrial Performance
              for IPv6 Internet-of-Things Networks", June 2019.

              ISA/IEC, "ISA100.11a, Wireless Systems for Automation,
              also IEC 62734", 2011, <http://www.isa100wci.org/en-

              www.hartcomm.org, "Industrial Communication Networks -
              Wireless Communication Network and Communication Profiles
              - WirelessHART - IEC 62591", 2010.

   [PCE]      IETF, "Path Computation Element",

   [CCAMP]    IETF, "Common Control and Measurement Plane",

   [TiSCH]    IETF, "IPv6 over the TSCH mode over 802.15.4",

Thubert, et al.         Expires 19 November 2020               [Page 53]

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   [RIH18]    Rihacek, C., Haindl, B., Fantappie, P., Pierattelli, S.,
              Gräupl, T., Schnell, M., and N. Fistas, "L-band Digital
              Aeronautical Communications System (LDACS) Activities in
              SESAR2020", Proceedings of the Integrated Communications
              Navigation and Surveillance Conference (ICNS) Herndon, VA,
              USA, April 2018.

   [GRA19]    Gräupl, T., Rihacek, C., and B. Haindl, "LDACS A/G
              Specification", SESAR2020 PJ14-02-01 D3.3.010, February

   [SAJ14]    Sajatovic, M., Günzel, H., and S. Müller, "WA04 D22 Test
              Report for Assessing LDACS1 Transmitter Impact upon DME/
              TACAN Receivers", April 2014.

   [GRA11]    Gräupl, T. and M. Ehammer, "L-DACS1 Data Link Layer
              Evolution of ATN/IPS", Proceedings of the 30th IEEE/AIAA
              Digital Avionics Systems Conference (DASC) Seattle, WA,
              USA, October 2011.

   [ICAO18]   International Civil Aviation Organization (ICAO), "L-Band
              Digital Aeronautical Communication System (LDACS)",
              International Standards and Recommended Practices Annex 10
              - Aeronautical Telecommunications, Vol. III -
              Communication Systems, July 2018.

   [GRA18]    al., T. G. E., "L-band Digital Aeronautical Communications
              System (LDACS) flight trials in the national German
              project MICONAV", Proceedings of the Integrated
              Communications, Navigation, Surveillance Conference
              (ICNS) Herndon, VA, USA, April 2018.

   [SCH19]    Schnell, M., "DLR tests digital communications
              technologies combined with additional navigation functions
              for the first time", 3 March 2019,

   [MAE19]    Mäurer, N. and C. Schmitt, "DLR tests digital
              communications technologies combined with additional
              navigation functions for the first time", Proceedings of
              the Integrated Communications, Navigation, Surveillance
              Conference (ICNS) Washington D.C., USA, April 2019.

   [TR37910]  "3GPP TR 37.910, Study on self evaluation towards IMT-2020

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   [TR38824]  "3GPP TR 38.824, Study on physical layer enhancements for
              NR ultra-reliable and low latency case (URLLC)",

   [TR38825]  "3GPP TR 38.825, Study on NR industrial Internet of Things

   [TS22104]  "3GPP TS 22.104, Service requirements for cyber-physical
              control applications in vertical domains",

   [TR22804]  "3GPP TR 22.804, Study on Communication for Automation in
              Vertical domains (CAV)",

   [TS23501]  "3GPP TS 23.501, System architecture for the 5G System

   [TS38300]  "3GPP TS 38.300, NR Overall description",

   [IMT2020]  "ITU towards IMT for 2020 and beyond",

   [RFC8655]  Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", RFC 8655,
              DOI 10.17487/RFC8655, October 2019,

              Varga, B., Farkas, J., Malis, A., and S. Bryant, "DetNet
              Data Plane: IP over IEEE 802.1 Time Sensitive Networking
              (TSN)", Work in Progress, Internet-Draft, draft-ietf-
              detnet-ip-over-tsn-02, 6 March 2020,

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              IEEE 802.1, "Time-Sensitive Networking (TSN) Task Group",

              IEEE, "IEEE Standard for Local and metropolitan area
              networks -- Timing and Synchronization for Time-Sensitive
              Applications", IEEE 802.1AS-2020,

              IEEE, "IEEE Standard for Local and metropolitan area
              networks -- Frame Replication and Elimination for
              Reliability", DOI 10.1109/IEEESTD.2017.8091139, IEEE

              IEEE, "IEEE Standard for Local and metropolitan area
              networks -- Bridges and Bridged Networks -- Amendment 25:
              Enhancements for Scheduled Traffic", IEEE 802.1Qbv-2015,

              IEEE, "IEEE Standard for Local and metropolitan area
              networks -- Bridges and Bridged Networks -- Amendment 31:
              Stream Reservation Protocol (SRP) Enhancements and
              Performance Improvements", IEEE 802.1Qcc-2018,

              IEEE, "IEEE Standard for Ethernet", IEEE 802.3-2018,

   [ETR5GTSN] Farkas, J., Varga, B., Miklos, G., and J. Sachs, "5G-TSN
              integration meets networking requirements for industrial
              automation", Ericsson Technology Review, Volume 9, No 7,
              August 2019, <https://www.ericsson.com/en/reports-and-

Authors' Addresses

   Pascal Thubert (editor)
   Cisco Systems, Inc
   Building D
   45 Allee des Ormes - BP1200

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   06254 MOUGINS - Sophia Antipolis

   Phone: +33 497 23 26 34
   Email: pthubert@cisco.com

   Dave Cavalcanti
   Intel Corporation
   2111 NE 25th Ave
   Hillsboro, OR,  97124
   United States of America

   Phone: 503 712 5566
   Email: dave.cavalcanti@intel.com

   Xavier Vilajosana
   Universitat Oberta de Catalunya
   156 Rambla Poblenou
   08018 Barcelona Catalonia

   Email: xvilajosana@uoc.edu

   Corinna Schmitt
   Research Institute CODE, UniBwM
   Werner-Heisenberg-Weg 28
   85577 Neubiberg

   Email: corinna.schmitt@unibw.de

   Janos Farkas
   Magyar tudosok korutja 11

   Email: janos.farkas@ericsson.com

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