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Versions: 00                                                            
Reliable and Available Wireless Working Group                 R.C. Sofia
Internet-Draft                                              fortiss GmbH
Intended status: Informational                               M. Kovatsch
Expires: 8 September 2021                            Huawei Technologies
                                                               P. Mendes
                                                            7 March 2021

         Requirements for Reliable Wireless Industrial Services


   This document provides an overview on communication requirements for
   handling reliable wireless services within the context of industrial
   environments.  The goal of the draft is to bring awareness to
   communication requirements of current and future wireless industrial
   services; how can they co-exist with wired infrastructures; key
   drivers for reliable wireless integration; relevant communication
   requirements to take into consideration; current and future
   challenges derived from the use of wireless.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the mailing list
   (raw@ietf.org), which is archived at

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   This Internet-Draft will expire on 4 September 2021.

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Table of Contents

   1.  Introduction
   2.  Conventions used in this document
   3.  Definitions
   4.  Wireless Industrial Services Today
     4.1.  Equipment and Process Control Services
     4.2.  Quality Supervision Services
     4.3.  Factory Resource Management Services
     4.4.  Display Services
     4.5.  Human Safety Services
     4.6.  Mobile Robotics Services
     4.7.  Power Grid Control
     4.8.  Wireless Avionics Intra-communication
   5.  Additional Reliable Wireless Industrial Services
     5.1.  AR/VR Services within Flexible Factories
       5.1.1.  Description
       5.1.2.  Wireless Integration Recommendations
       5.1.3.  Requirements Considerations
     5.2.  Decentralized Shop-floor Communication Services
       5.2.1.  Description
       5.2.2.  Wireless Integration Recommendations
       5.2.3.  Requirements Considerations
     5.3.  Autonomous Airborne Services
       5.3.1.  Wireless Integration Recommendations
       5.3.2.  Requirements Considerations
   6.  Security Considerations
   7.  IANA Considerations
   8.  Acknowledgments
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Authors' Addresses

1.  Introduction

   Within industrial environments, short-range wireless standards, such
   as IEEE 802.11ax, are gaining prominence as there exists an
   increasing need for flexibility in terms of infrastructure layout, of
   processes support.  Wireless, and specifically Wireless Fidelity (Wi-
   Fi), is now reaching a maturity point where the available
   transmission rates become highly competitive in comparison to wired
   environments, thus increasing flexibility, providing a lower cost and
   higher availability in scenarios requiring, for instance, mobility
   support.  There are, nonetheless, barriers to the integration of
   wireless in industrial environments.  Firstly, being wireless a
   shared medium, it experiences challenges such as interference and
   signal strength variability depending on its surroundings.  These
   features raise issues concerning critical services availability,
   resilience, and security support.  Secondly, wireless relies on
   probabilistic Quality of Service (QoS) and therefore requires tuning
   to be able to support time-sensitive traffic with bounded latency,
   low jitter, zero congestion loss.  However, the recent advancements
   of OFDMA-based wireless in the context of IEEE 802.11 standards such
   as 802.11ax and 802.11be bring in interesting features in the context
   of supporting critical industrial applications, e.g., a higher degree
   of flexibility in terms of resource management; frequency allocation
   aspects that can provide better traffic isolation, or even mechanisms
   that can assist a tighter time synchronization across wireless
   environments, thus providing the means to better support traffic in
   converged networks.  Still, being able to address the communication
   challenges that exist in industrial domains require a better
   understanding of communication requirements that the existing and
   future industrial applications may attain.  Hence, the focus of this
   draft is on discussing industrial application requirements, currently
   and for the future and how to best support time-sensitive
   applications and services within industrial converged networks.  For
   that purpose, the draft debates on wireless industrial services
   collected from related normative and informational references on the
   industrial domain; debates on key drivers for the integration of
   wireless; debates on specific wireless mechanisms that may assist
   such integration and challenges thereof; and elaborates on specific
   requirements to observe both for current wireless services as well as
   for a subset of future industrial wireless services.

2.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].
   In this document, these words will appear with that interpretation
   only when in ALL CAPS.  Lower case uses of these words are not to be
   interpreted as carrying significance described in RFC 2119.

3.  Definitions

   *  Latency (aka bounded latency), concerns the end-to-end
      transmission delay between a transmitter and a receiver, when a
      traffic flow is triggered by an application.  By definition,
      latency corresponds to the time interval between sending the first
      packet of a flow from a source to a destination, until the instant
      of reception of the last packet of that flow.

   *  Periodicity stands for whether or not the data transmission is
      executed in a periodic fashion and whenever possible, the specific
      periodicity per unit of time has been specified.

   *  "Transmit data size" corresponds to the data payload in bytes.

   *  Tolerance to packet loss is presented as "0" (zero congestion
      loss); tolerant (the application has tolerance to packet loss).
      Packet loss occurs when packets fail to reach a specific
      destination on a network.  Packet loss is usually measured as a
      percentage of packets loss in regards to the overall packets sent.
      In the context of deterministic networking and in particular, of
      Time-sensitive Networking (TSN), a packet is lost when it is not
      received within a specific deadline.

   *  "Time sync" refers to the need to ensure IEEE 1588

   *  "Node density" provides (wherever available) a glimpse into the
      number of end-nodes per 20mx20m.

4.  Wireless Industrial Services Today

   This section describes industrial applications where IEEE 802.11 is
   already being applied, derived from an analysis of related work.

   Industrial wireless services focused on the strengthening of
   industrial manufacturing environments have been intensively
   documented via the IEEE Nendica group [NENDICA], the Internet
   Industrial Consortium [IIC], the OPC FLC working group [OPCFLC].  The
   IEEE Nendica 2020 report [NENDICA] comprises several end-to-end use-
   cases and a technical analysis of the identified features and
   functions supported via wireless/wired deterministic environments.
   Based on surveys to industry, the report provides a first
   characterization of wireless services in factories (Wi-Fi 5),
   characterizing the scenarios in terms of aspects such as as payload
   size in bytes, communication rate, arrival time tolerance, node

   The IEEE 802.11 RTA report [IEEERTA] provides additional input
   concerning the support of wireless for time-sensitive and real-time
   applications.  For each category of application, the report provides
   a description, basic information concerning topology and packet flow/
   traffic model, summarizing the problem statement (main challenges).
   The industrial applications in this report are a subset and have also
   considered sources such as IEEE Nendica, IEC/IEEE 60802 Use-cases, as
   well as 3GPP TR 22.804.  The report aggregates the different services
   in 3 classes (A,B,C) and provides communication requirements for each
   class categorized as: bounded latency (worst-case one-way latency
   measured at the application layer); reliability (defined as the
   percentage of packets expected to be received within the latency
   bound); time synchronization needs (in the order of micro/
   milliseconds); throughput needs (high, moderate, low).  The report
   concludes with guidelines concerning implementation aspects, e.g.,
   traffic classification aspects and new capabilities to support real-
   time applications.

   The Avnu Alliance provides a white paper describing steps for the
   integration of TSN over WiFi [AVNU2020], briefly describing the
   integration of Wi-Fi in specific applications such as: closed loop
   control, mobile robots, power grid control, professional Audio/Video,
   gaming, AR/VR.  The document also raises awareness to the possibility
   of wireless replacing or being complementary to wired within
   connected cabines, i.e., in regards to the wiring harness within
   vehicles (cars, airplanes, trains), which are currently expensive and
   which require a complex onboarding.  Wireless can assist in lowering
   the costs, if it can be adapted to the critical latency, safety
   requirements and regulations.  Such cases would require 100
   micosecond level cycles, according to Avnu.  The communication
   requirements are summarised in terms of whether or not IEEE 1588
   synchronisation is required; the typical packet size (data payload);
   bounded latency; reliability.

   Manufacturing wireless use-cases have also been debated in the
   context of 5G ACIA [ACIA], NICT [NICT], and IETF Deterministic
   Networking [RFC8578].  These sources provide an overview on user
   stories, and debate on the challenges brought by the integration of
   wireless.  However, communication requirements are not presented in a
   systematic way.  Lastly, the IETF RAW working group has an active
   draft which provides an initial overview on the challenges of
   wireless industrial use-cases [IETFRAW-USECASES].

   Derived from the analysis of the aforementioned sources, this section
   provides a description of categories of applications, and respective
   communication requirements.  The following categories of applications
   are addressed:

   *  Equipment and process control.

   *  Quality supervision.

   *  Factory resource management.

   *  Display.

   *  Human safety.

   *  Industrial systems.

   *  Mobile robots.

   *  Drones/UAV control.

   *  Power grid control.

   *  Communication-based train networks.

   *  Mining industry.

   *  Connected cabin.

   The selected communication requirements and which are presented for
   each category of applications have been extracted from the different
   available related work.  The parameters are: bounded latency;
   periodicity; transmit data size; tolerance to packet loss; time
   synchronization needs; node density characterization.

4.1.  Equipment and Process Control Services

   This category of industrial wireless services refers to the data
   exchange required to send, for instance, commands to mobile robots/
   vehicles, production equipment, and also to receive status
   information.  Reasons for wireless integration concern: flexibility
   of deployment, reconfigurability, mobility, maintenance cost

   In this category, examples of applications and respective
   communication requirements are:

   *  Control of machines and robots.

      -  Bounded latency: below 10 ms.

      -  Periodic.

      -  Transmit data size (bytes): 10-400 (small).

      -  Tolerance to packet loss: 0.

      -  Time synchronization: IEEE 1588.

      -  Node density: 1 to 20 (per 20mx20m area).

   *  AGVs with rails

      -  Bounded latency: 10 ms-100ms.

      -  Periodic, once per minute.

      -  Transmit data size (bytes): 10-400 (small).

      -  Tolerance to packet loss: 0.

      -  Time synchronization: IEEE 1588.

      -  Node density: 1 to 20 (per 20mx20m area).

   *  AGVs without rails

      -  Bounded latency:1 s.

      -  Periodic, once per minute.

      -  Transmit data size (bytes): 10-400 (small).

      -  Tolerance to packet loss: 0.

      -  Time synchronization: IEEE 1588.

      -  Node density: 1 to 20 (per 20mx20m area).

   *  Hard-real time isochronous control, motion control

      -  Bounded latency: 250us - 1ms.

      -  Periodic.

      -  Transmit data size (bytes): 10-400 (small).

      -  Tolerance to packet loss: 0.

      -  Time synchronization: IEEE 1588.

      -  Node density: 1 to 20 (per 20mx20m area).

   *  Printing, packaging

      -  Bounded latency: below 2 ms.

      -  Transmit data size (bytes): 10-400 (small).

      -  Tolerance to packet loss: 0.

      -  Time synchronization: IEEE 1588.

      -  Node density: over 50 to 100.

   *  PLC to PLC communication

      -  Bounded latency: 100 us-50 ms.

      -  Transmit data size (bytes): 100-700.

      -  Tolerance to packet loss: 0.

      -  Time synchronization: IEEE 1588.

   *  Interactive video

      -  Bounded latency: 50 -10 ms.

      -  Time synchronization: 10-1[micro]s.

   *  Mobile robotics

      -  Bounded latency: 50 -10 ms.

   *  AR/VR, remote HMI

      -  Bounded latency: 10 - 1 ms.

      -  Time synchronization: ~1 [micro]s.

      -  Time synchronization: 10-1[micro]s.

   *  Machine, production line controls

      -  Bounded latency: 10 - 1 ms.

4.2.  Quality Supervision Services

   Quality supervision comprises industrial services that collect and
   assess information related to products and states of machines during
   production.  Reasons for wireless integration concern: flexibility of
   deployment, maintenance cost reduction.

   Examples of applications in this category, and their communication
   requirements are:

   *  Inline inspection

      -  Bounded latency: bellow 10ms.

      -  Time synchronization: 10-1[micro]s.

      -  Periodic, once per second.

      -  Transmit data size (bytes): 64-1M.

      -  Tolerance to packet loss: 0.

      -  Node density: 1-10 (per 20mx20m).

   *  Machine operation recording

      -  Bounded latency: over 100 s.

      -  Time synchronization: 10-1[micro]s.

      -  Periodic, once per second.

      -  Transmit data size (bytes): 64-1M.

      -  Tolerance to packet loss: 0.

      -  Node density: 1-10 (per 20mx20m).

   *  Logging

      -  Bounded latency: over 100s.

      -  Time synchronization: 10-1[micro]s.

      -  Transmit data size (bytes): 64-1M.

      -  Tolerance to packet loss: 0.

      -  Node density: 1-10 (per 20mx20m).

4.3.  Factory Resource Management Services

   Refers to capturing information about whether production is
   proceeding under proper environmental conditions, and whether staff
   and devices contributing to productivity enhancement are being
   managed appropriately.  Reasons for wireless integration concern:
   flexibility of deployment, reconfigurability, maintenance cost

   Services debated in this context are:

   *  Machine monitoring

      -  Bounded latency: 100ms-10s.

      -  Periodic.

      -  Time synchronization: 10-1[micro]s.

      -  Transmit data size (bytes): 10-10M.

      -  Tolerance to packet loss: 0.

      -  Node density: 1-30.

   *  Preventive maintenance

      -  Bounded latency: over 100ms.

      -  Periodic, once per event.

   *  Positioning, motion analysis

      -  Bounded latency: 50ms-10s.

      -  Periodic, once per second.

   *  Inventory control

      -  Bounded latency: 50ms-10s.

      -  Periodic, once per second.

   *  Facility control environment

      -  Bounded latency: 1s-50s.

      -  Periodic, once per minute.

   *  Checking status of material, small equipment

      -  Bounded latency: 100ms-1s.

      -  Sporadic, 1 to 10 times per 30 minutes.

4.4.  Display Services

   This category of services targets workers, allowing them to receive
   requested support information.  It also targets managers in regards
   to monitoring of production status and processes.  Reasons for
   wireless integration are: scalability, flexibility of deployment,
   mobility support.  Examples of services are:

   *  Work commands, e.g., wearable displays

      -  Bounded latency: 1-10s.

      -  Sporadic, once per 10s-1m.

      -  Transmit data size (bytes): 10-6K.

      -  Tolerance to packet loss: yes.

      -  Node density: 1-30

   *  Display information

      -  Bounded latency: 10s.

      -  Sporadic, once per hour.

      -  Transmit data size (bytes): 10-6K.

      -  Tolerance to packet loss: yes.

      -  Node density: 1-30.

   *  Supporting maintenance (video, audio)

      -  Bounded latency: 500ms.

      -  Sporadic, once per 100ms.

      -  Transmit data size (bytes): 10-6K.

      -  Tolerance to packet loss: yes.

      -  Node density: 1-30.

4.5.  Human Safety Services

   Refers to industrial wireless services that concern collecting data
   to infer about potential dangers to workers in industrial
   environments.  The need for wireless integration concerns: support
   for pervasive deployment; mobility.

   Examples of services are:

   *  Detection of dangerous situations/operations

      -  Bounded latency: 1s.

      -  Periodic, 10 per second (10 fps).

      -  Transmit data size (bytes): 2-100K.

      -  Tolerance to packet loss: yes.

      -  Node density: 1-50.

   *  Vital sign monitoring, dangerous behaviour detection

      -  Bounded latency: 1s-50s.

      -  Periodic, once per minute.

      -  Transmit data size (bytes): 2-100K.

      -  Tolerance to packet loss: 0.

      -  Node density: 1-30.

4.6.  Mobile Robotics Services

   Refers to services that support the communication between robots,
   e.g., task sharing; guidance control including data processing, AV,
   alerts.  Reasons to consider wireless integration are: the need to
   support mobility and reconfigurability.

   *  Video operated remote control

      -  Bounded latency: 10-100ms.

      -  Transmit data size (bytes): 15-150K.

      -  Tolerance to packet loss: yes.

      -  Node density: 2-100.

   *  Assembly of robots or milling machines

      -  Bounded latency: 4-8ms.

      -  Transmit data size (bytes): 40-250.

      -  Tolerance to packet loss: yes.

      -  Node density: 2-100.

   *  Operation of mobile cranes

      -  Bounded latency: 12ms.

      -  Periodic, once per 2-5ms.

      -  Transmit data size (bytes): 40-250.

      -  Tolerance to packet loss: yes.

      -  Node density: 2-100.

   *  Drone/UAV air monitoring

      -  Bounded latency: 100ms.

      -  Tolerance to packet loss: yes.

4.7.  Power Grid Control

   Power grid control concerns services that support communication links
   for predictive maintenance and to isolate faults on high voltage
   lines, transformers, reactors, etc.  Reasons to integrate wireless
   concern: wire replacement maintenance cost reduction.

   *  Bounded latency: 1-10ms.

   *  Transmit data size (bytes): 20-50.

   *  Time synchronization: IEEE 1588.

   *  Tolerance to packet loss: yes.

   *  Node density: 2-100.

4.8.  Wireless Avionics Intra-communication

   Wireless integration is also relevant to industrial environments in
   the context of replacing cabling.  Within the context of avionics
   [AVIONICS], _Wireless Avionics Intra-communication (WAIC)_ systems
   [WAIC] are expected to significantly benefit from determinist
   communications, given their higher criticality.  For instance, flight
   control systems, integrating a large number of endpoints (sensors and
   actuators), require high reliability and bounded latency to assist in
   estimating and controlling the state of the aircraft.  Real-time data
   needs to be delivered with strict deadlines for most control systems.

   The WAIC standardization process is still ongoing, without a clear
   indication about the frequencies that would be reserved for such
   systems, although the frequency band 4.2 GHz to 4.4 GHz is the one
   that currently seems most popular.  Nevertheless, independently of
   the allocated frequency bands, the determinisc guarantees required by
   WAIC services may be achieved by means of the integration of
   functionality developed in current wireless standards.

   However, the following requirements are expected to be supported by
   wireless technology in order to ensure the deterministic operation of
   WAIC systems:

   *  Must provide deterministic behaviour in short radio ranges (<

   *  Must use low transmit power levels for low rate (10mW) and high
      rate (50mW) applications.

   *  Must ensure good system reconfigurability.

   *  Must support dissimilar redundancy.

   In terms of potential KPIs, specific communication requirements can
   be identified:

   *  Latency: 20-40ms [PARK2020].

   *  Packet payload: small (e.g., 50 bytes) and variable bit rate

   *  Support between 125 to 4150 nodes [AVIONICS].

   *  Maximum distance between transmitter and receiver: 15m [AVIONICS].

   *  Aggregate average data rate of network (kbit/s): 394 to 18385

   *  Latency: below 5s for High data rate Inside (HI) applications

   *  Jitter: below 50ms for HI applications [AVIONICS].

   As an example of current standards that may support the deterministic
   requirements of WAIC system, we can point to IEEE 802.11ax, which is
   being devised to operate between 1 and 7GHz (in addition to 2.4 GHz
   and 5GHz).  The WAIC requirement for high reliability and bounded
   latency may be supported by 802.11ax capability of dividing the
   spectrum in frequency resource units (RUs), which are assigned to
   stations for reception and transmission by a central coordinating
   entity, the wireless Access Point.  Reliability can be achieved by
   assigning more than one RU to the same station, for instance (an
   aspect that is not covered by IEEE 802.11ax but already under
   discussion for IEEE 802.11be).  Through the central scheduling of the
   RUs contention overhead can be avoided, which increases efficiency in
   scenarios of dense deployments as is the case of WAIC applications.
   In this context, OFDMA and the concept of spatial reuse is relevant,
   to assist large-scale simultaneous transmission, while at the same
   time preventing collision and interference, and guaranteeing high
   throughput [ROBOTS1].

5.  Additional Reliable Wireless Industrial Services

   This section provides examples of additional wireless industrial
   services.  We have specifically selected three different examples of
   such use-cases: i) remote AR/VR for maintenance and control; ii)
   decentralized shop-floor communication and iii) wireless cabin intra-
   communications.  Based on these examples, wireless integration
   recommendations are debated and a list of specific requirements is

5.1.  AR/VR Services within Flexible Factories

5.1.1.  Description

   While Video is today integrated both into industrial automation
   systems, and also used with the shop-floor to assist the worker, the
   integration of AR/VR in the shop-floor in industrial environments is
   still in the beginning.  It is, however, being applied within the
   electric industry as a way to improve productivity and safety of
   workers, also overlaying real-time metadata over equipment under
   maintenance or operation.

   In this context, it is important to ensure that the AR/AV traffic
   does not interfere with the critical traffic of the production
   system, i.e., performance characteristics like latency and jitter for
   the critical traffic shall be independent from disturbances.
   Moreover, it is also important to provide the AR/VR application with
   low latency, also in the verge of mobility.

5.1.2.  Wireless Integration Recommendations

   The support of AR/AV in the context of remote maintenance
   environments is bound to increase within industrial environments,
   given the relevancy in terms of remote maintenance and equipment
   operations.  It is also relevant to consider its use within the
   context of worker safety and it can be foreseen that AV-based remote
   maintenance will, in the future, be supported via mobile devices
   carried by workers on the go.  Wireless is therefore a key
   communication asset for this type of applications.  In terms of
   traffic in a converged network, AR/AV is a bandwidth intensive real-
   time service.  It therefore requires specific handling (other than
   Best Effort, BE).  Moreover, the AR/AV traffic flows must not create
   disturbance when transmitted via wireless.  Hence, traffic isolation
   is an important aspect to ensure for this type of traffic profile.

   A third aspect to address in the future concerns the fact that there
   will most likely be the need to support multiple AR/AV streams from
   different end-users within a single Wireless Local Area Network
   (WLAN), thus increasing the need for traffic isolation.  A fourth
   aspect concerns the fact that VR systems, if not adequately support,
   result in VR sickness.  Specific network and non-network requirements
   have already been identified by IEEE 802, MPEG, 3GPP.  Such
   requirements contemplate, for instance, support of higher frame
   rates, reducing the motion-to-photon latency, higher data
   transmission rates, low jitter, etc.

5.1.3.  Requirements Considerations

   In such applications, to ensure minimum interference, a few aspects
   need to be ensured:

   *  The AR/AV traffic needs to be isolated in order to prevent
      interference, i.e., it SHOULD have a specific CoS assigned
      (downlink and uplink).

   *  Between wireless devices (stations) and AP, there is the need to
      ensure that the AR/AV traffic is handled in a way that does not
      hinder critical traffic.

   *  Low mobility SHOULD be supported.

   *  Multiple user support SHOULD be provided.

   *  VR sickness MUST be prevented [IEEERTA].

   *  A tight integration of the AR/VR systems with production systems
      SHOULD be address in way compatible with the deterministic wired
      infrastructure.  For instance, Audio Video Bridging (AVB) in the
      wired TSN infrastructure.  Specifically, AVB is usually blocked by
      the time-aware shaper, and impacted by: TAS, CBS, FIFO and FPNS
      (fixed priority non-preemptive scheduling).

   *  A software-based mechanism on the AP SHOULD support an adequate
      mapping of CoS to the wireless QoS (e.g., EDCA UPs).

   *  MAC layer contention MUST be mitigated for all wireless stations
      within the area (within the range of the same AP or not).

   Specific communication requirements:

   *  Latency: 3-10ms [IEEERTA].

   *  Bandwidth, 0.1-2Gbps [IEEERTA].

   *  Data payload, over 4Kbytes [IEEERTA].

5.2.  Decentralized Shop-floor Communication Services

5.2.1.  Description

   The increasing automation of industrial environments implies an
   increase in the number of integrated nodes, including mobile nodes.
   Wireless is, for instance, a key driver for scenarios involving
   mobile vehicles [NICT].  NICT also describes already production
   environments, in particular environments with elevated temperatures,
   where wireless communication is used to support safety of workers and
   to remotely monitor production status.  Such environments comprise
   different applications (e.g., safety of workers, mobile robots,
   factory resource management) and debate on the interconnection of
   different wireless technologies and devices, from PLCs, to autonomous
   mobile robots, e.g., UAVs, AGVs.  Wireless/wired integration
   mechanisms have also been debated in the cost of self-organizing
   production lines [DIETRICH2018].  Therefore, the notion of flexible
   and heterogeneous shop-floor communication is already present in
   industrial environments, based on hybrid wired/wireless systems and
   the integration of multi-AP environments.

5.2.2.  Wireless Integration Recommendations

   Prior related work debates on centralized communication architectures
   (infrastructure mode), and for this case, the issue of connectivity
   is usually circumvented via multiple AP coordination mechanisms.
   Within the context of multi-AP coordination and assuming TDMA-based
   communication, a well-organized schedule can prevent collisions
   [FERN2019].  Hence, for this specific type of scenario, the main
   issue concerns handling handovers in a timely and precise way,
   capable of providing deterministic guarantees.  However, with an
   increase on the number of nodes on a shop-floor, connectivity issues
   become more complex.

   Therefore, it is relevant to explore also the possibility of a
   "decentralized" approach to shop-floor communication, considering
   both mobile and static nodes.  In this case, and from a topology
   perspective, wireless industrial services are expected to be provided
   over both ad-hoc and infrastructure mode.  Within the ad-hoc
   communication areas, there is control-based traffic integrated with
   sensing (critical, non-critical), with real-time traffic, as well as
   time-triggered traffic.  Each node is responsible for managing its
   access to the medium, thus requiring a cooperative protocol approach.

5.2.3.  Requirements Considerations

   In such environment, connectivity becomes more complex requiring
   additional support:

   *  A wider variety of traffic profiles MUST be supported, thus
      increasing the management complexity.

   *  Devices communicating via ad-hoc mode MUST integrate a
      collaborative communication approach, e.g., relaying, cluster-
      based scheduling approach.

   *  Low mobility MUST be supported (e.g., up to 2 m/s within a BSS).

   *  Multi-AP coordination MUST still be integrated.

   *  Frequent handover MUST be supported, ideally with a make-before-
      break approach.

   *  Neighbor detection and coverage problem detection MUST be
      implemented in ad-hoc nodes.

   Specific communication requirements: * Latency: 20-40ms [ROBOTS1]. *
   Packet payload: small (e.g., 50 bytes) and variable bit rate

5.3.  Autonomous Airborne Services

   ### Description

   Over the last decade several services emerged that rely on the
   autonomous (total or partial) operation of airborne systems.
   Examples of such systems are: logistic drones; swarm of drones (e.g.
   for surveillance); urban Air Mobility [UAM18]; single Pilot operation
   of commercial aircrafts [BBN8436].

   Such autonomous airborne systems rely on advances in communications,
   navigation, and air traffic management to mitigate the significant
   workload of autonomous operations, namely by means of air-ground
   collaborative decision making.  Such decision making processes rely
   on expanding the role of ground operators, including tactical (re-
   routing) and emergency flight phases, as well as higher levels of
   decision support including systems monitoring in real-time.

   Such air-ground collaborative decision making process can only be
   possible with the support of a reliable wireless network able to
   assist in the required data exchange (of different types of traffic)
   within significant constraints in terms of delay and error avoidance.

5.3.1.  Wireless Integration Recommendations

   Independently of the type of application (logistics, surveillance,
   urban air mobility, single pilot operation), an autonomous airborne
   system can be models as a multi-agent system, in which agents need to
   use a wireless network to communicate reliably between them and in
   possible with a control entity.  The nature and position of such
   agentes differ from application to application.  For instance, all
   agents may be collocated in the same or different flying vehicles.

   A high-performance and reliable wireless network has an important
   role in meeting the challenges of autonomous airborne systems, such
   as coordination and collaboration strategies, control mechanisms, and
   mission planning algorithms.  Hence, wireless technologies plan a
   central role in the creation of the needed networking system,
   including air-to-air communications (single or multi-hop) but air-to-
   ground communications.

   Air-to-air communications allow all airborne agents to establish
   efficient communication, allowing the reception of error prune data
   exchanged within the required time frames.  For instance, in a swarm
   drones can either communicate with each other directly, or indirectly
   by constructing multi-hop communication paths with other drones.

   In what concerns air-to-ground communications, airborne agents
   communicate with a control center, such as a ground station, to
   obtain real-time updated information (e.g. mission related).  Air-to-
   ground communication is usually direct communication.

   The air-to-air and air-to-ground communications are combined through
   a communication architecture, which can be of different types.  In
   small autonomous systems (single drones used for logistics), a
   central control station is deployed with enough powerf to communicate
   with the drone.  In autonomous systems with a large number of agents,
   a decentralized approach should be used.

5.3.2.  Requirements Considerations

   When analysing the major properties of wireless communication
   architectures, the first priority should go to requirements of high
   coverage and maintaining connectivity.  The former plays an important
   role in gathering the information needed for the operation of the
   autonomous system, while maintaining connectivity ensures the real-
   time communication within the system.

   However, autonomous systems may operate in unknown environments, with
   the unpredicted appearance of threats and obstacles in time and
   space.  Hence such systems should rely on wireless technology that
   has a high level of reliability and availability.  For instance,
   wireless technology that is able to keep two neighbour agents
   connected, even when their direct link drops below the required
   minimum signal-to-noise ratio (SNR) or receive signal strength
   indicators (RSSI) range.  On a system level, wireless network
   technologies, such as routing, should be able to react cognitively to
   changes of the environment to adapt the communication system in order
   to ensure the needed coverage and connectivity levels.

   In this sense it is required the investigation of routing protocols
   able to ensure the desirable level or reliability and availability of
   complete system.  This means that the wireless routing function
   should fulfill a set of requirements, including: * Suitable for
   dynamic topologies. * Scalable with the number of networked agents. *
   Ensure low values of packet delays (KPI depends upon the specific
   application). * Ensure high values of packet delivery (KPI depends
   upon the specific application). * Ensure fast recovery in the
   presence of interrupted communications. * Ensure low cost in terms of
   the utilization of network resources (e.g. network queues,
   transmission opportunities). * Ensure high robustness to link

6.  Security Considerations

   This document describes industrial application communication
   requirements for the integration of reliable Wi-Fi technologies.  The
   different applications have security considerations which have been
   described in the respective sources [IEEERTA], [NICT], [IIC],
   [AVNU2020], [ACIA].

7.  IANA Considerations

   This document has no IANA actions.

8.  Acknowledgments

   The research leading to these results received funding from joint
   fortiss GmbH and Huawei project TSNWiFi (https://www.fortiss.org/en/r

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

9.2.  Informative References

   [ACIA]     5G ACIA, ., "5G for Connected Industries and Automation",
              November 2019.

   [AVIONICS] Fischione, P.Park, P.Di Marco, J.Nah, and C., "Wireless
              Avionics Intra-Communications, A Survey of Benefits,
              Challenges, and Solutions, pp. 1-24", 2020.

   [AVNU2020] Bush, S., "Avnu Alliance White Paper Wireless TSN-
              Definitions, Use Cases & Standards Roadmap", 2020.

   [BBN8436]  Pew, Deutsch, Stephen, and Richard W., "Single pilot
              commercial aircraft operation. BBN Report.", 2005.

              , & Fohler, G, Dietrich, S., May, G., von Hoyningen-Huene,
              J., Mueller, A., "Frame conversion schemes for cascaded
              wired/wireless communication networks of factory
              automation, Mobile Networks and Applications, 23(4),
              817-827", 2018.

   [FERN2019] Fernández Ganzabal, Z., "Analysis of the Impact of
              Wireless Mobile Devices in Critical Industrial
              Applications", May 2019.

   [IEEERTA]  Meng, K., "IEEE 802.11 Real Time Applications TIG Report",

              Bernardos, G.P.P.T.F.T.a.C., "RAW use cases," IETF draft -
              RAW working group", 2020,

   [IIC]      Linehan, M., "Time Sensitive Networks for Flexible
              Manufacturing Testbed Characterization and Mapping of
              Converged Traffic Types", 2020.

   [NENDICA]  Zein, Ed, N., "IEEE 802 Nendica Report, Flexible Factory
              IoT-Use Cases and Communication Requirements for Wired and
              Wireless Bridged Networks", 2020.

   [NICT]     NICT, "Wireless use cases and communication requirements
              in factories ( abridged edition ), Flex. Factories Proj",
              February 2018.

   [OPCFLC]   "OPC Foundation Field Level Communications (FLC)
              Initiative", September 2020,

   [PARK2020] Park, Pangun, et al, ., "Wireless Avionics Intra-
              Communications, A Survey of Benefits, Challenges, and
              Solutions. IEEE Internet of Things Journal", 2020.

   [RFC8578]  Grossman, E., Ed., "Deterministic Networking Use Cases",
              RFC 8578, DOI 10.17487/RFC8578, May 2019,

   [ROBOTS1]  Hoebeke, J.Haxhibeqiri, E.A.Jarchlo, I.Moerman, and J.,
              "Flexible Wi-Fi Communication among Mobile Robots in
              Indoor Industrial Environments, Mob. Inf. Syst.", 2018.

   [UAM18]    Shamiyeh, Michael, Raoul Rothfeld, and Mirko Hornung, .,
              "A performance benchmark of recent personal air vehicle
              concepts for urban air mobility.  Proceedings of the 31st
              Congress of the International Council of the Aeronautical
              Sciences, Belo Horizonte, Brazil", 2018.

   [WAIC]     International Telecommunication Union, "Technical
              characteristics and operational objectives for wireless
              avionics intra-communications, Policy, vol. 2197, p. 58,".

Authors' Addresses

   Rute C. Sofia
   fortiss GmbH
   Guerickestr. 25
   80805 Munich

   Email: sofia@fortiss.org

   Matthias Kovatsch
   Huawei Technologies
   Riesstr. 25 C, 3.0G
   80992 Munich

   Email: ietf@kovatsch.net

   Paulo Milheiro Mendes
   Willy-Messerschmitt Strasse 1
   81663 Munich

   Email: paulo.mendes@airbus.com