lpwan Working Group E. Ramos
Internet-Draft Ericsson
Intended status: Standards Track A. Minaburo
Expires: 20 November 2022 Acklio
19 May 2022
SCHC over NBIoT
draft-ietf-lpwan-schc-over-nbiot-08
Abstract
The Static Context Header Compression and Fragmentation (SCHC)
specification describes header compression and fragmentation
functionalities for LPWAN (Low Power Wide Area Networks)
technologies. The Narrowband Internet of Things (NB-IoT)
architecture may adapt SCHC to improve its capacities.
This document describes the use of SCHC over the NB-IoT wireless
access and provides use-cases for efficient parameterization.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Data Transmission in the 3GPP Architecture . . . . . . . . . 5
4.1. Use of SCHC over the Radio link . . . . . . . . . . . . . 6
4.1.1. SCHC Entities Placing . . . . . . . . . . . . . . . . 6
4.2. Use of SCHC over the No-Access Stratum (NAS) . . . . . . 7
4.2.1. SCHC Entities Placing . . . . . . . . . . . . . . . . 8
4.2.2. Parameters for Static Context Header Compression and
Fragmentation (SCHC) for the Section 4.1 and
Section 4.2. . . . . . . . . . . . . . . . . . . . . 9
4.3. End-to-End Compression . . . . . . . . . . . . . . . . . 11
4.3.1. SCHC Entities Placing . . . . . . . . . . . . . . . . 11
4.3.2. Parameters for Static Context Header Compression and
Fragmentation (SCHC) . . . . . . . . . . . . . . . . 12
5. Padding . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6. Security considerations . . . . . . . . . . . . . . . . . . . 14
7. Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1. NB-IoT User Plane protocol architecture . . . . . . . . . 14
7.1.1. Packet Data Convergence Protocol (PDCP) . . . . . . . 14
7.1.2. Radio Link Protocol (RLC) . . . . . . . . . . . . . . 15
7.1.3. Medium Access Control (MAC) . . . . . . . . . . . . . 16
7.2. NB-IoT Data over NAS (DoNAS) . . . . . . . . . . . . . . 17
8. Normative References . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
The Static Context Header Compression (SCHC) [RFC8724] defines a
header compression scheme, and fragmentation functionality, suitable
for the Low Power Wide Area Networks (LPWAN) networks described in
[RFC8376].
In a Narrowband Internet of Things (NB-IoT) network, header
compression efficiently brings Internet connectivity to the Device -
User Equipment (Dev-UE). This document describes the SCHC parameters
used to support the static context header compression and
fragmentation over the NB-IoT wireless access. This document assumes
functionality for NB-IoT of 3GPP release 15 (3GPPR15). Otherwise,
the text explicitly mentions other versions' functionality.
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2. Terminology
This document will follow the terms defined in [RFC8724], in
[RFC8376], and the [TGPP23720].
* CIoT. Cellular IoT.
* NGW-C-SGN. Network Gateway - CIoT Serving Gateway Node.
* Dev-UE. Device - User Equipment.
* RGW-eNB. Radio Gateway - Node B. Base Station that controls the
UE.
* EPC. Evolved Packet Connectivity. Core network of 3GPP LTE
systems.
* EUTRAN. Evolved Universal Terrestrial Radio Access Network.
Radio access network of LTE-based systems.
* NGW-MME. Network Gateway - Mobility Management Entity. An entity
in charge of handling mobility of the Dev-UE.
* NB-IoT. Narrowband IoT. A 3GPP LPWAN technology based on the LTE
architecture but with additional optimization for IoT and using a
Narrowband spectrum frequency.
* NGW-SGW. Network Gateway - Serving Gateway. It routes and
forwards the user data packets through the access network.
* HSS. Home Subscriber Server. It is a database that performs
mobility management.
* NGW-PGW. Network Gateway - Packet Data Node Gateway. An
interface between the internal with the external network.
* PDU. Protocol Data Unit. A data packet including headers that
are transmitted between entities through a protocol.
* SDU. Service Data Unit. A data packet (PDU) from higher layer
protocols used by lower layer protocols as a payload of their own
PDUs.
* IWK-SCEF. InterWorking Service Capabilities Exposure Function.
It is used in roaming scenarios, it is located in the Visited PLMN
and serves for interconnection with the SCEF of the Home PLMN.
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* NGW-SCEF. Network Gateway - Service Capability Exposure Function.
EPC node for exposure of 3GPP network service capabilities to 3rd
party applications.
3. Architecture
The Narrowband Internet of Things (NB-IoT) architecture has a complex
structure. It relies on different NGWs from different providers and
can send data via different paths, each path with different
characteristics in terms of bandwidths, acknowledgments, and layer
two reliability and segmentation.
Figure 1 shows this architecture, where the Network Gateway Cellular
Internet of Things Serving Gateway Node (NGW-CSGN) optimizes co-
locating entities in different paths. For example, a Dev-UE using
the path formed by the Network Gateway Mobility Management Entity
(NGW-MME), the NGW-CSGW, and Network Gateway Packet Data Node Gateway
(NGW-PGW) may get a limited bandwidth transmission from few bytes/s
to one thousand bytes/s only.
Another node introduced in the NB-IoT architecture is the Network
Gateway Service Capability Exposure Function (NGW-SCEF), which
securely exposes service and network capabilities to entities
external to the network operator. OMA and OneM2M define the
northbound APIS [TGPP33203]. In this case, the path is small for
data transmission. The main functions of the NGW-SCEF are:
Connectivity path and Device Monitoring.
+---+ +------+
|Dev| \ +-----+ ----| HSS |
|-UE| \ | NGW | +------+
+---+ | |-MME |\__
\ / +-----+ \
+---+ \+-----+ / | +------+
|Dev| ----| RGW |- | | NGW- |
|-UE| |-eNB | | | SCEF |---------+
+---+ /+-----+ \ | +------+ |
/ \ +------+ |
/ \| NGW- | +-----+ +-----------+
+---+ / | CSGW |--| NGW-|---|Application|
|Dev| | | | PGW | | Server |
|-UE| +------+ +-----+ +-----------+
+---+
Figure 1: 3GPP network architecture
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4. Data Transmission in the 3GPP Architecture
NB-IoT networks deal with end-to-end user data and in-band signalling
between the nodes and functions to configure, control, and monitor
the system functions and behaviors. The signalling data uses a
different path with specific protocols, handling processes, and
entities but can transport end-to-end user data for IoT services. In
contrast, the end-to-end application only transports end-to-end data.
The maximum recommended 3GPP MTU size is 1358 Bytes. The radio
network protocols limit the packet sizes over the air, including
radio protocol overhead, to 1600 Bytes. However, the recommended MTU
is smaller to avoid fragmentation in the network backbone due to the
payload encryption size (multiple of 16) and the additional core
transport overhead handling.
3GPP standardizes NB-IoT and, in general, the cellular technologies
interfaces and functions. Therefore the introduction of SCHC
entities to Dev-UE, RGW-eNB, and NGW-CSGN needs to be specified in
the NB-IoT standard, which implies that standard specifying SCHC
support would not be backward compatible. A terminal or a network
supporting a version of the standard without SCHC or without an
implementation capability (in case of not being standardized as
mandatory capability) cannot utilize SCHC with this approach.
SCHC could be deployed differently depending on where the header
compression and the fragmentation are applied. The SCHC
functionalities can be used over the radio transmission only, between
the Dev-UE and the RGW-eNB. Alternatively, the packets transmitted
over the path can use SCHC. Else, when the transmissions go over the
NGW-MME or NGW-SCEF, the NGW-CSGN uses SCHC entity. For these two
cases, the functions need to be standardized by 3GPP.
Another possibility is to apply SCHC functionalities to the end-to-
end connection or at least up to the operator network edge. SCHC
functionalities are available in the application layer of the Dev-UE
and the Application Servers or a broker function at the edge of the
operator network. The radio network transmits the packets as non-IP
traffic using IP tunnelling or SCEF services. Since this option does
not necessarily require 3GPP standardization, it is possible to also
benefit legacy devices with SCHC by using the non-IP transmission
features of the operator network.
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4.1. Use of SCHC over the Radio link
Deploying SCHC only over the radio link would require placing it as
part of the protocol stack for data transfer between the Dev-UE and
the RGW-eNB. This stack is the functional layer responsible for
transporting data over the wireless connection and managing radio
resources. There is support for features such as reliability,
segmentation, and concatenation. The transmissions use link
adaptation, meaning that the system will optimize the transport
format used according to the radio conditions, the number of bits to
transmit, and the power and interference constraints. That means
that the number of bits transmitted over the air depends on the
Modulation and Coding Schemes (MCS) selected. The transmissions of
Transport Block (TB) happen in the physical layer at network
synchronized intervals called Transmission Time Interval (TTI). Each
Transport Block has a different MCS and number of bits available to
transmit. The MAC layer [TGPP36321] defines the Transport Blocks
characteristics. The Radio link stack shown in Figure 2 comprises
the Packet Data Convergence Protocol (PDCP) [TGPP36323], Radio Link
Protocol (RLC) [TGPP36322], Medium Access Control protocol (MAC)
[TGPP36321], and the Physical Layer [TGPP36201]. The
Appendix gives more details of these protocols.
4.1.1. SCHC Entities Placing
The current architecture provides support for header compression in
the PDCP layer using RoHC [RFC5795]. Therefore SCHC header
compression entities can be deployed similarly without the need for
significant changes in the 3GPP specifications.
In this scenario, the RLC layer takes care of fragmentation unless
for the Transparent Mode. When packets exceed the Transport Block
size at transmission, SCHC fragmentation is unnecessary and should
not be used to avoid the additional protocol overhead. The RLC
Transparent Mode is not commonly used while sending IP packets in the
Radio link. However, given the case in the future, SCHC
fragmentation may be used. In that case, a SCHC tile would match the
minimum transport block size minus the PDCP and MAC headers.
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+---------+ +---------+ |
|IP/non-IP+------------------------------+IP/non-IP+->+
+---------+ | +---------------+ | +---------+ |
| PDCP +-------+ PDCP | GTP|U +------+ GTP-U |->+
| (SCHC) + + (SCHC)| + + | |
+---------+ | +---------------+ | +---------+ |
| RLC +-------+ RLC |UDP/IP +------+ UDP/IP +->+
+---------+ | +---------------+ | +---------+ |
| MAC +-------+ MAC | L2 +------+ L2 +->+
+---------+ | +---------------+ | +---------+ |
| PHY +-------+ PHY | PHY +------+ PHY +->+
+---------+ +---------------+ +---------+ |
C-Uu/ S1-U SGi
Dev-UE RGW-eNB NGW-CSGN
Radio Link
Figure 2: SCHC over the Radio link
4.2. Use of SCHC over the No-Access Stratum (NAS)
The NGW-MME conveys mainly control signaling between the Dev-UE and
the cellular network [TGPP24301]. The network transports this
traffic on top of the radio link.
This kind of flow supports data transmissions to reduce the overhead
when transmitting infrequent small quantities of data. This
transmission is known as Data over No-Access Stratum (DoNAS) or
Control Plane CIoT EPS optimization. In DoNAS, the Dev-UE uses the
pre-established security and can piggyback small uplink data into the
initial uplink message and uses an additional message to receive a
downlink small data response.
The NGW-MME performs the data encryption from the network side in a
DoNAS PDU. Depending on the data type signaled indication (IP or
non-IP data), the network allocates an IP address or establishes a
direct forwarding path. DoNAS is regulated under rate control upon
previous agreement, meaning that a maximum number of bits per unit of
time is agreed upon per device subscription beforehand and configured
in the device.
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The system will use DoNAS when a terminal in a power-saving state
requires a short transmission and receives an acknowledgment or short
feedback from the network. Depending on the size of buffered data to
transmit, the Dev-UE might deploy the connected mode transmissions
instead, limiting and controlling the DoNAS transmissions to
predefined thresholds and a good resource optimization balance for
the terminal and the network. The support for mobility of DoNAS is
present but produces additional overhead. The Appendix gives
additional details of DoNAS.
4.2.1. SCHC Entities Placing
In this scenario, SCHC may reside in the Non-Access Stratum (NAS)
protocol layer. The same principles as for Radio link transmissions
apply here as well. Because the NAS protocol already uses RoHC it
can adapt SCHC for header compression too. The main difference
compared to the radio link is the physical placing of the SCHC
entities. On the network side, the NGW-MME resides in the core
network and is the terminating node for NAS instead of the RGW-eNB.
+--------+ +--------+--------+ + +--------+
| IP/ +--+-----------------+--+ IP/ | IP/ +-----+ IP/ |
| Non-IP | | | | Non-IP | Non-IP | | | Non-IP |
+--------+ | | +-----------------+ | +--------+
| NAS +-----------------------+ NAS |GTP-C/U +-----+GTP-C/U |
|(SCHC) | | | | (SCHC) | | | | |
+--------+ | +-----------+ | +-----------------+ | +--------+
| RRC +-----+RRC |S1|AP+-----+ S1|AP | | | | |
+--------+ | +-----------+ | +--------+ UDP +-----+ UDP |
| PDCP* +-----+PDCP*|SCTP +-----+ SCTP | | | | |
+--------+ | +-----------+ | +-----------------+ | +--------+
| RLC +-----+ RLC | IP +-----+ IP | IP +-----+ IP |
+--------+ | +-----------+ | +-----------------+ | +--------+
| MAC +-----+ MAC | L2 +-----+ L2 | L2 +-----+ L2 |
+--------+ | +-----------+ | +-----------------+ | +--------+
| PHY +--+--+ PHY | PHY +--+--+ PHY | PHY +-----+ PHY |
+--------+ +-----+-----+ +--------+--------+ | +--------+
C-Uu/ S1-lite SGi
Dev-UE RGW-eNB NGW-MME NGW-PGW
*PDCP is bypassed until AS security is activated TGPP36300.
Figure 3: SCHC entities placement in the 3GPP CIOT radio protocol
architecture for DoNAS transmissions
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4.2.2. Parameters for Static Context Header Compression and
Fragmentation (SCHC) for the Section 4.1 and Section 4.2.
These scenarios MUST use SCHC header compresion capability to improve
the transmission of IPv6 packets. The 3GPP Architecture currently
provides Header Compression using the [RFC5795] but the use of SCHC
for IoT application MUST be considered to improve the devices
connectivity.
* SCHC Context initialization RRC (Radio Resource Control) protocol
is the main tool used to configure the parameters of the Radio
link. It will configure SCHC and the static context distribution
as it has made for RoHC operation [TGPP36323].
* SCHC Rules The network operator in these scenarios defines the
number of rules in a context. The operator must be aware of the
type of IP traffic that the device will carry out. Implying that
the operator might use provision sets of rules compatible with the
use case of the device. For devices acting as gateways of other
devices, several rules may match the diversity of devices and
protocols used by the devices associated with the gateway.
Meanwhile, simpler devices (for example, an electricity meter) may
have a predetermined set of fixed protocols and parameters.
Additionally, the deployment of IPv6 addresses may force different
rules to deal with each case.
* RuleID There is a reasonable assumption of 9 bytes of radio
protocol overhead for these transmission scenarios in NB-IoT,
where PDCP uses 5 bytes due to header and integrity protection,
and RLC and MAC use 4 bytes. The minimum physical Transport
Blocks (TB) that can withhold this overhead value according to
3GPP Release 15 specifications are 88, 104, 120, and 144 bits. A
transmission optimization may require only one physical layer
transmission. SCHC overhead should not exceed the available
number of effective bits of the smallest physical TB available.
The packets handled by 3GPP networks are byte-aligned, and
therefore the minimum payload possible (including padding) is 8
bits. Therefore in order to use the smallest TB, the maximum SCHC
header size is 12 bits. These 12 bits must include the
Compression Residue in addition to the RuleID. On the other hand,
more complex NB-IoT devices (such as a capillarity gateway) might
require additional bits to handle the variety and multiple
parameters of higher-layer protocols deployed. In that sense, the
operator may want to have flexibility on the number and type of
rules supported by each device independently, and consequently,
these scenarios require a configurable value. The configuration
may be part of the operation profile agreed together with the
content distribution. The RuleID field size may range from 2
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bits, resulting in 4 rules to an 8-bit value that would yield up
to 256 rules that can be used together with the operators and
seems quite a reasonable maximum limit even for a device acting as
a NAT. More bits could be configured, but it should consider the
byte-alignment of the expected Compression Residue. In the
minimum TB size case, 2 bits of RuleID leave only 6 bits available
for Compression Residue.
* SCHC MAX_PACKET_SIZE The Radio Link can handle the fragmentation
of SCHC packets if needed, including reliability. Hence the
packet size is limited by the MTU handled by the radio protocols
which corresponds to 1600 bytes for 3GPP Release 15.
* Fragmentation For the Section 4.1 and Section 4.2 scenarios, the
SCHC fragmentation functions are disabled. The RLC layer of NB-
IoT can segment packets in suitable units that fit the selected
transport blocks for transmissions of the physical layer. The
blocks selection is made according to the link adaptation input
function in the MAC layer and the quantity of data in the buffer.
The link adaptation layer may produce different results at each
Time Transmission Interval (TTI), resulting in varying physical
transport blocks that depend on the network load, interference,
number of bits transmitted, and QoS. Even if setting a value that
allows the construction of data units following the SCHC tiles
principle, the protocol overhead may be greater or equal than
allowing the Radio link protocols to take care of the
fragmentation natively.
* Fragmentation in RLC Transparent Mode If RLC operates in
Transparent Mode, there could be a case to activate a
fragmentation function together with a light reliability function
such as the ACK-Always mode. In practice, it is uncommon to
transmit radio link data using this configuration. It mainly
targets signaling transmissions. In those cases, the MAC layer
mechanisms ensure reliability, such as repetitions or automatic
retransmissions, and additional reliability might only generate
protocol overhead.
SCHC may reduce radio network protocols overhead in future
operations, support reliable transmissions, and transmit compressed
data with fewer possible transmissions by using fixed or limited
transport blocks compatible with the tiling SCHC fragmentation
handling. For SCHC fragmentation parameters see section
Section 4.3.2.
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4.3. End-to-End Compression
The Non-IP Data Delivery (NIDD) services of 3GPP enable the
transmission of SCHC packets compressed by the application layer.
The packets can be delivered using IP-tunnels to the 3GPP network or
NGW-SCEF functions (i.e., API calls). In both cases, as compression
occurs before transmission, the network will not understand the
packet, and the network does not have context information of this
compression. Therefore the network will treat the packet as Non-IP
traffic and deliver it to the other side without any other protocol
stack element, directly under the L2.
4.3.1. SCHC Entities Placing
In the two scenarios using End-to-End compression, SCHC entities are
located almost on top of the stack. The NB-IoT connectivity services
implement SCHC in the Dev, an in the Application Server. The IP
tunneling scenario requires that the Application Server send the
compressed packet over an IP connection terminated by the 3GPP core
network. If the transmission uses the NGW-SCEF services, it is
possible to utilize an API call to transfer the SCHC packets between
the core network and the Application Server. Also, an IP tunnel
could be established by the Application Server if negotiated with the
NGW-SCEF.
+---------+ XXXXXXXXXXXXXXXXXXXXXXXX +--------+
| SCHC | XXX XXX | SCHC |
|(Non-IP) +-----XX........................XX....+--*---+(Non-IP)|
+---------+ XX +----+ XX | | +--------+
| | XX |SCEF+-------+ | | |
| | XXX 3GPP RAN & +----+ XXX +---+ UDP |
| | XXX CORE NETWORK XXX | | |
| L2 +---+XX +------------+ | +--------+
| | XX |IP TUNNELING+--+ | |
| | XXX +------------+ +---+ IP |
+---------+ XXXX XXXX | +--------+
| PHY +------+ XXXXXXXXXXXXXXXXXXXXXXX +---+ PHY |
+---------+ +--------+
UE AS
Figure 4: SCHC entities placed when using Non-IP Delivery (NIDD)
3GPP Sevices
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4.3.2. Parameters for Static Context Header Compression and
Fragmentation (SCHC)
These scenarios may use SCHC header compresion capability to improve
the transmission of IPv6 packets. The use of SCHC for IoT
application MUST be considered to improve the devices connectivity.
* SCHC Context initialization. The application layer handles the
static context; consequently, the context distribution must be
according to the application's capabilities, perhaps utilizing IP
data transmissions up to context initialization. Also, the static
contexts delivery may use the same IP tunneling or NGW-SCEF
services used later for the SCHC packets transport.
* SCHC Rules. Even when the transmission content is not visible for
the 3GPP network, the same limitations as for Section 4.1 and
Section 4.2 transmissions apply in these scenarios in terms of
aiming to use the minimum number of transmissions and minimize the
protocol overhead.
* Rule ID Similar to the case of Section 4.1 and Section 4.2, the
RuleID size can be dynamically set before the context delivery.
For example, negotiated between the applications when choosing a
profile according to the type of traffic and application deployed.
The same considerations related to the transport block size and
performance mentioned for the Section 4.1 and Section 4.2 must be
followed when choosing a size value for the RuleID field.
* SCHC MAX_PACKET_SIZE In these scenarios, the maximum recommended
MTU size that applies is 1358 Bytes since the SCHC packets (and
fragments) are traversing the whole 3GPP network infrastructure
(core and radio), not only the radio as the IP transmissions case.
* Fragmentation Packets larger than 1358 bytes need the SCHC
fragmentation function. Since the 3GPP uses reliability
functions, the No-ACK fragmentation mode may be enough in point-
to-point connections. Nevertheless, additional considerations are
described below for more complex cases.
* Fragmentation modes A global service assigns a QoS to the packets
depending on the billing. Packets with very low QoS may get lost
before they arrive in the 3GPP radio network transmission, for
example, in between the links of a capillarity gateway or due to
buffer overflow handling in a backhaul connection. The use of
SCHC fragmentation with the ACK-on-Error mode is recommended to
secure additional reliability on the packets transmitted with a
small trade-off on additional transmissions to signal the end-to-
end arrival of the packets if no transport protocol takes care of
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retransmission.
Also, the ACK-on-Error mode is even desirable to keep track of all
the SCHC packets delivered. In that case, the fragmentation
function could be active for all packets transmitted by the
applications. SCHC ACK-on-Error fragmentation may be active in
transmitting non-IP packets on the NGW-MME. A non-IP packet will
use SCHC reserved RuleID for non-compressing packets as [RFC8724]
allows it.
* Fragmentation Parameters SCHC profile will have specific Rules for
the fragmentation modes. The rule will identify, which
fragmentation mode is in use, and section Section 4.2.2 defines
the RuleID size.
SCHC parametrization considers that NBIoT aligns the bit and uses
padding and the size of the Transfer Block. SCHC will try to reduce
padding to optimize the compression of the information. The Header
size needs to be multiple of 4, and the Tiles may keep a fixed value
of 4 or 8 bits to avoid padding except for transfer block equals 16
bits where Tiles may be of 2 bits. The transfer block size has a
wide range of values. Two configurations may be used for the
fragmentation parameters.
* For Transfer Blocks smaller or equal to 300bits using a 8 bits-
Header_size configuration, with the size of the header fields as
follows:
- RuleID from 1 - 3 bits,
- DTag 1 bit,
- FCN 3 bits,
- W 1 bits.
* For Transfer Blocks bigger than 300 bits using a 16 bits-
Header_size configuration, with the size of the header fields as
follows:
- RulesID from 1 to 8 or 10 bits,
- DTag 1 or 2 bits,
- FCN 3 bits,
- W 2 or 3 bits.
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The IoT devices communicate with small data transfer and have a
battery life of 10 years. These devices use the Power Save Mode and
the Idle Mode DRX, which govern how often the device wakes up, stays
up, and is reachable. Table 10.5.163a in {3GPP-TS_24.088} specifies
a range for the radio timers as N to 3N in increments of one where
the units of N can be 1 hour or 10 hours. To adapt SCHC to the NB-
IoT activities, the Inactivity Timer and the Retransmission Timer be
set based on these limits.
5. Padding
NB-IoT and 3GPP wireless access, in general, assumes byte-aligned
payload. Therefore the L2 word for NB-IoT MUST be considered 8 bits,
and the padding treatment should use this value accordingly.
6. Security considerations
This document does not add any security considerations and follows
the [RFC8724] and the 3GPP access security document specified in
[TGPP33203].
7. Appendix
7.1. NB-IoT User Plane protocol architecture
7.1.1. Packet Data Convergence Protocol (PDCP)
Each of the Radio Bearers (RB) is associated with one PDCP entity.
Moreover, a PDCP entity is associated with one or two RLC entities
depending on the unidirectional or bi-directional characteristics of
the RB and RLC mode used. A PDCP entity is associated with either a
control plane or a user plane with independent configuration and
functions. The maximum supported size for NB-IoT of a PDCP SDU is
1600 octets. The primary services and functions of the PDCP sublayer
for NB-IoT for the user plane include:
* Header compression and decompression using ROHC (Robust Header
Compression)
* Transfer of user and control data to higher and lower layers
* Duplicate detection of lower layer SDUs when re-establishing
connection (when RLC with Acknowledge Mode in use for User Plane
only)
* Ciphering and deciphering
* Timer-based SDU discard in uplink
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7.1.2. Radio Link Protocol (RLC)
RLC is a layer-2 protocol that operates between the UE and the base
station (eNB). It supports the packet delivery from higher layers to
MAC, creating packets transmitted over the air, optimizing the
Transport Block utilization. RLC flow of data packets is
unidirectional, and it is composed of a transmitter located in the
transmission device and a receiver located in the destination device.
Therefore to configure bi-directional flows, two sets of entities,
one in each direction (downlink and uplink), must be configured and
effectively peered to each other. The peering allows the
transmission of control packets (ex., status reports) between
entities. RLC can be configured for data transfer in one of the
following modes:
* Transparent Mode (TM). RLC does not segment or concatenate SDUs
from higher layers in this mode and does not include any header to
the payload. RLC receives SDUs from upper layers when acting as a
transmitter and transmits directly to its flow RLC receiver via
lower layers. Similarly, a TM RLC receiver would only deliver
without processing the packets to higher layers upon reception.
* Unacknowledged Mode (UM). This mode provides support for
segmentation and concatenation of payload. The RLC packet's size
depends on the indication given at a particular transmission
opportunity by the lower layer (MAC) and is octets aligned. The
packet delivery to the receiver does not include reliability
support, and the loss of a segment from a packet means a complete
packet loss. Also, in the case of lower layer retransmissions,
there is no support for re-segmentation in case of change of the
radio conditions triggering the selection of a smaller transport
block. Additionally, it provides PDU duplication detection and
discards, reordering of out-of-sequence, and loss detection.
* Acknowledged Mode (AM). In addition to the same functions
supported by UM, this mode also adds a moving windows-based
reliability service on top of the lower layer services. It also
supports re-segmentation, and it requires bidirectional
communication to exchange acknowledgment reports called RLC Status
Report and trigger retransmissions. This model also supports
protocol error detection. The mode used depends on the operator
configuration for the type of data to be transmitted. For
example, data transmissions supporting mobility or requiring high
reliability would be most likely configured using AM. Meanwhile,
streaming and real-time data would be mapped to a UM
configuration.
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7.1.3. Medium Access Control (MAC)
MAC provides a mapping between the higher layers abstraction called
Logical Channels comprised by the previously described protocols to
the Physical layer channels (transport channels). Additionally, MAC
may multiplex packets from different Logical Channels and prioritize
what to fit into one Transport Block if there is data and space
available to maximize data transmission efficiency. MAC also
provides error correction and reliability support through HARQ,
transport format selection, and scheduling information reporting from
the terminal to the network. MAC also adds the necessary padding and
piggyback control elements when possible and the higher layers data.
<Max. 1600 bytes>
+---+ +---+ +------+
Application |AP1| |AP1| | AP2 |
(IP/non-IP) |PDU| |PDU| | PDU |
+---+ +---+ +------+
| | | | | |
PDCP +--------+ +--------+ +-----------+
|PDCP|AP1| |PDCP|AP1| |PDCP| AP2 |
|Head|PDU| |Head|PDU| |Head| PDU |
+--------+ +--------+ +--------+--\
| | | | | | | | |\ `----\
+---------------------------+ | |(1)| `-----\(2)'-\
RLC |RLC |PDCP|AP1|RLC |PDCP|AP1| +-------------+ +----|---+
|Head|Head|PDU|Head|Head|PDU| |RLC |PDCP|AP2| |RLC |AP2|
+-------------|-------------+ |Head|Head|PDU| |Head|PDU|
| | | | | +---------|---+ +--------+
| | | LCID1 | | / / / / /
/ / / _/ _// _/ _/ / LCID2 /
| | | | | / _/ _/ / ___/
| | | | || | | / /
+------------------------------------------+ +-----------+---+
MAC |MAC|RLC|PDCP|AP1|RLC|PDCP|AP1|RLC|PDCP|AP2| |MAC|RLC|AP2|Pad|
|Hea|Hea|Hea |PDU|Hea|Hea |PDU|Hea|Hea |PDU| |Hea|Hea|PDU|din|
|der|der|der | |der|der | |der|der | | |der|der| |g |
+------------------------------------------+ +-----------+---+
TB1 TB2
Figure 5: Example of User Plane packet encapsulation for two
transport blocks
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7.2. NB-IoT Data over NAS (DoNAS)
The Access Stratum (AS) protocol stack used by DoNAS is somehow
particular. Since the security associations are not established yet
in the radio network, to reduce the protocol overhead, PDCP (Packet
Data Convergence Protocol) is bypassed until AS security is
activated. RLC (Radio Link Control protocol) uses by default the AM
mode, but depending on the network's features and the terminal, it
may change to other modes by the network operator. For example, the
transparent mode does not add any header or process the payload to
reduce the overhead, but the MTU would be limited by the transport
block used to transmit the data, which is a couple of thousand bits
maximum. If UM (only Release 15 compatible terminals) is used, the
RLC mechanisms of reliability are disabled, and only the reliability
provided by the MAC layer by Hybrid Automatic Repeat reQuest (HARQ)
is available. In this case, the protocol overhead might be smaller
than the AM case because of the lack of status reporting but with the
same support for segmentation up to 16000 Bytes. NAS packets are
encapsulated within an RRC (Radio Resource Control) TGPP36331
message.
Depending on the data type indication signaled (IP or non-IP data),
the network allocates an IP address or establishes a direct
forwarding path. DoNAS is regulated under rate control upon previous
agreement, meaning that a maximum number of bits per unit of time is
agreed upon per device subscription beforehand and configured in the
device. The use of DoNAS is typically expected when a terminal in a
power-saving state requires a short transmission and receiving an
acknowledgment or short feedback from the network. Depending on the
size of buffered data to transmit, the UE might be instructed to
deploy the connected mode transmissions instead, limiting and
controlling the DoNAS transmissions to predefined thresholds and a
good resource optimization balance for the terminal the network. The
support for mobility of DoNAS is present but produces additional
overhead.
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+--------+ +--------+ +--------+
| | | | | | +-----------------+
| UE | | C-BS | | C-SGN | |Roaming Scenarios|
+----|---+ +--------+ +--------+ | +--------+ |
| | | | | | |
+----------------|------------|+ | | P-GW | |
| Attach | | +--------+ |
+------------------------------+ | | |
| | | | | |
+------|------------|--------+ | | | |
|RRC Connection Establishment| | | | |
|with NAS PDU transmission | | | | |
|& Ack Rsp | | | | |
+----------------------------+ | | | |
| | | | | |
| |Initial UE | | | |
| |message | | | |
| |----------->| | | |
| | | | | |
| | +---------------------+| | |
| | |Checks Integrity || | |
| | |protection, decrypts || | |
| | |data || | |
| | +---------------------+| | |
| | | Small data packet |
| | |------------------------------->
| | | Small data packet |
| | |<-------------------------------
| | +----------|---------+ | | |
| | Integrity protection,| | | |
| | encrypts data | | | |
| | +--------------------+ | | |
| | | | | |
| |Downlink NAS| | | |
| |message | | | |
| |<-----------| | | |
+-----------------------+ | | | |
|Small Data Delivery, | | | | |
|RRC connection release | | | | |
+-----------------------+ | | | |
| |
| |
+-----------------+
Figure 6: DoNAS transmission sequence from an Uplink initiated access
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+---+ +---+ +---+ +----+
Application |AP1| |AP1| |AP2| |AP2 |
(IP/non-IP) |PDU| |PDU| |PDU| ............... |PDU |
+---+ +---+ +---+ +----+
| |/ / | \ | |
NAS /RRC +--------+---|---+----+ +---------+
|NAS/|AP1|AP1|AP2|NAS/| |NAS/|AP2 |
|RRC |PDU|PDU|PDU|RRC | |RRC |PDU |
+--------+-|-+---+----+ +---------|
| |\ | | |
|<--Max. 1600 bytes-->|__ |_ |
| | \__ \___ \_ \_
| | \ \ \__ \_
+---------------|+-----|----------+ \ \
RLC |RLC | NAS/RRC ||RLC | NAS/RRC | +----|-------+
|Head| PDU(1/2)||Head | PDU (2/2)| |RLC |NAS/RRC|
+---------------++----------------+ |Head|PDU |
| | | \ | +------------+
| | LCID1 | \ | | /
| | | \ \ | |
| | | \ \ | |
| | | \ \ \ |
+----+----+----------++-----|----+---------++----+---------|---+
MAC |MAC |RLC | RLC ||MAC |RLC | RLC ||MAC | RLC |Pad|
|Head|Head| PAYLOAD ||Head |Head| PAYLOAD ||Head| PDU | |
+----+----+----------++-----+----+---------++----+---------+---+
TB1 TB2 TB3
Figure 7: Example of User Plane packet encapsulation for Data
over NAS
8. Normative References
[RFC5795] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
Header Compression (ROHC) Framework", RFC 5795,
DOI 10.17487/RFC5795, March 2010,
<https://www.rfc-editor.org/info/rfc5795>.
[RFC8376] Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN)
Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018,
<https://www.rfc-editor.org/info/rfc8376>.
[RFC8724] Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC.
Zuniga, "SCHC: Generic Framework for Static Context Header
Compression and Fragmentation", RFC 8724,
DOI 10.17487/RFC8724, April 2020,
<https://www.rfc-editor.org/info/rfc8724>.
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[TGPP23720] Meredith, J., "TR 23.720 v13.0.0 - Study on architecture
enhancements for Cellular Internet of Things", TR 23.720
v13.0.0, 2015,
<https://www.3gpp.org/ftp/Specs/archive/23_series/23.720
/23720-d00.zip>.
[TGPP33203] Soveri, M., "TR 33.203 v13.0.1 : 3G security; Access
security fro IP-based services", TR 33.203 v13.0.1, 2020
<https://www.3gpp.org/ftp/Specs/archive/33_series/33.203
/33203-d10.zip>.
[TGPP36321] Chung, Y., "TR 36.321 v13.2.0 : Evolved Universal
Terrestrial Radio Access (E-UTRA); Medium Access Control
(MAC) protocol specification", TR 36.321 V13.2.0, 2016,
<https://www.3gpp.org/ftp/Specs/archive/36_series/36.321
/36321-d20.zip>.
[TGPP36323] Chung, Y., "TS 36.323 v13.2.0 : Evolved Universal
Terrestrial Radio Access (E-UTRA); Packet Data Convergence
Protocol (PDCP) specification", TS 36.323 v13.2.0, 2016,
<https://www.3gpp.org/ftp/Specs/archive/36_series/36.323
/36323-d20.zip>.
[TGPP24301] Firmin. F., "TS 24.301 v13.2.0 : Non-Access-Stratum (NAS)
protocol for Evolved Packet System (EPS); Stage 3, TS
24.301 v13.2.0, 2015.
<https://www.3gpp.org/ftp/Specs/archive/24_series/24.301
/24301-d20.zip
Authors' Addresses
Edgar Ramos
Ericsson
Hirsalantie 11
FI- 02420 Jorvas, Kirkkonummi
Finland
Email: edgar.ramos@ericsson.com
Ana Minaburo
Acklio
1137A Avenue des Champs Blancs
35510 Cesson-Sevigne Cedex
France
Email: ana@ackl.io
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