lpwan Working Group E. Ramos
Internet-Draft Ericsson
Intended status: Informational A. Minaburo
Expires: April 25, 2022 Acklio
October 22, 2021
SCHC over NB-IoT
draft-ietf-lpwan-schc-over-nbiot-06
Abstract
The Static Context Header Compression (SCHC) specification describes
header compression and fragmentation functionalities for LPWAN (Low
Power Wide Area Networks) technologies.
The Narrow Band 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 elements for efficient parameterization.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Data Transmission . . . . . . . . . . . . . . . . . . . . . . 5
5. IP based Data Transmission . . . . . . . . . . . . . . . . . 6
5.1. SCHC over the Radio link . . . . . . . . . . . . . . . . 6
5.1.1. SCHC Entities Placing . . . . . . . . . . . . . . . . 6
5.2. SCHC over No-Access Stratum (NAS) . . . . . . . . . . . . 7
5.2.1. SCHC Entities Placing . . . . . . . . . . . . . . . . 8
5.3. Parameters for Static Context Header Compression (SCHC) . 9
5.3.1. SCHC Context initialization . . . . . . . . . . . . . 9
5.3.2. SCHC Rules . . . . . . . . . . . . . . . . . . . . . 9
5.3.3. Rule ID . . . . . . . . . . . . . . . . . . . . . . . 9
5.3.4. SCHC MAX_PACKET_SIZE . . . . . . . . . . . . . . . . 10
5.3.5. Fragmentation . . . . . . . . . . . . . . . . . . . . 10
6. End-to-End Compression . . . . . . . . . . . . . . . . . . . 10
6.1. SCHC Entities Placing . . . . . . . . . . . . . . . . . . 11
6.2. Parameters for Static Context Header Compression . . . . 11
6.2.1. SCHC Context initialization . . . . . . . . . . . . . 11
6.2.2. SCHC Rules . . . . . . . . . . . . . . . . . . . . . 12
6.2.3. Rule ID . . . . . . . . . . . . . . . . . . . . . . . 12
6.2.4. SCHC MAX_PACKET_SIZE . . . . . . . . . . . . . . . . 12
6.3. Fragmentation . . . . . . . . . . . . . . . . . . . . . . 12
6.3.1. Fragmentation modes . . . . . . . . . . . . . . . . . 12
6.3.2. Fragmentation Parameters . . . . . . . . . . . . . . 13
7. Padding . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
8. Security considerations . . . . . . . . . . . . . . . . . . . 13
9. 3GPP References . . . . . . . . . . . . . . . . . . . . . . . 14
10. Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . 14
10.1. NB-IoT User Plane protocol architecture . . . . . . . . 14
10.1.1. Packet Data Convergence Protocol (PDCP) . . . . . . 14
10.1.2. Radio Link Protocol (RLC) . . . . . . . . . . . . . 15
10.1.3. Medium Access Control (MAC) . . . . . . . . . . . . 16
10.2. NB-IoT Data over NAS (DoNAS) . . . . . . . . . . . . . . 17
11. Normative References . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
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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 defined in
[RFC8376].
In an NB-IoT network, header compression efficiently brings Internet
connectivity to the node. This document describes the SCHC
parameters used to performs the static context header compression
into the NB-IoT wireless access. This document assumes functionality
for NB-IoT of 3GPP release 15. Otherwise, the text explicitly
mentions other versions' functionality.
2. Terminology
This document will follow the terms defined in [RFC8724], in
[RFC8376], and the TGPP23720.
o CIoT. Cellular IoT
o NGW-C-SGN. Network Gateway - CIoT Serving Gateway Node
o Dev-UE. Device - User Equipment
o RGW-eNB. Radio Gateway - Node B. Base Station that controls the
UE
o EPC. Evolved Packet Connectivity. Core network of 3GPP LTE
systems.
o EUTRAN. Evolved Universal Terrestrial Radio Access Network.
Radio network from LTE based systems.
o NGW-MME. Network Gateway - Mobility Management Entity. Handle
mobility of the UE
o NB-IoT. Narrow Band IoT. Referring to 3GPP LPWAN technology
based in LTE architecture but with additional optimization for IoT
and using a Narrow Band spectrum frequency.
o NGW-SGW. Network Gateway - Serving Gateway. Routes and forwards
the user data packets through the access network
o HSS. Home Subscriber Server. It is a database that performs
mobility management
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o NGW-PGW. Network Gateway - Packet Data Node Gateway. An
interface between the internal with the external network
o PDU. Protocol Data Unit. Data packets including headers that are
transmitted between entities through a protocol.
o SDU. Service Data Unit. Data packets (PDUs) from higher layers
protocols used by lower layer protocols as a payload of their own
PDUs that has not yet been encapsulated.
o IWK-SCEF. InterWorking Service Capabilities Exposure Function.
Used in roaming scenarios and serves for interconnection with the
SCEF of the Home PLMN and is located in the Visited PLMN
o NGW-SCEF. Network Gateway - Service Capability Exposure Function.
EPC node for exposure of 3GPP network service capabilities to 3rd
party applications.
3. Architecture
+---+ +------+
|Dev| \ +-----+ ----| HSS |
+---+ \ | NGW | +------+
| |-MME |\__
\ / +-----+ \
+---+ \+-----+ / | +------+
|Dev| ----| RGW |- | | NGW- |
+---+ |-eNB | | | SCEF |---------+
/+-----+ \ | +------+ |
/ \ +------+ |
/ \| NGW- | +-----+ +-----------+
+---+ / | CSGW |--| NGW-|---|Application|
|Dev| | | | PGW | | Server |
+---+ +------+ +-----+ +-----------+
Figure 1: 3GPP network architecture
The Narrow Band Internet of Things (NB-IoT) architecture has a more
complex structure. It relies on different NGWs from different
providers and can send data by different paths, each path with
different characteristics such as 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-
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locating entities in different paths. For example, a Dev using the
path form 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 NBIoT 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:
o Connectivity path
o Device Monitoring
4. Data Transmission
NB-IoT networks deal with end-to-end user data and in-band signaling
between the nodes and functions to configure, control, and monitor
the system functions and behaviors. The signaling 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 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 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, 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 capability
implementation (in case of not being standardized as mandatory
capability) cannot utilize the compression services 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 and the RGW-eNB. Alternatively, the packets transmitted over
the end-to-end link can use SCHC. Else, when the transmissions over
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the NGW-MME or NGW-SCEF, the NGW-CSGN uses SCHC entity. For these
two cases, the functions are 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 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 tunneling 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.
5. IP based Data Transmission
5.1. 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 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. A Transport Block (TB)
transmissions 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 Figure 2 stack 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.
5.1.1. SCHC Entities Placing
The current architecture provides support for header compression in
PDCP with RoHC [RFC5795]. Therefore SCHC entities can be deployed
similarly without the need for significant changes in the 3GPP
specifications.
In this scenario, RLC takes care of fragmentation unless for the
transparent mode. When packets exceed the transport block size at
the time of transmission, SCHC fragmentation is unnecessary and
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should not be used to avoid the additional protocol overhead. It is
not common to configure RLC in Transparent Mode for IP-based data.
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.
+---------+ +---------+ |
|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 RGW-eNB NGW-CSGN
Figure 2: SCHC over the Radio link
5.2. SCHC over No-Access Stratum (NAS)
The NGW-MME conveys mainly control signaling between the Dev 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 uses the pre-
established security and piggyback small uplink data into the initial
uplink message and uses an additional message to receive 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 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.
5.2.1. SCHC Entities Placing
SCHC may reside in the Non-Access Stratum (NAS) protocol layer in
this scenario. The same principles as for Radio link transmissions
apply here as well. The main difference is the physical placing of
the SCHC entities on the network side as the NGW-MME resides in the
core network and is the terminating node for NAS instead of the 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 RGW-eNB NGW-MME NGW-PGW
*PDCP is bypassed until AS security is activated TGPP36300.
Figure 3
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5.3. Parameters for Static Context Header Compression (SCHC)
5.3.1. 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].
5.3.2. 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 IPv4
addresses and IPv6 may force different rules to deal with each case.
5.3.3. Rule ID
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 is 12 bits. These 12 bits must include the
Compression Residue in addition to the Rule ID. 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 Rule ID
field size may range from 2 bits, resulting in 4 rules to an 8 bits
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
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consider the byte-alignment of the expected Compression Residue. In
the minimum TB size case, 2 bits of Rule Id leave only 6 bits
available for Compression Residue.
5.3.4. 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 that correspond to 1600 bytes
for 3GPP Release 15.
5.3.5. Fragmentation
For these 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.
5.3.5.1. Fragmentation in 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 small data
with fewer possible transmissions by using fixed or limited transport
blocks compatible with the tiling SCHC fragmentation handling.
6. 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
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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 Dev without any other stack element,
directly under the L2.
6.1. SCHC Entities Placing
In the two scenarios using End-to-End compression, SCHC entities are
located almost on top of the stack. In the Dev, an application using
the NB-IoT connectivity services may implement SCHC and 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
6.2. Parameters for Static Context Header Compression
6.2.1. 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.
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6.2.2. SCHC Rules
Even when the transmission content is not visible for the 3GPP
network, the same limitations as for IP-based data transmissions
applies in these scenarios in terms of aiming to use the minimum
number of transmission and minimize the protocol overhead.
6.2.3. Rule ID
Similar to the case of IP transmissions, the Rule ID 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
IP type of traffic must be followed when choosing a size value for
the Rule ID field.
6.2.4. 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.
6.3. Fragmentation
In principle, packets larger than 1358 bytes need the 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.
6.3.1. 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 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 for the transmission of
non-IP packets on the NGW-MME. If these packets are considering to
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use SCHC with the RuleID for non-compressing packets as {RFC8724}
allows it.
6.3.2. Fragmentation Parameters
SCHC profile with the fragmentation mode will have specific Rules.
SCHC defines the Rule ID according to the fragmentation mode; 2 bits
could recognize all the fragmentation modes or another solution
depending on the Rules implementation.
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. For the other parameters, the
transfer block size has a wide range that needs two configurations.
o For Transfer Blocks smaller than 300bits: 8 bits-Header_size
configuration, with the size of the header fields as follows: Rule
ID 3 bits, DTag 1 bit, FCN 3 bits, W 1 bits.
o For Transfer Blocks bigger than 300 bits: 16 bits-Header_size
configuration, with the size of the header fields as follows:
Rules ID 8 - 10 bits, DTag 1 or 2 bits, FCN 3 bits, W 2 or 3 bits.
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 may
use these limits.
7. 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.
8. Security considerations
3GPP access security is specified in [TGPP33203].
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9. 3GPP References
o TGPP23720 3GPP, "TR 23.720 v13.0.0 - Study on architecture
enhancements for Cellular Internet of Things", 2016.
o TGPP33203 3GPP, "TS 33.203 v13.1.0 - 3G security; Access security
for IP-based services", 2016.
o TGPP36321 3GPP, "TS 36.321 v13.2.0 - Evolved Universal Terrestrial
Radio Access (E-UTRA); Medium Access Control (MAC) protocol
specification", 2016
o TGPP36323 3GPP, "TS 36.323 v13.2.0 - Evolved Universal Terrestrial
Radio Access (E-UTRA); Packet Data Convergence Protocol (PDCP)
specification", 2016.
o TGPP36331 3GPP, "TS 36.331 v13.2.0 - Evolved Universal Terrestrial
Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol
specification", 2016.
o TGPP36300 3GPP, "TS 36.300 v15.1.0 - Evolved Universal Terrestrial
Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio
Access Network (E-UTRAN); Overall description; Stage 2", 2018
o TGPP24301 3GPP "TS 24.301 v15.2.0 - Non-Access-Stratum (NAS)
protocol for Evolved Packet System (EPS); Stage 3", 2018
o TGPP24088 3GPP, "TS 24.088 v12.9.0 - Mobile radio interface Layer
3 specification;Core network protocols; Stage 3", 2015.
10. Appendix
10.1. NB-IoT User Plane protocol architecture
10.1.1. 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:
o Header compression and decompression using ROHC (Robust Header
Compression)
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o Transfer of user and control data to higher and lower layers
o Duplicate detection of lower layer SDUs when re-establishing
connection (when RLC with Acknowledge Mode in use for User Plane
only)
o Ciphering and deciphering
o Timer-based SDU discard in uplink
10.1.2. 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:
o 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.
o 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.
o 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
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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.
10.1.3. 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.
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<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
10.2. 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
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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
11. 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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Authors' Addresses
Edgar Ramos
Ericsson
Hirsalantie 11
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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