6. 5G
This section preserves the RFC text for RAW technologies, including Wi-Fi 6/7, IEEE 802.11, TSCH, 6TiSCH, 5G NR, TSN/TSC integration, UE, gNB, RAN, UPF, PDU sessions, LDACS, PHY/MAC terms, figures, tables, and security considerations.
Original RFC Text
6. 5G
5G technology enables deterministic communication. Based on the
centralized admission control and the scheduling of the wireless
resources, licensed or unlicensed, Quality of Service (QoS) such as
latency and reliability can be guaranteed. 5G contains several
features to achieve ultra-reliable and low-latency performance (e.g.,
support for different OFDM numerologies and slot durations), as well
as fast processing capabilities and redundancy techniques that lead
to achievable latency numbers of below 1 ms with 99.999% or higher
confidence.
5G also includes features to support industrial IoT use cases, e.g.,
via the integration of 5G with TSN. This includes 5G capabilities
for each TSN component, latency, resource management, time
synchronization, and reliability. Furthermore, 5G support for TSN
can be leveraged when 5G is used as the subnet technology for DetNet,
in combination with or instead of TSN, which is the primary subnet
for DetNet. In addition, the support for integration with TSN
reliability was added to 5G by making DetNet reliability also
applicable, due to the commonalities between TSN and DetNet
reliability. Moreover, providing IP service is native to 5G, and
3GPP Release 18 adds direct support for DetNet to 5G.
Overall, 5G provides scheduled wireless segments with high
reliability and availability. In addition, 5G includes capabilities
for integration to IP networks. This makes 5G a suitable technology
upon which to apply RAW.
6.1. Provenance and Documents
The 3rd Generation Partnership Project (3GPP) incorporates many
companies whose business is related to cellular network operation as
well as network equipment and device manufacturing. All generations
of 3GPP technologies provide scheduled wireless segments, primarily
in licensed spectrum, which is beneficial for reliability and
availability.
In 2016, the 3GPP started to design New Radio (NR) technology
belonging to the fifth generation (5G) of cellular networks. NR has
been designed from the beginning to not only address enhanced Mobile
Broadband (eMBB) services for consumer devices such as smart phones
or tablets, but it is also tailored for future IoT communication and
connected cyber-physical systems. In addition to eMBB, requirement
categories have been defined on Massive Machine-Type Communication
(M-MTC) for a large number of connected devices/sensors and on Ultra-
Reliable Low-Latency Communications (URLLC) for connected control
systems and critical communication as illustrated in Figure 5. It is
the URLLC capabilities that make 5G a great candidate for reliable
low-latency communication. With these three cornerstones, NR is a
complete solution supporting the connectivity needs of consumers,
enterprises, and the public sector for both wide-area and local-area
(e.g., indoor) deployments. A general overview of NR can be found in
[TS38300].
enhanced
Mobile Broadband
^
/ \
/ \
/ \
/ \
/ 5G \
/ \
/ \
/ \
+-----------------+
Massive Ultra-Reliable
Machine-Type Low-Latency
Communication Communication
Figure 5: 5G Application Areas
As a result of releasing the first NR specification in 2018 (Release
15), it has been proven by many companies that NR is a URLLC-capable
technology and can deliver data packets at 10^-5 packet error rate
within a 1 ms latency budget [TR37910]. Those evaluations were
consolidated and forwarded to ITU to be included in the work on
[IMT2020].
In order to understand communication requirements for automation in
vertical domains, 3GPP studied different use cases [TR22804] and
released a technical specification with reliability, availability,
and latency demands for a variety of applications [TS22104].
As an evolution of NR, multiple studies that focus on radio aspects
have been conducted in scope of 3GPP Release 16, including the
following two:
1. "Study on physical layer enhancements for NR ultra-reliable and
low latency case (URLLC)" [TR38824]
2. "Study on NR industrial Internet of Things (IoT)" [TR38825]
As a result of these studies, further enhancements to NR have been
standardized in 3GPP Release 16 and are available in [TS38300] and
continued in 3GPP Release 17 standardization (according to
[RP210854]).
In addition, several enhancements have been made on the system
architecture level, which are reflected in "System architecture for
the 5G System (5GS)" [TS23501]. These enhancements include multiple
features in support of Time-Sensitive Communications (TSC) by Release
16 and Release 17. Further improvements, such as support for DetNet
[TR2370046], are provided in Release 18.
The adoption and the use of 5G is facilitated by multiple
organizations. For instance, the 5G Alliance for Connected
Industries and Automation (5G-ACIA) brings together widely varying 5G
stakeholders including Information and Communication Technology (ICT)
players and Operational Technology (OT) companies (e.g., industrial
automation enterprises, machine builders, and end users). Another
example is the 5G Automotive Association (5GAA), which bridges ICT
and automotive technology companies to develop end-to-end solutions
for future mobility and transportation services.
6.2. General Characteristics
The 5G Radio Access Network (5G RAN) with its NR interface includes
several features to achieve Quality of Service (QoS), such as a
guaranteed low latency or tolerable packet error rates for selected
data flows. Determinism is achieved by centralized admission control
and scheduling of the wireless frequency resources, which are
typically licensed frequency bands assigned to a network operator.
NR enables short transmission slots in a radio subframe, which
benefits low-latency applications. NR also introduces mini-slots,
where prioritized transmissions can be started without waiting for
slot boundaries, further reducing latency. As part of giving
priority and faster radio access to URLLC traffic, NR introduces
preemption, where URLLC data transmission can preempt ongoing non-
URLLC transmissions. Additionally, NR applies very fast processing,
enabling retransmissions even within short latency bounds.
NR defines extra-robust transmission modes for increased reliability
for both data and control radio channels. Reliability is further
improved by various techniques, such as multi-antenna transmission,
the use of multiple frequency carriers in parallel, and packet
duplication over independent radio links. NR also provides full
mobility support, which is an important reliability aspect not only
for devices that are moving, but also for devices located in a
changing environment.
Network slicing is seen as one of the key features for 5G, allowing
vertical industries to take advantage of 5G networks and services.
Network slicing is about transforming a Public Land Mobile Network
(PLMN) from a single network to a network where logical partitions
are created, with appropriate network isolation, resources, optimized
topology, and specific configurations to serve various service
requirements. An operator can configure and manage the mobile
network to support various types of services enabled by 5G (e.g.,
eMBB and URLLC), depending on the different needs of customers.
Exposure of capabilities of 5G systems to the network or applications
outside the 3GPP domain have been added to Release 16 [TS23501].
Applications can access 5G capabilities like communication service
monitoring and network maintenance via exposure interfaces.
For several generations of mobile networks, 3GPP has considered how
the communication system should work on a global scale with billions
of users, taking into account resilience aspects, privacy regulation,
protection of data, encryption, access and core network security, as
well as interconnect. Security requirements evolve as demands on
trustworthiness increase. For example, this has led to the
introduction of enhanced privacy protection features in 5G. 5G also
employs strong security algorithms, encryption of traffic, protection
of signaling, and protection of interfaces.
One particular strength of mobile networks is the authentication,
based on well-proven algorithms and tightly coupled with a global
identity management infrastructure. Since 3G, there is also mutual
authentication, allowing the network to authenticate the device and
the device to authenticate the network. Another strength is secure
solutions for storage and distribution of keys, fulfilling regulatory
requirements and allowing international roaming. When connecting to
5G, the user meets the entire communication system, where security is
the result of standardization, product security, deployment,
operations, and management as well as incident-handling capabilities.
The mobile networks approach the entirety in a rather coordinated
fashion, which is beneficial for security.
6.3. Deployment and Spectrum
The 5G system allows deployment in a vast spectrum range, addressing
use cases in both wide-area and local-area networks. Furthermore, 5G
can be configured for public and non-public access.
When it comes to spectrum, NR allows combining the merits of many
frequency bands, such as the high bandwidths in millimeter waves
(mmWaves) for extreme capacity locally and the broad coverage when
using mid- and low-frequency bands to address wide-area scenarios.
URLLC is achievable in all these bands. Spectrum can be either
licensed, which means that the license holder is the only authorized
user of that spectrum range, or unlicensed, which means that anyone
who wants to use the spectrum can do so.
A prerequisite for critical communication is performance
predictability, which can be achieved by full control of access to
the spectrum, which 5G provides. Licensed spectrum guarantees
control over spectrum usage by the system, making it a preferable
option for critical communication. However, unlicensed spectrum can
provide an additional resource for scaling non-critical
communications. While NR was initially developed for usage of
licensed spectrum, the functionality to also access unlicensed
spectrum was introduced in 3GPP Release 16. Moreover, URLLC features
are enhanced in Release 17 [RP210854] to be better applicable to
unlicensed spectrum.
Licensed spectrum dedicated to mobile communications has been
allocated to mobile service providers, i.e., issued as longer-term
licenses by national administrations around the world. These
licenses have often been associated with coverage requirements and
issued across whole countries or large regions. Besides this,
configured as a non-public network (NPN) deployment, 5G can also
provide network services to a non-operator defined organization and
its premises such as a factory deployment. With this isolation, QoS
requirements as well as security requirements can be achieved. An
integration with a public network, if required, is also possible.
The non-public (local) network can thus be interconnected with a
public network, allowing devices to roam between the networks.
In an alternative model, some countries are now in the process of
allocating parts of the 5G spectrum for local use to industries.
These non-service providers then have the choice to apply for a local
license themselves and operate their own network or to cooperate with
a public network operator or service provider.
6.4. Applicability to Deterministic Flows
6.4.1. System Architecture
The 5G system [TS23501] consists of the User Equipment (UE) at the
terminal side, the Radio Access Network (RAN) with the gNodeB (gNB)
as radio base station node, and the Core Network (CN), which is
connected to the external Data Network (DN). The CN is based on a
service-based architecture with the following central functions:
Access and Mobility Management Function (AMF), Session Management
Function (SMF), and User Plane Function (UPF) as illustrated in
Figure 6. (Note that this document only explains key functions;
however, Figure 6 provides a more detailed view, and [SYSTOVER5G]
summarizes the functions and provides the full definitions of the
acronyms used in the figure.)
The gNB's main responsibility is radio resource management, including
admission control and scheduling, mobility control, and radio
measurement handling. The AMF handles the UE's connection status and
security, while the SMF controls the UE's data sessions. The UPF
handles the user plane traffic.
The SMF can instantiate various Packet Data Unit (PDU) sessions for
the UE, each associated with a set of QoS flows, i.e., with different
QoS profiles). Segregation of those sessions is also possible; for
example, resource isolation in the RAN and CN can be defined
(slicing).
+----+ +---+ +---+ +---+ +---+ +---+
|NSSF| |NEF| |NRF| |PCF| |UDM| |AF |
+--+-+ +-+-+ +-+-+ +-+-+ +-+-+ +-+-+
| | | | | |
Nnssf| Nnef| Nnrf| Npcf| Nudm| Naf|
| | | | | |
---+------+-+-----+-+------------+--+-----+-+---
| | | |
Nausf| Nausf| Nsmf| |
| | | |
+--+-+ +-+-+ +-+-+ +-+-+
|AUSF| |AMF| |SMF| |SCP|
+----+ +++-+ +-+-+ +---+
/ | |
/ | |
/ | |
N1 N2 N4
/ | |
/ | |
/ | |
+--+-+ +--+--+ +--+---+ +----+
| UE +---+(R)AN+--N3--+ UPF +--N6--+ DN |
+----+ +-----+ ++----++ +----+
| |
+-N9-+
Figure 6: 5G System Architecture
To allow UE mobility across cells/gNBs, handover mechanisms are
supported in NR. For an established connection (i.e., connected mode
mobility), a gNB can configure a UE to report measurements of
received signal strength and quality of its own and neighboring
cells, periodically or based on events. Based on these measurement
reports, the gNB decides to hand over a UE to another target cell/
gNB. Before triggering the handover, it is handshaked with the
target gNB based on network signaling. A handover command is then
sent to the UE, and the UE switches its connection to the target
cell/gNB. The Packet Data Convergence Protocol (PDCP) of the UE can
be configured to avoid data loss in this procedure, i.e., to handle
retransmissions if needed. Data forwarding is possible between
source and target gNB as well. To improve the mobility performance
further (i.e., to avoid connection failures due to too-late
handovers), the mechanism of conditional handover is introduced in
Release 16 specifications. Therein, a conditional handover command,
defining a triggering point, can be sent to the UE before the UE
enters a handover situation. A further improvement that has been
introduced in Release 16 is the Dual Active Protocol Stack (DAPS),
where the UE maintains the connection to the source cell while
connecting to the target cell. This way, potential interruptions in
packet delivery can be avoided entirely.
6.4.2. Overview of the Radio Protocol Stack
The protocol architecture for NR consists of the Layer 1 Physical
(PHY) layer and, as part of Layer 2, the sublayers of Medium Access
Control (MAC), Radio Link Control (RLC), Packet Data Convergence
Protocol (PDCP), and Service Data Adaption Protocol (SDAP).
The PHY layer handles actions related to signal processing, such as
encoding/decoding of data and control bits, modulation, antenna
precoding, and mapping.
The MAC sublayer handles multiplexing and priority handling of
logical channels (associated with QoS flows) to transport blocks for
PHY transmission, as well as scheduling information reporting and
error correction through Hybrid Automated Repeat Request (HARQ).
The RLC sublayer handles sequence numbering of higher-layer packets,
retransmissions through Automated Repeat Request (ARQ), if
configured, as well as segmentation and reassembly and duplicate
detection.
The PDCP sublayer consists of functionalities for ciphering/
deciphering, integrity protection/verification, reordering and in-
order delivery, and duplication and duplicate handling for higher-
layer packets. This sublayer also acts as the anchor protocol to
support handovers.
The SDAP sublayer provides services to map QoS flows, as established
by the 5G core network, to data radio bearers (associated with
logical channels), as used in the 5G RAN.
Additionally, in RAN, the Radio Resource Control (RRC) protocol
handles the access control and configuration signaling for the
aforementioned protocol layers. RRC messages are considered Layer 3
and are thus also transmitted via those radio protocol layers.
To provide low latency and high reliability for one transmission link
(i.e., to transport data or control signaling of one radio bearer via
one carrier), several features have been introduced on the user plane
protocols for PHY and Layer 2, as explained below.
6.4.3. Radio (PHY)
NR is designed with native support of antenna arrays utilizing
benefits from beamforming, transmissions over multiple MIMO layers,
and advanced receiver algorithms allowing effective interference
cancellation. Those antenna techniques are the basis for high signal
quality and the effectiveness of spectral usage. Spatial diversity
with up to four MIMO layers in UL and up to eight MIMO layers in DL
is supported. Together with spatial-domain multiplexing, antenna
arrays can focus power in the desired direction to form beams. NR
supports beam management mechanisms to find the best suitable beam
for UE initially and when it is moving. In addition, gNBs can
coordinate their respective downlink (DL) and uplink (UL)
transmissions over the backhaul network, keeping interference
reasonably low, and even make transmissions or receptions from
multiple points (multi-TRP). Multi-TRP can be used for repetition of
a data packet in time, in frequency, or over multiple MIMO layers,
which can improve reliability even further.
Any DL transmission to a UE starts from resource allocation signaling
over the Physical Downlink Control Channel (PDCCH). If it is
successfully received, the UE will know about the scheduled
transmission and may receive data over the Physical Downlink Shared
Channel (PDSCH). If retransmission is required according to the HARQ
scheme, a signaling of negative acknowledgement (NACK) on the
Physical Uplink Control Channel (PUCCH) is involved, and PDCCH
together with PDSCH transmissions (possibly with additional
redundancy bits) are transmitted and soft-combined with previously
received bits. Otherwise, if no valid control signaling for
scheduling data is received, nothing is transmitted on PUCCH
(discontinuous transmission (DTX)), and upon detecting DTX, the base
station will retransmit the initial data.
An UL transmission normally starts from a Scheduling Request (SR), a
signaling message from the UE to the base station sent via PUCCH.
Once the scheduler is informed about buffer data in the UE (e.g., by
SR), the UE transmits a data packet on the Physical Uplink Shared
Channel (PUSCH). Pre-scheduling, not relying on SR, is also possible
(see Section 6.4.4).
Since transmission of data packets requires usage of control and data
channels, there are several methods to maintain the needed
reliability. NR uses Low Density Parity Check (LDPC) codes for data
channels, polar codes for PDCCH, as well as orthogonal sequences and
polar codes for PUCCH. For ultra-reliability of data channels, very
robust (low-spectral efficiency) Modulation and Coding Scheme (MCS)
tables are introduced containing very low (down to 1/20) LDPC code
rates using Binary Phase-Shift Keying (BPSK) or Quadrature Phase-
Shift Keying (QPSK). Also, PDCCH and PUCCH channels support multiple
code rates including very low ones for the channel robustness.
A connected UE reports DL quality to gNB by sending Channel State
Information (CSI) reports via PUCCH while UL quality is measured
directly at gNB. For both UL and DL, gNB selects the desired MCS
number and signals it to the UE by Downlink Control Information (DCI)
via PDCCH channel. For URLLC services, the UE can assist the gNB by
advising that MCS targeting a 10^-5 Block Error Rate (BLER) are used.
Robust link adaptation algorithms can maintain the needed level of
reliability, considering a given latency bound.
Low latency on the PHY layer is provided by short transmission
duration, which is possible by using high Subcarrier Spacing (SCS)
and the allocation of only one or a few Orthogonal Frequency Division
Multiplexing (OFDM) symbols. For example, the shortest latency for
the worst case is 0.23 ms in DL and 0.24 ms in UL (according to
Section 5.7.1 in [TR37910]). Moreover, if the initial transmission
has failed, HARQ feedback can quickly be provided and an HARQ
retransmission scheduled.
Dynamic multiplexing of data associated with different services is
highly desirable for efficient use of system resources and to
maximize system capacity. Assignment of resources for eMBB is
usually done with regular (longer) transmission slots, which can lead
to blocking of low-latency services. To overcome the blocking, eMBB
resources can be preempted and reassigned to URLLC services. In this
way, spectrally efficient assignments for eMBB can be ensured while
providing the flexibility required to ensure a bounded latency for
URLLC services. In DL, the gNB can notify the eMBB UE about
preemption after it has happened, while in UL there are two
preemption mechanisms: special signaling to cancel eMBB transmission
and URLLC dynamic power boost to suppress eMBB transmission.
6.4.4. Scheduling and QoS (MAC)
One integral part of the 5G system is the Quality of Service (QoS)
framework [TS23501]. QoS flows are set up by the 5G system for
certain IP or Ethernet packet flows, so that packets of each flow
receive the same forwarding treatment (i.e., in scheduling and
admission control). For example, QoS flows can be associated with
different priority levels, packet delay budgets, and tolerable packet
error rates. Since radio resources are centrally scheduled in NR,
the admission control function can ensure that only QoS flows for
which QoS targets can be reached are admitted.
NR transmissions in both UL and DL are scheduled by the gNB
[TS38300]. This ensures radio resource efficiency and fairness in
resource usage of the users, and it enables differentiated treatment
of the data flows of the users according to the QoS targets of the
flows. Those QoS flows are handled as data radio bearers or logical
channels in NR RAN scheduling.
The gNB can dynamically assign DL and UL radio resources to users,
indicating the resources as DL assignments or UL grants via control
channel to the UE. Radio resources are defined as blocks of OFDM
symbols in spectral domain and time domain. Different lengths are
supported in time domain, (i.e., multiple slot or mini-slot lengths).
Resources of multiple frequency carriers can be aggregated and
jointly scheduled to the UE.
Scheduling decisions are based, e.g., on channel quality measured on
reference signals and reported by the UE (cf. periodical CSI reports
for DL channel quality). The transmission reliability can be chosen
in the scheduling algorithm, i.e., chosen by link adaptation where an
appropriate transmission format (e.g., robustness of modulation and
coding scheme, controlled UL power) is selected for the radio channel
condition of the UE. Retransmissions, based on HARQ feedback, are
also controlled by the scheduler. The feedback transmission in HARQ
loop introduces delays, but there are methods to minimize it by using
short transmission formats, sub-slot feedback reporting, and PUCCH
carrier switching. If needed to avoid HARQ round-trip time delays,
repeated transmissions can be also scheduled beforehand, to the cost
of reduced spectral efficiency.
In dynamic DL scheduling, transmission can be initiated immediately
when DL data becomes available in the gNB. However, for dynamic UL
scheduling, when data becomes available but no UL resources are
available yet, the UE indicates the need for UL resources to the gNB
via a (single bit) scheduling request message in the UL control
channel. When UL resources are scheduled, the UE can transmit its
data and may include a buffer status report that indicates the exact
amount of data per logical channel still left to be sent. More UL
resources may be scheduled accordingly. To avoid the latency
introduced in the scheduling request loop, UL radio resources can
also be pre-scheduled.
In particular, for periodical traffic patterns, the pre-scheduling
can rely on the scheduling features DL Semi-Persistent Scheduling
(SPS) and UL Configured Grant (CG). With these features,
periodically recurring resources can be assigned in DL and UL.
Multiple parallels of those configurations are supported in order to
serve multiple parallel traffic flows of the same UE.
To support QoS enforcement in the case of mixed traffic with
different QoS requirements, several features have recently been
introduced. These features allow different periodical critical QoS
flows to be served, together with best-effort transmissions, by the
same UE. These features include the following:
* UL logical channel transmission restrictions, allowing logical
channels of certain QoS to only be mapped to intended UL resources
of a certain frequency carrier, slot length, or CG configuration.
* intra-UE preemption and multiplexing, allowing critical UL
transmissions to either preempt non-critical transmissions or be
multiplexed with non-critical transmissions keeping different
reliability targets.
When multiple frequency carriers are aggregated, duplicate parallel
transmissions can be employed (beside repeated transmissions on one
carrier). This is possible in the Carrier Aggregation (CA)
architecture where those carriers originate from the same gNB or in
the Dual Connectivity (DC) architecture where the carriers originate
from different gNBs (i.e., the UE is connected to two gNBs in this
case). In both cases, transmission reliability is improved by this
means of providing frequency diversity.
In addition to licensed spectrum, a 5G system can also utilize
unlicensed spectrum to offload non-critical traffic. This version of
NR, called NR-U, is part of 3GPP Release 16. The central scheduling
approach also applies for unlicensed radio resources and the
mandatory channel access mechanisms for unlicensed spectrum (e.g.,
Listen Before Talk (LBT) is supported in NR-U). This way, by using
NR, operators have and can control access to both licensed and
unlicensed frequency resources.
6.4.5. Time-Sensitive Communications (TSC)
Recent 3GPP releases have introduced various features to support
multiple aspects of Time-Sensitive Communication (TSC), which
includes Time-Sensitive Networking (TSN) and beyond, as described in
this section.
The main objective of TSN is to provide guaranteed data delivery
within a guaranteed time window (i.e., bounded low latency). IEEE
802.1 TSN [IEEE802.1TSN] is a set of open standards that provide
features to enable deterministic communication on standard IEEE 802.3
Ethernet [IEEE802.3]. TSN standards can be seen as a toolbox for
traffic shaping, resource management, time synchronization, and
reliability.
A TSN stream is a data flow between one end station (talker) to
another end station (listener). In the centralized configuration
model, TSN bridges are configured by the Central Network Controller
(CNC) [IEEE802.1Qcc] to provide deterministic connectivity for the
TSN stream through the network. Time-based traffic shaping provided
by scheduled traffic [IEEE802.1Qbv] may be used to achieve bounded
low latency. The TSN tool for time synchronization is the
generalized Precision Time Protocol (gPTP) [IEEE802.1AS], which
provides reliable time synchronization that can be used by end
stations and by other TSN tools (e.g., scheduled traffic
[IEEE802.1Qbv]). High availability, as a result of ultra-
reliability, is provided for data flows by the Frame Replication and
Elimination for Reliability (FRER) mechanism [IEEE802.1CB].
3GPP Release 16 includes integration of 5G with TSN, i.e., specifies
functions for the 5G System (5GS) to deliver TSN streams so that
their QoS requirements are met. A key aspect of the integration is
that, from the rest of the network, the 5GS appears as a set of TSN
bridges (in particular, one virtual bridge per User Plane Function
(UPF) on the user plane). The 5GS includes TSN Translator (TT)
functionality for the adaptation of the 5GS to the TSN bridged
network and for hiding the 5GS internal procedures. The 5GS provides
the following components:
1. interface to TSN controller, as per [IEEE802.1Qcc] for the fully
centralized configuration model
2. time synchronization via reception and transmission of gPTP PDUs
[IEEE802.1AS]
3. low latency, which allows integration with scheduled traffic
[IEEE802.1Qbv]
4. reliability, which allows integration with FRER [IEEE802.1CB]
3GPP Release 17 [TS23501] introduced enhancements to generalize
support for TSC beyond TSN. This includes IP communications to
provide time-sensitive services (e.g., to Video, Imaging, and Audio
for Professional Applications (VIAPA)). The system model of 5G
acting as a "TSN bridge" in Release 16 has been reused to enable the
5GS acting as a "TSC node" in a more generic sense (which includes
TSN bridge and IP node). In the case of TSC that does not involve
TSN, requirements are given via exposure interfaces, and the control
plane provides the service based on QoS and time synchronization
requests from an Application Function (AF).
Figure 7 shows an illustration of 5G-TSN integration where an
industrial controller (Ind Ctrlr) is connected to industrial Input/
Output devices (I/O dev) via 5G. The 5GS can directly transport
Ethernet frames since Release 15; thus, end-to-end Ethernet
connectivity is provided. The 5GS implements the required interfaces
towards the TSN controller functions such as the CNC, thus adapting
to the settings of the TSN network. A 5G user plane virtual bridge
interconnects TSN bridges or connects end stations (e.g., I/O devices
to the TSN network). TTs, i.e., the Device-Side TSN Translator (DS-
TT) at the UE and the Network-Side TSN Translator (NW-TT) at the UPF,
have a key role in the interconnection. Note that the introduction
of 5G brings flexibility in various aspects, e.g., a more flexible
network topology because a wireless hop can replace several wireline
hops, thus significantly reducing the number of hops end to end.
[TSN5G] dives more into the integration of 5G with TSN.
+------------------------------+
| 5G System |
| +---+|
| +-+ +-+ +-+ +-+ +-+ |TSN||
| | | | | | | | | | | |AF |......+
| +++ +++ +++ +++ +++ +-+-+| .
| | | | | | | | .
| -+---+---++--+-+-+--+-+- | .
| | | | | | +--+--+
| +++ +++ +++ +++ | | TSN |
| | | | | | | | | | |Ctrlr+.......+
| +++ +++ +++ +++ | +--+--+ .
| | . .
| | . .
| +..........................+ | . .
| . Virtual Bridge . | . .
+---+ | . +--+--+ +---+ +---+--+ . | +--+---+ .
|I/O+----------------+DS|UE+---+RAN+-+UPF|NW+------+ TSN +----+ .
|dev| | . |TT| | | | | |TT| . | |bridge| | .
+---+ | . +--+--+ +---+ +---+--+ . | +------+ | .
| +..........................+ | . +-+-+-+
| | . | Ind |
| +..........................+ | . |Ctrlr|
| . Virtual Bridge . | . +-+---+
+---+ +------+ | . +--+--+ +---+ +---+--+ . | +--+---+ |
|I/O+--+ TSN +------+DS|UE+---+RAN+-+UPF|NW+------+ TSN +----+
|dev| |bridge| | . |TT| | | | | |TT| . | |bridge|
+---+ +------+ | . +--+--+ +---+ +---+--+ . | +------+
| +..........................+ |
+------------------------------+
<----------------- end-to-end Ethernet ------------------->
Figure 7: 5G - TSN Integration
NR supports accurate reference time synchronization in 1 µs accuracy
level. Since NR is a scheduled system, an NR UE and a gNB are
tightly synchronized to their OFDM symbol structures. A 5G internal
reference time can be provided to the UE via broadcast or unicast
signaling, associating a known OFDM symbol to this reference clock.
The 5G internal reference time can be shared within the 5G network
(i.e., radio and core network components). Release 16 has introduced
interworking with gPTP for multiple time domains, where the 5GS acts
as a virtual gPTP time-aware system and supports the forwarding of
gPTP time synchronization information between end stations and
bridges through the 5G user plane TTs. These account for the
residence time of the 5GS in the time synchronization procedure. One
special option is when the 5GS internal reference time is not only
used within the 5GS, but also to the rest of the devices in the
deployment, including connected TSN bridges and end stations.
Release 17 includes further improvements (i.e., methods for
propagation delay compensation in RAN), further improving the
accuracy for time synchronization over the air, as well as the
possibility for the TSN grandmaster clock to reside on the UE side.
More extensions and flexibility were added to the time
synchronization service, making it general for TSC and providing
additional support for other types of clocks and time distribution
such as boundary clocks and transparent clocks (both peer-to-peer and
end-to-end) aside from the time-aware system used for TSN.
Additionally, it is possible to use internal access stratum signaling
to distribute timing (and not the usual (g)PTP messages), for which
the required accuracy can be provided by the AF [TS23501]. The same
time synchronization service is expected to be further extended and
enhanced in Release 18 to support Timing Resiliency (according to
study item [SP211634]), where the 5G system can provide a backup or
alternative timing source for the failure of the local GNSS source
(or other primary timing source) used by the vertical.
IETF DetNet is the technology to support time-sensitive
communications at the IP layer. 3GPP Release 18 includes a study
[TR2370046] on interworking between 5G and DetNet. Along the TSC
framework introduced for Release 17, the 5GS acts as a DetNet node
for the support of DetNet; see Figure 7.1-1 in [TR2370046]. The
study provides details on how the 5GS is exposed by the Time
Sensitive Communication and Time Synchronization Function (TSCTSF) to
the DetNet controller as a router on a per-UPF granularity (similarly
to the per-UPF Virtual TSN Bridge granularity shown in Figure 7). In
particular, it lists the parameters that are provided by the TSCTSF
to the DetNet controller. The study also includes how the TSCTSF
maps DetNet flow parameters to 5G QoS parameters. Note that TSN is
the primary subnetwork technology for DetNet. Thus, the work on
DetNet over TSN, e.g., [RFC9023], can be leveraged via the TSN
support built in 5G.
Redundancy architectures were specified in order to provide
reliability against any kind of failure on the radio link or nodes in
the RAN and the core network. Redundant user plane paths can be
provided based on the dual connectivity architecture, where the UE
sets up two PDU sessions towards the same data network, and the 5G
system makes the paths of the two PDU sessions independent as
illustrated in Figure 8. There are two PDU sessions involved in the
solution: The first spans from the UE via gNB1 to UPF1, acting as the
first PDU session anchor, while the second spans from the UE via gNB2
to UPF2, acting as second the PDU session anchor.
The independent paths may continue beyond the 3GPP network.
Redundancy Handling Functions (RHFs) are deployed outside of the 5GS,
i.e., in Host A (the device) and in Host B (the network). RHF can
implement replication and elimination functions as per [IEEE802.1CB]
or the Packet Replication, Elimination, and Ordering Functions
(PREOF) of IETF DetNet [RFC8655].
+........+
. Device . +------+ +------+ +------+
. . + gNB1 +--N3--+ UPF1 |--N6--+ |
. ./+------+ +------+ | |
. +----+ / | |
. | |/. | |
. | UE + . | DN |
. | |\. | |
. +----+ \ | |
. .\+------+ +------+ | |
+........+ + gNB2 +--N3--+ UPF2 |--N6--+ |
+------+ +------+ +------+
Figure 8: Reliability with Single UE
An alternative solution is that multiple UEs per device are used for
user plane redundancy as illustrated in Figure 9. Each UE sets up a
PDU session. The 5GS ensures that the PDU sessions of the different
UEs are handled independently internal to the 5GS. There is no
single point of failure in this solution, which also includes RHF
outside of the 5G system, e.g., as per FRER [IEEE802.1CB] or PREOF
[RFC8655] specifications.
+.........+
. Device .
. .
. +----+ . +------+ +------+ +------+
. | UE +-----+ gNB1 +--N3--+ UPF1 |--N6--+ |
. +----+ . +------+ +------+ | |
. . | DN |
. +----+ . +------+ +------+ | |
. | UE +-----+ gNB2 +--N3--+ UPF2 |--N6--+ |
. +----+ . +------+ +------+ +------+
. .
+.........+
Figure 9: Reliability with Dual UE
Note that the abstraction provided by the RHF and the location of the
RHF outside of the 5G system allow 5G to support integration for
reliability with both TSN FRER [IEEE802.1CB] and DetNet PREOF
[RFC8655], as they both rely on the same concept.