Integrated Access and Backhaul (IAB) allows using wireless backhaul to connect small cells and avoid wiring clutter. IAB nodes terminate the gNB-DU and support UE functionality towards the IAB donor. The IAB donor terminates the gNB-CU and lower layers for the IAB node. Backhaul Adaptation Protocol (BAP) routes packets across the backhaul topology using routing IDs. IP addresses for F1 and OAM are discovered for IAB nodes. Signaling and user plane traffic is mapped to backhaul RLC channels. Open issues in Rel-16 include BAP specification and topology adaptation procedures. Rel-17 may address inter-donor migration and topological redundancy.

1. Integrated Access and Backhaul
Architecture and Agreements
Sridhar Bhaskaran
19th December, 2019
Presented for 5G-KS

2. Agenda
➔ Why Integrated Access and Backhaul?
➔ Terminologies
➔ IAB – end to end architecture
➔ Role of Backhaul Adaptation Protocol
(BAP)
➔ IAB node integration to IAB donor
➔ IP address discovery for IAB node for
F1 communication with donor CU
➔ UE to network signalling via IAB
network
➔ UE to network data plane via IAB
network

3. Why IAB?
● Gigabit broadband requires fiber to
home
● For mmWave NR small cell at home,
backhaul wired connectivity required
● Wiring clutter
● mmWave has huge bandwidth
● Can be potentially used as self
backhaul to avoid wiring clutter
● Useful for dense urban and also for
providing broadband to remote
places
Source: RP-192519

4. Background on Split gNB
3GPP Rel-15 supports split gNB
architecture
3GPP adopted option-2 split
Split is between RLC and PDCP layer
CU is centralized unit
DU is distributed unit
gNB-DU gNB-DU gNB-DU gNB-DU gNB-DU
F1 F1 F1 F1 F1
Xn
5GC
N2/N3N2/N3
Uu Uu Uu Uu Uu
* Figure does not show O-RAN option 7.2 split of the DU
CP CPUPUP
gNB-CU gNB-CU

5. Background - Stack for split gNB
PHY
MAC
RLC
IP
SCTP
F1-AP
UDP
GTPU
IP
SCTP
F1AP
PDCP-C
RRC
IP
UDP
GTPU
gNB-DU
gNB-CU-UP
gNB-CU-CP

6. Terminologies
IAB-node:
➔ An NR base station terminating the gNB-DU
functionality towards the UEs (access link) and
towards gNB-CU
➔ IAB node also support a sub-set of the UE
functionality towards its parent nodes and Core
Network (referred to as MT functionality)
IAB-donor:
➔ An NR base station which terminates gNB-CU
functionality towards IAB-node and N2/Xn
interfaces towards rest of the network.
➔ The IAB-donor is also terminating RRC/PDCP of the
MT functionality of the IAB node as well as lower
layers (donor DU) in case of single hop.

7. IAB End to End Architecture
➔ IAB node DU needs to get an IP
address for F1-C and F1-U traffic
➔ F1 traffic from IAB node DU
terminates at IAB donor CU
➔ IAB donor DU maps the F1-C /
F1-U traffic between IAB donor CU
and IAB node DU to BH RLC based
on DSCP/IPv6 flow label @ IP
header
➔ IAB donor DU sets up required BH
RLC channel for F1-C traffic based
on cpTrafficType IE in F1-C
message (different BH RLC for
different SRBs - see later slide)

8. IAB Deployment Architecture Options

9. Protocol Stack Architecture - Control Plane

10. Protocol Stack Architecture - User Plane

11. Role of BAP
➔ Functions of BAP
◆ Transfer of data
◆ Routing of packets to next hop - across
backhaul topology
● Routing uses a BAP routing ID @
BAP header
◆ Determination of BAP destination and
path for packets from upper layer
◆ Determination of egress RLC channels
for packets routed to next hop
◆ Flow control feedback signalling
Source: R2-1913254
➔ BAP entity routes based on a routingTableConfig
➔ Each entry of the table contains a BAP address, optionally a BAP path ID and
next hop ID
➔ BAP layer maps data received on ingress to an egress logical channel based
on logicalChannelMappingConfigUL and logicalChannelMappingConfigDL
configured in IAB node
➔ BAP routing ID = 20 bits = 10 bit BAP address + 10 bit BAP path ID for DL
[See RP-192518]
➔ BAP address for an IAB node configured via RRC
➔ BH routing table configuration at IAB node DU setup via F1-C for both UL
and DL (see R3-197785)

12. Backhaul Routing Configuration
➔ BH Routing information configuration provided by donor CU to each IAB node
DU via F1 signalling
➔ Configuration contains
◆ BAP Routing ID (20 bits)
◆ Next hop BAP address (10 bits)
Source: R3-197785

13. BAP Routing in DL
Source: R3-192425

14. BAP Routing in UL
Source: R3-192425

15. IAB Node Onboarding
Source: R3-196504

16. IP Address Discovery for IAB Node for F1 Communication
➔ IAB node’s DU part involved in F1 communication with IAB donor CU
➔ How does IAB node’s DU get an IP address for F1 communication?
➔ Current agreements in RAN3
◆ DU address needs to be different from IAB node’s MT address
◆ DU IP address needs to be routable on IAB node DU-->next hop IAB node BAP → IAB donor DU
→ IAB donor CU path
◆ IAB node DU IP address can be assigned by donor DU or by donor CU or by OAM
◆ The donor CU or donor DU can use OAM or DHCP to allocate IAB node IP address
◆ If DHCP is used donor DU may act as DHCP server or as DHCP proxy
◆ IAB node can request one or more IP addresses from donor CU via RRC
◆ CU can obtain IAB node IP address from donor DU via F1AP

17. IP Address Discovery for IAB Node for OAM
➔ IAB node uses its MT part to act as a UE to setup a PDU session with 5GC and
get an IP address.
➔ This IP address can be used for OAM connectivity of IAB node to OAM
servers.
➔ The IP connectivity may also be provided using Backhaul IP layer (i.e the IP
layer used for backhaul F1-C/F1-U traffic)
➔ OAM traffic from IAB nodes include
◆ Configuration to IAB node
◆ Alarms and statistics from IAB node
◆ Software download to IAB node
➔ Different types of OAM traffic can use different DRBs between IAB-MT and
serving DU and different BH RLC channels with different QoS parameters

18. BH RLC Channel Establishment Procedure
Source: R3-196504

19. Bearer Mapping
➔ 1:1 bearer mapping:
◆ a separate backhaul RLC channel is configured for each F1-U tunnel from the
access IAB node; where 1 F1-U tunnel is for UE DRB
◆ enables the scheduler to enforce bearer-specific QoS requirements such as
GBR;
◆ for F1-C, a separate backhaul RLC channel can be configured for each F1-C SCTP
stream to enforce prioritization of signalling messages.
➔ N:1 bearer mapping:
◆ multiple F1-U and/or F1-C associations are aggregated onto the same backhaul
RLC channel.

20. Bearer Mapping
➔ The Donor IAB (CU function) will setup a number of BH RLC channels between the IAB
node and the IAB nodes parent (e.g. an intermediate IAB node or the Donor DU)
➔ The BH RLC channels will not be visible in the CN
➔ The BH RLC channels will be associated with different QoS profiles and support
backhauling of end user traffic and signaling towards the IAB node.
N:1 Mapping 1:1 Mapping

21. BH RLF Handling
➔ IAB node can experience
radio link failure (RLF) with
parent IAB node
➔ Recovery from RLF is based
on parent node migration /
topology adaptation (see next
slides)
➔ Further stage 3 work on this
is pending in Rel-16
Source: 3GPP TR 38.874

22. Parent Node Migration / Topology Adaptation

23. Intra CU Topology Adaptation Procedure
Source: R3-196504

24. IAB Topology Redundancy
Source: R3-196781
Need for topology redundancy
➔ Backhaul links are more
critical since it carries traffic
of multiple UEs
➔ The links should be
redundant to offer resiliency
against RLF
➔ NR supports dual connectivity
➔ Why not use dual connectivity
to offer robustness in the
backhaul?

25. UE to Network Signaling via IAB Nodes

26. Mapping UE RRC signaling to SCTP streams
Problem Statement
➔ How to differentiate UE’s RRC signalling from UE’s initial access messages (SRB 0) when they are carried on F1 links
hop by hop via BH RLC?
➔ Should separate SCTP streams be used for SRB0 and other SRBs?

27. Mapping UE RRC signaling to SCTP streams - Solution
➔ Way 1
◆ One BH RLC CH is shared by all F1APs with DL SRB0 of all UEs
◆ Each UE uses a dedicated BH RLC CH to convey other UE-associated F1APs
➔ Way 2
◆ One BH RLC CH is shared by all F1APs with DL SRB0 of all UEs
◆ Another BH RLC CH is shared by all other F1APs of all UEs
➔ Way Forward
◆ Mapping of control plane traffic type to BH RLC channel ID is provided as part of F1 UE CONTEXT SETUP
◆ Mappnig of control plane traffic type (CP1, CP2, CP3 …) to specific SRB / RRC messages is configurable
Source: R3-197123, R3-197657

28. UE to Network User Plane Traffic via IAB Nodes

29. IAB - Status as of TSG-RAN#84
Open Issues in Rel-16
➔ Stage 3 aspects of F1 signaling for BH RLC channel setup, routing table setup
and QoS/bearer mapping
➔ Stage 3 aspects of BAP protocol
➔ Specification of procedures for IAB node integration and topology adaptation
➔ Specification of BH RLF handling
➔ Specification of flow control and congestion handling over backhaul link
➔ Security protection over wireless backhaul link

30. IAB - Release 17
Open Issues (to be pushed to Rel-17)
➔ Inter IAB donor CU/DU migration
➔ Improve topological robustness / redundancy using dual connectivity
➔ Support for partial simultaneous Tx/Rx by co-located IAB-MT / IAB-DU
➔ Specification of IAB node timing mode(s), UL/DL power control extensions,
cross link interference measurements for BH links
Source: RP-193251

31. Thank You!