4 General

38.3403GPPBackhaul Adaptation Protocol (BAP) specificationNRRelease 17TS

4.1 Introduction

The present document describes the functionalities of BAP.

4.2 Architecture

4.2.1 BAP structure

Figure 4.2.1-1 represents one possible structure for the BAP sublayer; it should not restrict implementation. The figure is based on the radio interface protocol architecture defined in TS 38.300 [2].

Figure 4.2.1-1: BAP layer, structure view

The BAP sublayer is configured by upper layers TS 38.331 [3] and TS 38.473 [5].

4.2.2 BAP entities

On the IAB-node, the BAP sublayer contains one BAP entity at the MT function and a separate collocated BAP entity at the DU function. On the IAB-donor-DU, the BAP sublayer contains only one BAP entity. Each BAP entity has a transmitting part and a receiving part.

NOTE: The modelling of BAP entities does not restrict internal implementation of IAB-nodes, i.e. the exact modelling of BAP sublayer may vary for different IAB-node implementations.

The transmitting part of the BAP entity has a corresponding receiving part of a BAP entity at the IAB-node or IAB-donor-DU across the BH link.

Figure 4.2.2-1 shows one example of the functional view of the BAP sublayer. This functional view should not restrict implementation. The figure is based on the radio interface protocol architecture defined in TS 38.300 [2].

In the example of Figure 4.2.2-1, the receiving part on the BAP entity delivers BAP PDUs to the transmitting part on the collocated BAP entity. Alternatively, the receiving part may deliver BAP SDUs to the collocated transmitting part. When passing BAP SDUs, the receiving part removes the BAP header and the transmitting part adds the BAP header with the same BAP header content as carried on the BAP PDU header prior to removal. Passing BAP SDUs in this manner is therefore functionally equivalent to passing BAP PDUs, in implementation. The following specification therefore refers to the passing of BAP Data Packets.

Besides, BAP entity generates, delivers/receives BAP Control PDU(s) as described in clause 6.1.2. BAP Control PDU can only be exchanged between peer BAP entities across the BH link.

In addition to the functions shown in Figure 4.2.2-1, for a boundary IAB-node, the transmitting part of the BAP entity may perform BAP header rewriting operation, in accordance with clause 5.2.1.

Figure 4.2.2-1. Example of functional view of BAP sublayer

4.3 Services

4.3.1 Services provided to upper layers

The following services are provided by the BAP sublayer to upper layers:

– data transfer.

4.3.2 Services expected from lower layers

A BAP sublayer expects the following services from lower layers per RLC entity (for a detailed description see TS 38.322 [4]):

– acknowledged data transfer service;

– unacknowledged data transfer service.

4.4 Functions

The BAP sublayer supports the following functions:

– Data transfer;

– Determination of BAP destination and path for packets from upper layers;

– Determination of egress BH RLC channels for packets routed to next hop;

– Routing of packets to next hop;

– BAP header rewriting;

– Differentiating traffic to be delivered to upper layers from traffic to be delivered to egress link;

– Flow control feedback and polling signalling;

– Handling of BH RLF related indications;

4.5 Configurations

The configuration of the BAP entity includes:

– The IAB-node’s BAP address via RRC.

– The IAB-donor-DU’s BAP address via F1AP.

– Mapping from next hop BAP address to downstream egress link via F1AP.

– Mapping from next hop BAP address to upstream egress link via RRC.

– Mapping from upper layer traffic to BAP routing ID in BAP header via F1AP and RRC.

– The BAP routing entries via F1AP.

– Mapping to egress BH RLC channels via F1AP and RRC.

– Flow control feedback type(s) to be provided, if any, via RRC.

BH RLC channels are configured via RRC on the IAB-MT, and via F1AP on the IAB-DU/IAB-donor-DU.

For F1AP configurations, the following mapping, which are derived from the original F1AP signaling, are used in procedure:

– Uplink Traffic to Routing ID Mapping Configuration.

– Downlink Traffic to Routing ID Mapping Configuration.

– BH Routing Configuration.

– BH RLC Channel Mapping Configuration.

– Uplink Traffic to BH RLC Channel Mapping Configuration.

– Downlink Traffic to BH RLC Channel Mapping Configuration.

– Header Rewriting Configuration.