4 General

38.3513GPPNRRelease 17Sidelink Relay Adaptation Protocol (SRAP) SpecificationTS

4.1 Introduction

The objective is to describe the SRAP architecture and the SRAP entities from a functional point of view.

4.2 SRAP architecture

4.2.1 General

This clause describes a model of the SRAP, i.e., it does not specify or restrict implementations.

4.2.2 SRAP entities

Figure 4.2.2-1 represents one possible structure for the SRAP sublayer. The figure is based on the radio interface protocol architecture defined in TS 38.300 [2].

Figure 4.2.2-1: SRAP structure overview

On the U2N Relay UE, the SRAP sublayer contains one SRAP entity at Uu interface and a separate collocated SRAP entity at the PC5 interface. On the U2N Remote UE, the SRAP sublayer contains only one SRAP entity at the PC5 interface.

Each SRAP entity has a transmitting part and a receiving part. Across the PC5 interface, the transmitting part of the SRAP entity at the U2N Remote UE has a corresponding receiving part of an SRAP entity at the U2N Relay UE, and vice versa. Across the Uu interface, the transmitting part of the SRAP entity at the U2N Relay UE has a corresponding receiving part of an SRAP entity at the gNB, and vice versa.

Figure 4.2.2-2 and Figure 4.2.2-3 represents the functional view of the SRAP entity for the SRAP sublayer at PC5 interface and at Uu interface respectively.

Figure 4.2.2-2: Example of functional view of SRAP sublayer at PC5 interface

Figure 4.2.2-3: Example of functional view of SRAP sublayer at Uu interface

In the example of Figure 4.2.2-2 and Figure 4.2.2-3, at relay UE:

– The receiving part on the SRAP entity of Uu interface delivers SRAP Data PDUs to the transmitting part on the collocated SRAP entity of PC5 interface, and the receiving part on the SRAP entity of PC5 interface delivers SRAP Data PDUs to the transmitting part on the collocated SRAP entity of Uu interface, except for data packet for SRB0 (i.e., received from SL-RLC0 as specified in TS 38.331 [3]). As an alternative, the receiving part may deliver SRAP SDUs to the transmitting part on the collocated SRAP entity. When passing SRAP SDUs, the receiving part removes the SRAP header and the transmitting part adds the SRAP header with the same SRAP header content as carried on the SRAP Data PDU header prior to removal. Passing SRAP SDUs in this manner is therefore functionally equivalent to passing SRAP Data PDUs, in implementation. The following specification therefore refers to the passing of SRAP data packets.

– For UL data packet corresponding to SRB0, the receiving part on the SRAP entity of PC5 interface delivers SRAP SDUs to the transmitting part on the collocated SRAP entity of Uu interface, and the transmitting part on the SRAP entity of Uu interface adds the SRAP header in accordance with clause 5.3.3.

– For DL data packet corresponding to SRB0, the receiving part on the SRAP entity of Uu interface delivers SRAP Data PDUs to the transmitting part on the collocated SRAP entity of PC5 interface, and the transmitting part on the SRAP entity of PC5 interface removes the SRAP header in accordance with clause 5.2.2. As an alternative for handling DL data packet corresponding to SRB0, the receiving part on the SRAP entity of Uu interface remove the SRAP header and delivers SRAP SDUs to the transmitting part on the collocated SRAP entity of PC5 interface.

4.3 Services

4.3.1 Services provided to upper layers

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

– Data transfer.

4.3.2 Services expected from lower layers

An SRAP 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 SRAP sublayer supports the following functions:

– Data transfer;

– Determination of UE ID field and BEARER ID field for data packets;

– Determination of egress link;

– Determination of egress RLC channel.

4.5 Configurations

The configuration of the SRAP entity for U2N Remote UE includes:

– Mapping from a radio bearer identified by BEARER ID field to egress PC5 Relay RLC channel via RRC;

– The local identity via RRC.

The configuration of the SRAP entity for U2N Relay UE includes:

– The local identity for each U2N Remote UE via RRC;

– Mapping from UE ID field and BEARER ID field to egress Uu Relay RLC channel for each U2N Remote UE via RRC;

– Mapping from UE ID field and BEARER ID field to egress PC5 Relay RLC channel for each U2N Remote UE via RRC.