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TS 38.351
5G New Radio —
Sidelink Relay Adaptation Protocol (SRAP) Specification

3GPP‑Page   ETSI‑search   CONTENT↓
V18.1.0 (PDF)  2023/12  21 p.
V17.7.0  2023/09  20 p.
Rapporteur:
Dr. Lu, Qianxi
Guangdong OPPO Mobile Telecom

Content for  TS 38.351  Word version:  17.3.0

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1  Scope  p. 6

The present document provides description of the Sidelink Relay Adaptation Protocol (SRAP).

2  References  p. 6

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.
  • References are either specific (identified by date of publication, edition number, version number, etc.) or non-specific.
  • For a specific reference, subsequent revisions do not apply.
  • For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.
[1]
TR 21.905: "Vocabulary for 3GPP Specifications".
[2]
TS 38.300: "NG Radio Access Network; Overall description".
[3]
TS 38.331: "NR Radio Resource Control (RRC); Protocol Specification".
[4]
TS 38.322: "NR Radio Link Control (RLC) protocol specification".
[5]
TS 38.323: "NR; Packet Data Convergence Protocol (PDCP) specification".
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3  Definitions of terms, symbols and abbreviations  p. 6

3.1  Terms  p. 6

For the purposes of the present document, the terms given in TR 21.905 and the following apply. A term defined in the present document takes precedence over the definition of the same term, if any, in TR 21.905.
Egress RLC channel:
a RLC channel on which a packet is transmitted by a U2N Relay UE, a U2N Remote UE or a network node.
Egress link:
a radio link on which a packet is transmitted by a U2N Relay UE, a U2N Remote UE or a network node.
Ingress RLC channel:
a RLC channel on which a packet is received from a U2N Relay UE, a U2N Remote UE or a network node.
Ingress link:
a radio link on which a packet is received from a U2N Relay UE, a U2N Remote UE or a network node.
U2N Relay UE:
a UE that provides functionality to support connectivity to the network for U2N Remote UE(s).
U2N Remote UE:
a UE that communicates with the network via a U2N Relay UE.
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3.2  Abbreviations  p. 6

For the purposes of the present document, the abbreviations given in TR 21.905 and the following apply. An abbreviation defined in the present document takes precedence over the definition of the same abbreviation, if any, in TR 21.905.
SRAP
Sidelink Relay Adaptation Protocol
U2N
UE-to-Network

4  General  p. 7

4.1  Introduction  p. 7

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

4.2  SRAP architecture  p. 7

4.2.1  General  p. 7

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

4.2.2  SRAP entities  p. 7

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.
Copy of original 3GPP image for 3GPP TS 38.351, Fig. 4.2.2-1: SRAP structure overview
Figure 4.2.2-1: SRAP structure overview
(⇒ copy of original 3GPP image)
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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.
Copy of original 3GPP image for 3GPP TS 38.351, Fig. 4.2.2-2: Example of functional view of SRAP sublayer at PC5 interface
Figure 4.2.2-2: Example of functional view of SRAP sublayer at PC5 interface
(⇒ copy of original 3GPP image)
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Copy of original 3GPP image for 3GPP TS 38.351, Fig. 4.2.2-3: Example of functional view of SRAP sublayer at Uu interface
Figure 4.2.2-3: Example of functional view of SRAP sublayer at Uu interface
(⇒ copy of original 3GPP image)
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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). 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 of the relay UE 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 not shown in Figure 4.2.2-2 or Figure 4.2.2-3, the receiving part on the SRAP entity of Uu interface removes the SRAP header and delivers SRAP SDUs to the transmitting part on the collocated SRAP entity of PC5 interface.
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4.3  Services  p. 10

4.3.1  Services provided to upper layers  p. 10

The following services are provided by the SRAP sublayer to upper layers:
  • Data transfer.

4.3.2  Services expected from lower layers  p. 10

An SRAP sublayer expects the following services from lower layers per RLC entity (for a detailed description see TS 38.322):
  • Acknowledged data transfer service;
  • Unacknowledged data transfer service.

4.4  Functions  p. 10

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  p. 10

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.
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5  Procedures  p. 10

5.1  SRAP entity handling  p. 10

5.1.1  SRAP entity establishment  p. 10

When upper layers request establishment of an SRAP entity, UE shall:
  • establish an SRAP entity;
  • follow the procedures in clause 5.

5.1.2  SRAP entity release  p. 11

When upper layers request release of an SRAP entity, UE shall:
  • release the SRAP entity and the related SRAP configurations.

5.2  DL Data transfer  p. 11

5.2.1  Receiving operation of U2N Relay UE  p. 11

Upon receiving an SRAP Data PDU from lower layer, the receiving part of the SRAP entity on the Uu interface of U2N Relay UE shall:
  • deliver the SRAP data packet to the transmitting part of the collocated SRAP entity on the PC5 interface.

5.2.2  Transmitting operation of U2N Relay UE  p. 11

The transmitting part of the SRAP entity on the PC5 interface of U2N Relay UE receives SRAP data packets from the receiving part of the SRAP entity on the Uu interface of the same U2N Relay UE, and construct SRAP Data PDUs as needed (see clause 4.2.2).
When the transmitting part of the SRAP entity on the PC5 interface has an SRAP Data PDU to transmit, the transmitting part of the SRAP entity on the PC5 interface shall:
  • Determine the egress link in accordance with clause 5.2.2.1;
  • Determine the egress RLC channel in accordance with clause 5.2.2.2;
  • if the SRAP Data PDU is for SRB0 (the BEARER ID field is 0, and the bearer is identified as SRB based on sl-RemoteUE-RB-Identity associated with the entry containing the sl-EgressRLC-ChannelUu which matches the LCID of the Uu Relay RLC Channel from which the SRAP Data PDU is received):
    • Remove the SRAP header from the SRAP Data PDU;
  • Submit this SRAP Data PDU to the determined egress RLC channel of the determined egress link.
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5.2.2.1  Egress link determination  p. 11

For a SRAP Data PDU to be transmitted, SRAP entity shall:
  • if there is an entry in sl-SRAP-ConfigRelay, whose sl-LocalIdentity matches the UE ID field in SRAP Data PDU:
    • Determine the egress link on PC5 interface corresponding to sl-L2IdentityRemote configured for the concerned sl-LocalIdentity as specified in TS 38.331.

5.2.2.2  Egress RLC channel determination  p. 11

For a SRAP Data PDU to be transmitted, the SRAP entity shall:
  • if the SRAP Data PDU is for SRB0 (the BEARER ID field is 0 and the bearer is identified as SRB based on sl-RemoteUE-RB-Identity associated with the entry containing the sl-EgressRLC-ChannelUu which matches the LCID of the Uu Relay RLC Channel from which the SRAP Data PDU is received):
    • Determine the egress PC5 Relay RLC channel in the determined egress link corresponding to logicalChannelIdentity for SL-RLC0 as specified in TS 38.331;
  • else if there is an entry in sl-SRAP-Config-Relay, whose sl-LocalIdentity matches the UE ID field in SRAP Data PDU, which includes an sl-RemoteUE-RB-Identity that matches the SRB identity or DRB identity of the SRAP Data PDU determined by the BEARER ID field (For the BEARER ID shared by both SRB and DRB, SRB and DRB are differentiated based on sl-RemoteUE-RB-Identity associated with the entry containing the sl-EgressRLC-ChannelUu which matches the LCID of the Uu Relay RLC Channel from which the SRAP Data PDU is received):
    • Determine the egress PC5 Relay RLC channel in the determined egress link corresponding to sl-EgressRLC-ChannelPC5 configured for the concerned sl-LocalIdentity and concerned sl-RemoteUE-RB-Identity as specified in TS 38.331.
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5.2.3  Receiving operation of U2N Remote UE  p. 12

Upon receiving an SRAP Data PDU from lower layer, the receiving part of the SRAP entity shall:
  • if the SRAP Data PDU is not for SRB0 (not received from SL-RLC0 as specified in TS 38.331):
    • if sl-RemoteUE-RB-Identity is configured:
      • remove the SRAP header of this SRAP Data PDU and deliver the SRAP SDU to upper layer entity corresponding to the BEARER ID field of this SRAP Data PDU (For the BEARER ID shared by both SRB and DRB, SRB and DRB are differentiated based on sl-RemoteUE-RB-Identity associated with the entry containing the sl-EgressRLC-ChannelPC5 which matches LCID of the PC5 Relay RLC Channel from which the SRAP Data PDU is received, and for DRB, the upper layer entity for BEARER ID plus 1);
    • else
      • remove the SRAP header of this SRAP Data PDU and deliver the SRAP SDU to PDCP entity of SRB1 by ignoring the UE ID field and BEARER ID field of this SRAP Data PDU;
  • else:
    • deliver the SRAP SDU (i.e., same as SRAP PDU for SRB0) to upper layer, i.e., RRC layer entity (TS 38.331).
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5.3  UL Data transfer  p. 12

5.3.1  Transmitting operation of U2N Remote UE  p. 12

The transmitting part of the SRAP entity on the PC5 interface of U2N Remote UE can receive SRAP SDU from upper layer and constructs SRAP Data PDU.
Upon receiving an SRAP SDU from upper layer, the transmitting part of the SRAP entity on the PC5 interface shall:
  • if the SRAP SDU is not for SRB0:
    • Determine the UE ID field and BEARER ID field in accordance with clause 5.3.1.1;
    • Construct an SRAP Data PDU with SRAP header, where the UE ID field and BEARER ID field are set to the determined values, in accordance with clause 6.2.2;
  • else:
    • Construct an SRAP Data PDU without SRAP header in accordance with clause 6.2.2.
  • Determine the egress RLC channel in accordance with clause 5.3.1.2;
  • Submit this SRAP Data PDU to the determined egress RLC channel.
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5.3.1.1  UE ID field and BEARER ID field determination  p. 12

For an SRAP SDU received from upper layer, the SRAP entity shall:
  • Determine the UE ID field corresponding to sl-LocalIdentity, configured as specified in TS 38.331;
  • Determine the BEARER ID field corresponding to SRB identity for SRB (i.e., set the BEARER ID field to srb-Identity), or corresponding to DRB identity minus 1 for DRB (i.e., set the BEARER ID field to drb-Identity minus 1), from which the SRAP SDU is received, configured as specified in TS 38.331.
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5.3.1.2  Egress RLC channel determination  p. 13

For a SRAP Data PDU to be transmitted, the SRAP entity shall:
  • if the SRAP Data PDU is for SRB0:
    • Determine the egress PC5 Relay RLC channel in the link with U2N Relay UE corresponding to logicalChannelIdentity for SL-RLC0 as specified in TS 38.331;
  • else if there is an entry in sl-SRAP-ConfigRemote, whose sl-RemoteUE-RB-Identity matches the SRB identity or DRB identity of the SRAP Data PDU:
    • Determine the egress PC5 Relay RLC channel of the link with U2N Relay UE corresponding to sl-EgressRLC-ChannelPC5 configured for the concerned sl-RemoteUE-RB-Identity as specified in TS 38.331.
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5.3.2  Receiving operation of U2N Relay UE  p. 13

Upon receiving an SRAP Data PDU from lower layer, the receiving part of the SRAP entity on the PC5 interface shall:
  • deliver the SRAP data packet to the transmitting part of the collocated SRAP entity on the Uu interface.

5.3.3  Transmitting operation of U2N Relay UE  p. 13

The transmitting part of the SRAP entity on the Uu interface of U2N Relay UE can receive SRAP data packets from the receiving part of the SRAP entity on the PC5 interface of the same U2N Relay UE, and construct SRAP Data PDUs as needed (see clause 4.2.2).
When the transmitting part of the SRAP entity on the Uu interface has an SRAP Data PDU to transmit, the transmitting part of the SRAP entity on the Uu interface shall:
  • if the SRAP Data PDU is received from SL-RLC0 as specified in TS 38.331:
    • Determine the UE ID field and BEARER ID field in accordance with clause 5.3.3.1;
    • Construct an SRAP Data PDU with SRAP header, where the UE ID field and BEARER ID field are set to the determined values, in accordance with clause 6.2.2;
  • Determine the egress RLC channel in accordance with clause 5.3.3.2;
  • Submit this SRAP Data PDU to the determined egress RLC channel.
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5.3.3.1  UE ID field and BEARER ID field determination  p. 13

For an SRAP Data PDU received from SL-RLC0 as specified in TS 38.331, the SRAP entity shall:
  • if there is an entry in sl-RemoteUE-ToAddModList, whose sl-L2IdentityRemote matches the Layer-2 ID of the remote UE from which the SRAP Data PDU is received:
    • Determine the UE ID field corresponding to sl-LocalIdentity configured for the concerned sl-L2IdentityRemote as specified in TS 38.331;
    • Determine the BEARER ID field as 0 (i.e., set BEARER ID field as 0).
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5.3.3.2  Egress RLC channel determination  p. 13

For a SRAP Data PDU to be transmitted, the SRAP entity shall:
  • if there is an entry in sl-SRAP-ConfigRelay, whose sl-LocalIdentity matches the UE ID field in SRAP Data PDU, and which includes an sl-RemoteUE-RB-Identity matches SRB identity or DRB identity of the SRAP Data PDU determined by the BEARER ID field (For the BEARER ID shared by both SRB and DRB, SRB and DRB are differentiated based on sl-RemoteUE-RB-Identity associated with the entry containing the sl-EgressRLC-ChannelPC5 which matches LCID of the PC5 Relay RLC Channel from which the SRAP Data PDU is received):
    • Determine the egress Uu Relay RLC channel corresponding to sl-EgressRLC-ChannelUu configured for the concerned sl-LocalIdentity and concerned sl-RemoteUE-RB-Identity as specified in TS 38.331.
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5.4  Handling of unknown, unforeseen, and erroneous protocol data  p. 14

For U2N Remote UE, if sl-LocalIdentity and sl-RemoteUE-RB-Identity are both configured, when a SRAP Data PDU with SRAP header that contains a UE ID field or BEARER ID field which does not match sl-LocalIdentity or sl-RemoteUE-RB-Identity included in sl-SRAP-ConfigRemote is received, the SRAP entity shall:
  • discard the received SRAP Data PDU.
For U2N Relay UE, when a SRAP Data PDU with SRAP header that contains a UE ID field or BEARER ID field which does not match sl-LocalIdentity or sl-RemoteUE-RB-Identity included in sl-SRAP-ConfigRelay is received except in the case where the SRAP Data PDU from SL-RLC1 as specified in TS 38.331 is the first SRAP Data PDU received from a U2N Remote UE, or when a SRAP Data PDU that contains a UE ID which does not match the concerned sl-LocalIdentity corresponding to sl-L2IdentityRemote of the ingress link is received by U2N Relay UE, the SRAP entity shall:
  • discard the received SRAP Data PDU.
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6  Protocol data units, formats, and parameters  p. 14

6.1  Protocol data units  p. 14

6.1.1  Data PDU  p. 14

The SRAP Data PDU is used to convey the following with or without the PDU header:
  • upper layer data.

6.2  Formats  p. 14

6.2.1  General  p. 14

An SRAP Data PDU is a bit string that is byte aligned (i.e. multiple of 8 bits) in length. The formats of SRAP Data PDUs are described in clause 6.2.2 and their parameters are described in clause 6.3.

6.2.2  Data PDU  p. 14

Figure 6.2.2-1 shows the format of the SRAP Data PDU with SRAP header being configured. This SRAP Data PDU format is applicable to SRAP SDU except those for SRB0 delivered over PC5 interface.
Copy of original 3GPP image for 3GPP TS 38.351, Fig. 6.2.2-1: SRAP Data PDU format with SRAP header
Figure 6.2.2-1: SRAP Data PDU format with SRAP header
(⇒ copy of original 3GPP image)
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Figure 6.2.2-2 shows the format of the SRAP Data PDU consisting only of a data field without any SRAP header. This SRAP Data PDU format is applicable to SRAP SDU for SRB0 delivered over PC5 interface.
Copy of original 3GPP image for 3GPP TS 38.351, Fig. 6.2.2-2: SRAP Data PDU format without SRAP header
Figure 6.2.2-2: SRAP Data PDU format without SRAP header
(⇒ copy of original 3GPP image)
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6.3  Parameters  p. 15

6.3.1  General  p. 15

If not otherwise mentioned in the definition of each field the bits in the parameters shall be interpreted as follows: the left most bit is the first and most significant and the right most bit is the last and least significant bit.
Unless otherwise mentioned, integers are encoded in standard binary encoding for unsigned integers. In all cases the bits appear ordered from MSB to LSB when read in the PDU.

6.3.2  UE ID  p. 15

Length:
8 bits.
This field carries local identity of U2N Remote UE.

6.3.3  BEARER ID  p. 15

Length:
5 bits.
This field carries Uu radio bearer identity for U2N Remote UE.

6.3.4  Data  p. 15

Length:
Variable
This field carries the SRAP SDU (i.e. PDCP PDU or RRC PDU).

6.3.5  R  p. 16

Length:
1 bit
Reserved. In this release, reserved bits shall be set to 0. Reserved bits shall be ignored by the receiver.

6.3.6  D/C  p. 16

Length:
1 bit
This field indicates whether the corresponding SRAP PDU is an SRAP Data PDU or an SRAP Control PDU (not used in this release).
Bit Description
0SRAP Data PDU
1SRAP Control PDU (not used in this release)

$  Change history  p. 17

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