Description
The Sidelink Relay Adaptation Protocol (SRAP) is a key protocol layer within the 5G New Radio (NR) sidelink protocol stack, specifically defined for relay operations between User Equipments (UEs). It operates as a sublayer of the Packet Data Convergence Protocol (PDCP) layer for the sidelink relay user plane. SRAP is responsible for adapting data packets for transmission over the sidelink relay interface, which is a direct device-to-device (D2D) communication link. It performs functions such as adding and removing SRAP headers, which contain necessary information for relay routing and identification, and may handle segmentation and reassembly of data units to match the characteristics of the sidelink radio bearer.
Architecturally, SRAP sits between the higher-layer services (e.g., IP packets or non-IP data) and the lower-layer sidelink radio protocols (like RLC and MAC). It is utilized in scenarios where a remote UE communicates with the network via a relay UE, which acts as an intermediary. The relay UE has a direct connection to the gNB (Uu interface) and also establishes a sidelink (PC5 interface) with the remote UE. SRAP operates on both the remote UE and the relay UE to ensure data packets are correctly formatted for the sidelink leg of the communication path. It works in conjunction with other sidelink protocols, such as Sidelink Radio Link Control (SL-RLC) and Sidelink Medium Access Control (SL-MAC), to provide reliable data transfer.
The protocol's operation involves the SRAP entity processing Service Data Units (SDUs) from the upper layers. It adds a SRAP header that includes fields like a Destination Layer-2 ID and possibly a Source Layer-2 ID for addressing within the sidelink. This header enables the relay UE to identify the target remote UE or to forward data towards the network. On the receiving side, the SRAP entity removes the header and delivers the SDU to the appropriate upper-layer entity. SRAP is designed to be transparent to the core network, meaning the network treats the remote UE as if it is directly connected, simplifying network management. Its role is crucial in enabling efficient relay-based communication, which is a foundational capability for Proximity Services (ProSe), public safety networks, and Vehicle-to-Everything (V2X) scenarios where direct network coverage may be limited.
Purpose & Motivation
SRAP was introduced to standardize and enhance relay functionality in 5G NR sidelink communications, addressing the need for reliable device-to-device relay without continuous network coverage. Prior to 3GPP Release 17, sidelink communications in LTE and early 5G primarily supported direct communication but had limited standardized relay mechanisms, especially for user plane data. This posed challenges for public safety applications, where first responders might operate in areas with damaged or absent network infrastructure, and for V2X scenarios where vehicles need to extend communication range. SRAP provides a protocol framework to efficiently adapt data for relay transmission, solving the problem of seamless connectivity extension.
The creation of SRAP was motivated by the evolution of Proximity Services (ProSe) and the increasing demand for advanced V2X and industrial IoT applications. In Release 16, 5G NR sidelink was enhanced for V2X, but relay support was not fully fleshed out in the user plane protocol stack. SRAP fills this gap by defining a dedicated adaptation layer that handles the intricacies of relay routing over the PC5 interface. It addresses limitations of previous ad-hoc or non-standardized relay approaches by ensuring interoperability, efficient header usage, and integration with the existing NR sidelink architecture. This enables UEs to dynamically act as relays, extending network coverage and improving service reliability in challenging environments.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (166 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the SRAP (Sidelink Relay Adaptation Protocol) function was newly introduced for L2 U2N Relay communication, defining the roles of U2N Relay UEs (including First, Intermediate, and Last U2N Relay UEs) and U2N Remote UEs that communicate via an indirect path. The specification details the protocol for establishing PC5 connections to enable a U2N Remote UE's network connectivity through a relay, with the first U2N Relay UE requiring its own network connection before relaying begins. This is specified in the newly created TS 38.351.
- Delay budget report and MAC CE adaptation for NR for TS 38.300 TS 38.300CR0042
- Correction to description of bandwidth adaptation TS 38.300CR0051
- Correction of BWP adaptation TS 38.300CR0107
- - correction to ASN.1: addiming a missing change to "WriteReplaceWarningResponseIEs F1AP-PROTOCOL-IES ::= {" TS 38.473
In Release 16, the SRAP function was newly specified to support Layer 2 UE-to-Network (U2N) relay communication, defining the protocol for data adaptation between the Uu and PC5 interfaces in relay chains. It introduced roles such as the Last U2N Relay UE, Intermediate U2N Relay UE, and First U2N Relay UE to enable single-hop and multi-hop sidelink relaying. This protocol facilitates the establishment of an indirect path for a U2N Remote UE to connect to the network via one or more relay UEs.
- Introduction of 5G V2X with NR Sidelink TS 38.300CR0204
- Introduction of 5G V2X with NR sidelink TS 38.331CR1493
- Correction for NR sidelink communication TS 38.300CR0245
- Stage-2 corrections for NR sidelink communication TS 38.300CR0323
- 38.323 corrections on Sidelink TS 38.323CR0056
- Correction on RRC parameters for 5G V2X with NR sidelink TS 38.331CR1992
+ 7 more changes
In Release 17, the SRAP (Sidelink Relay Adaptation Protocol) function was introduced to enable Layer 2 UE-to-Network (U2N) relay communication, as detailed in the new TS 38.351 specification. It defined the roles of the First, Intermediate, and Last U2N Relay UEs to establish multi-hop relay paths for U2N Remote UEs. Furthermore, the protocol clarified the service interfaces between SRAP and PDCP for managing sidelink relay connections.
- Introduction of Sidelink Relay TS 38.300CR0403
- Introduction of SL Relay in 38.323 TS 38.323CR0086
- RRC CR for NR Sidelink enhancement TS 38.331CR2902
- Introduction of SL relay TS 38.331CR2910
- BL CR to TS38.401 on Rel-17 Sidelink Relay TS 38.401CR0195
- Introduction of SideLink Relay TS 38.473CR0842
+ 57 more changes
In Release 18, the SRAP function was enhanced as part of the broader NR sidelink relay enhancements, which introduced support for multi-hop L2 U2N Relay communication involving Intermediate U2N Relay UEs and First U2N Relay UEs. These enhancements specifically defined the roles and connection establishment procedures for these relay types, requiring them to first establish a network connection before relaying traffic for U2N Remote UEs. Additionally, the release introduced support for U2U Relays and included corrections related to handover failure scenarios involving unavailable candidate relay UEs.
- Introduction of sidelink positioning in 38300 TS 38.300CR0722
- Introduction of NR Sidelink Evolution TS 38.300CR0728
- Introduction of NR sidelink relay enhancements TS 38.300CR0771
- Introduction of NR sidelink PDCP duplication in TS 38.323 TS 38.323CR0126
- Introduction of Enhanced NR Sidelink Relay TS 38.323CR0127
- Introduction of Rel-18 SL relay enhancements TS 38.331CR4441
+ 64 more changes
In Release 19, the SRAP function was enhanced to introduce multi-hop L2 U2N relay operation, supporting intermediate and first U2N Relay UEs that establish network connectivity before relaying traffic. The protocol specifications, including TS 38.323 and TS 38.351, were updated to define the procedures and architecture for this multi-hop sidelink relay capability. Additionally, corrections and miscellaneous updates were made to SRAP to address the new multi-hop U2N relay functionality.
- ProSe support in NPN [ProSe_NPN] TS 38.300CR0957
- Introduction of Multi-path Relay Enhancement [N3C_M_Relay] TS 38.300CR0991
- Introduction of NR sidelink multi-hop U2N Relay TS 38.300CR1028
- Introduction of NR Femto Architecture and Protocol Aspects TS 38.300CR1035
- Introduction of multi-hop U2N relay in TS 38.323 TS 38.323CR0150
- ProSe support in NPN [ProSe_NPN] TS 38.331CR5209
+ 10 more changes
Explore further
Broader topics and technologies where SRAP plays a role.
Defining Specifications
3GPP specifications that define or reference SRAP, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 38.300 vj00 | NG-RAN Overall Description | Rel-19 |
| TS 38.323 vj00 | Packet Data Convergence Protocol (PDCP) | Rel-19 |
| TS 38.331 vj00 | NR Radio Resource Control (RRC) Protocol Specification | Rel-19 |
| TS 38.351 vj00 | Sidelink Relay Adaptation Protocol (SRAP) | Rel-19 |
| TS 38.401 vj10 | NG-RAN Architecture Specification | Rel-19 |
| TS 38.473 vj10 | 5G F1 Application Protocol (F1AP) | Rel-19 |