- SDAP stands for Service Data Adaptation Protocol.
- It is a sub-layer of 5G NR radio protocol stack.
- It is placed the top of the PDCP Layer (for user plane) in the Radio Protocol Stack in 5G NR.
- It is present/applicable only to the user plane.
- It receives the DL data from the UPF ( User Plane Function of 5GC ) via NG-U interface, it means SDAP sub-layer is required when Base Station is connected to the 5GC.
Services provided to upper layers :
The SDAP sublayer provides its service to the user plane upper layers. i.e. transfer of user plane data.
Services expected from lower layers :
An SDAP entity expects the following services from lower layers:
- user plane data transfer service
- in-order delivery except when out of order delivery is configured by RRC (3GPP TS 38.331)
The main services and functions of SDAP are :
- Transfer of user plane data
- The mapping between a QoS flow and a data radio bearer for both DL and UL
- Marking QoS flow ID (QFI) in both DL and UL packets.
- A single protocol entity of SDAP is configured for each individual PDU session.
- Reflective QoS flow to DRB mapping for the UL SDAP data PDUs.
SDAP Layer Structural View:
The structural view of the SDAP layer given in the below diagram, which will give a very high-level idea about SDAP layer.
Now, on seeing above diagram, there are some questions are coming in minds like “What is the SDAP entity?”, “What is the SDAP SAP?”, “What is the QoS Flow?”, “What is the PDU Session?”, “What is the PDCP-SAP?”, “What is the SDAP SDU/PDU?” and “What is the PDCP SDU/PDU?”. Don’t worry about these questions,I’ll will explain.
The SDAP entities are located in the SDAP sublayer. Several SDAP entities may be defined for a UE. There is an SDAP entity configured for each individual PDU session
What is the SDAP entity? A SDAP entity which is used to perform the SDAP sub-layer functions (like The mapping between a QoS flow and a data radio bearer(DRB), Reflective QoS flow to DRB mapping, Add/Remove SDAP Header if configured.
What is the PDCP SAP? SAP stands for Service Access Points. A SAP is logical connection (interface) between any two layers. Here PDCP SAP is interface between SDAP & PDCP.
What is the QoS Flow? The QoS flow is the lowest level granularity within the 5G and where policy and charging are enforced.
What is the SDAP SDU/PDU? SDU stands for Service Data Unit and PDU stands for Packet Data Unit. Input of SDAP sub-layer called as SDAP SDU and output of SDAP sub-layer called as SDAP PDU or PDCP SDU.
The functional diagram for SDAP layer:
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3GPP TS 37.324 Rel 15