5G NR: SDAP Layer

                                                   


SDAP is responsible for QoS Flow handling across the 5G air interface. In particular, SDAP will map a specific QoS Flow within a PDU Session to a corresponding Data Radio Bearer (which has been established with the appropriate level of QoS). In addition, SDAP will mark the transmitted packets with the correct QFI (QoS Flow ID), ensuring that the packet receives the correct forwarding treatment as it traverses the 5G System.
For each PDU Session, a single protocol entity of SDAP will be configured. The exception to this is Dual Connectivity, whereby the Master Cell Group and Secondary Cell Group will each have a separate SDAP configuration on the device.

NR Radio Protocol Stack Architecture is almost same as LTE Radio Protocol Stack Architecture. If you are already familiar with LTE protocol stack or general concept of radio protocol stack, you would not need to spend too much time to understand this Layer.

I would not describe much details on each component of the protocol stack in this video. It is too much to describe everything in a single page. The purpose of this video is to provide you with some understanding of the radio protocol stack of 5G-NR.

SDAP is a completely new layer added to NR on top of PDCP. Main role of SDAP is to apply a sophisticated QoS for each of data stream.

As in LTE / WCDMA, NR radio protocol stack has two different stacks depending on the type of data that is processed by the stack. If the data is Signaling message, it goes through the C-plane stack and if it is user data, it goes through U-Plane stack.

In LTE U-Plane and C-Plane is made up of a common structure: PHY <-> MAC <-> RLC <-> PDCP but In 5G-NR, case of U-Plane, a layer called SDAP is sitting at the top of the radio stack and the SDP is connected to UPF (User Plane Function). In case of C-Plane, the two layers RRC and NAS are sitting at the top of the stack.

Take a look at the L2 (layer 2) structure of the NR U-Plane radio protocol. The structure of L2 downlink stack can be illustrated as shown below. Except the new layer called SDAP, you would notice that the overall structure is almost identical to LTE L2 structure. NR support carrier aggregation from the beginning, data for each carrier is processed separately for each carrier in SDAP, PDCP, RLC and multiplexed/scheduled in the common MAC layer. This is also same as LTE Rel 10 or higher.

Following is L2 structure of NR U-Plane Uplink radio protocol. Basic structure is same as downlink structure except that Uplink does not support carrier aggregation.