SDAP — Maps a QoS flow to a radio bearer

The protocol stack

Where it sits

What it is

What it is. The sublayer that maps a QoS flow onto a data radio bearer. TS 37.324 — note the 37, not 38: it is shared with LTE's numbering series. It is the one sublayer 5G added that LTE did not have, and it is by far the smallest specification of the five, at nineteen pages.

The functions, from TS 37.324 clause 4.4:

Why 5G needed a new sublayer for this. In LTE, quality of service was a property of a bearer: to give traffic different treatment you established another bearer end to end, all the way into the core. Bearers were coarse, slow to set up, and the radio had no way to distinguish two kinds of traffic inside one.

5G moved QoS into the core as flows, which are finer-grained and cheaper to create, and then needed something at the bottom of the stack to decide which flow rides which radio bearer. That something is SDAP. It is small because its job is small: read a marking, choose a bearer.

Reflective mapping is the neat part. Rather than configuring the uplink mapping explicitly, the device can infer it by observing how the network mapped the downlink — so the rule arrives without a signalling message.

Read on

This concept was first written up in ref-stack, which reads the whole group as one argument.