TM's Blog for 3GPP Spec

When you study the 5G user plane protocol stack, you must observe that the SDAP is added. Why does the NR system need SDAP?

In LTE system, one EPS bearer corresponds to a specific Quality of Service (QoS). For example, when you turn on your smartphone. The device could establish two EPS bearers with different QoS (one default bearer for internet the other for IP Multimedia Subsystem (IMS) signaling). If you want to make a phone call, the device will establish the third EPS bearer with higher QoS for VoLTE service. However, the concept of the EPS bearer is replaced by the PDU session in 5G NR.

A PDU session could contain more than one QoS flow. Different from the previous example, your smartphone can only establish one PDU session for IMS signaling. When you want to make a phone call, the device can just new a QoS flow for VoLTE service in the PDU session you already established. (no need to establish a new session). However, a problem derives from the change: how to map QoS flows to Radio Bearers (RBs)? SDAP is the answer