An Introduction to 5G

12Air Interface Layer 2

This chapter addresses the air interface's higher‐level transport protocols, which form layer 2 of an OSI protocol stack. We will work our way upwards from the bottom of the protocol stack, beginning with the medium access control (MAC) protocol, and progressing through radio link control (RLC), the packet data convergence protocol (PDCP) and the service data adaptation protocol (SDAP). The protocols are simpler than the air interface's physical layer: the first three have not changed much in comparison with LTE, while the last one is new to 5G.

12.1 Medium Access Control

12.1.1 Protocol Architecture

The medium access control protocol schedules transmissions on the air interface and controls the low‐level operation of the physical layer [1]. There is one MAC entity per node, so a mobile in 5G dual connectivity has two 5G MAC entities, one each for the master and secondary. Figure 12.1 shows the protocol architecture from the viewpoint of the mobile.

Figure 12.1 Architecture of the mobile's medium access control protocol. BCH: broadcast channel; DL‐SCH: downlink shared channel; PCH: paging channel; RACH: random access channel; UL‐SCH: uplink shared channel.

Source: Adapted from 3GPP TS 38.321.

Above the MAC protocol, each logical channel carries data from a single radio bearer. In the transmitter, the radio link control protocol stores outgoing data ...