Overview

The MAC layer, as the name implies, determines how different types of information coming from the higher layers over different logical channels should be transmitted over a physical channel on a radio frame (that is, the medium), and controls the timing of those transmissions [7], [15]. It provides the following services to the upper layers: data transfer, reallocation of radio resources and redefinition of MAC parameters, and measurement of the traffic volume and signal quality, and reporting the results to the RRC layer.

The MAC layer interacts with the RLC sublayer over a number of logical channels. Data flows on each logical channel are associated with a certain priority based on the attributes of the radio bearer service and the RLC buffer status. For example, if a particular UE is running two applications simultaneously, say, voice and a file transfer in the background, the RRC sublayer may assign a different priority to each of the two applications. Similarly, multiple UEs may be assigned relative priorities as well. To meet these priority requirements, the MAC layer may use some scheduling algorithms and map, say, high-priority data to a high-bit-rate transport format and low-priority data to a low-bit-rate transport format. Thus, the responsibility of the MAC layer is to map each logical channel onto a transport channel, selecting, on the basis of the associated priorities, an appropriate transport format within a Transport Format Combination (TFC) set that is assigned by the RRC layer. When multiple users access...