Three layers in UU

The air interface is

layered into three protocol layers:

• The physical layer (layer 1, L1);

• The data link layer (layer 2, L2);

• Network layer (layer 3, L3).

The physical layer interfaces the medium access control (MAC) sublayer of

layer 2 and the radio resource control (RRC) layer of layer 3. The physical layer offers

different transport channels to MAC. A transport channel is characterized by how the

information is transferred over the radio interface. Transport channels are channel coded

and then mapped to the physical channels specified in the physical layer. MAC offers

different logical channels to the radio link control (RLC) sublayer of layer 2. A logical

channel is characterized by the type of information transferred.

Layer 2 is split into following sublayers: MAC, RLC, packet data convergence

protocol (PDCP) and broadcast/multicast control (BMC). Layer 3 and RLC are divided

into control and user planes. PDCP and BMC exist in the user plane only. In the control

plane, layer 3 is partitioned into sublayers where the lowest sublayer, denoted as RRC,

interfaces with layer 2. The RLC sublayer provides ARQ functionality closely coupled

with the radio transmission technique used.