25 series - Uu interface protocol stack

Description

The Uu interface is the radio interface between the mobile and the radio access network.

The protocol stack has two planes: the user plane carries the data streams of interest to the user, while the control plane carries the network's signalling messages. The user plane is divided further, to distinguish data streams to and from the circuit-switched, packet-switched and broadcast control domains.

The protocols are as follows:

• RRC: This is the main signalling protocol in the Uu interface. It defines the signalling messages that are exchanged between the mobile and the radio access network.
• BMC: This is an extra interface for the cell broadcast service. It distributes cell broadcast messages from the network, and collects them at the mobile.
• PDCP: This is an extra interface for packet data. It carries out functions such as header compression of IP packets.
• RLC: This manages the radio link between the mobile and the network, for example by optionally re-transmitting data packets that have not been received correctly.
• MAC: This carries out low-level manipulation and control of the physical layer. For example, it prioritises the transmission of different data streams from the mobile or Node B, to ensure that each one has an appropriate data rate.
• Physical layer: This carries out the low-level transmission and reception. It has two modes of operation, FDD and TDD, which are described on separate pages.

Roughly speaking, the physical layer is implemented in the Node B, while the higher-layer protocols are implemented in the RNC. The details are more complicated, however.

The Uu interface also carries non-access stratum signalling messages between the mobile and the fixed network, which are forwarded transparently by the radio access network. These messages are defined in the 24 series.

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