This layer 2 sublayer is responsible for transferring messages from the network that are to be broadcast or multicast to all mobile stations in a cell. The user data that comes from the higher layers are temporarily saved in the BMC, if necessary, until they are scheduled for transmission. The transfer takes place using the unacknowledged mode service of the RLC on a common traffic channel to the MAC layer. ^[22] User data that is not intended to be broadcast over a cell is passed transparently by the BMC. As for other functions performed, this entity on the network side periodically estimates the volume of the cell broadcast traffic and forwards the information to the RRC layer using an indication primitive.

There are two types of messages in BMC-the cell broadcast message from the network and the schedule message that gives the location of broadcast messages in the next schedule period as well as the location of the schedule message for the following period. The BMC layer, on the UE side, determines which messages to forward to its upper layer and sends an indication to the RRC layer.

The BMC message format is very similar to that of an ISDN call control message and consists of a number of information elements. As an example, the information elements of a cell broadcast message are shown in Figure 6-31.

Figure 6-31: The cell broadcast message format