In 3GPP specifications, several types of data transfer services are offered by MAC. Each logical channel type is defined by the type of information that it transfers.
Logical channels can be classified into two general groups:
- Control channels (for transferring control plane information)
- Traffic channels (for transferring user plane information)
Control Channels:Control channels are used for the transfer of control plane information only. The control channels offered by MAC are:
- Broadcast Control Channel (BCCH)
- A downlink channel for broadcasting system control information.
- Paging Control Channel (PCCH)
- A downlink channel that transfers paging information and system information change notifications.
This channel is used for paging when the network does not know the location cell of the UE.
- Common Control Channel (CCCH)
- Channel for transmitting control information between UEs and network. This channel is used for UEs having no RRC connection with the network.
- Dedicated Control Channel (DCCH)
- A point-to-point bi-directional channel that transmits dedicated control information between a UE and the network. Used by UEs having an RRC connection.
Traffic Channels:Traffic channels are used for the transfer of user plane information only. The traffic channels offered by MAC are:
- Dedicated Traffic Channel (DTCH)
- A point-to-point channel, dedicated to one UE, for the transfer of user information. A DTCH can exist in both uplink and downlink.
- Multicast Traffic Channel (MTCH) (from Release 9)
- A point-to-multipoint downlink channel for transmitting traffic data from the network to the UE. This channel is only used by UEs that receive MBMS.
The figures below depict the mapping between logical channels, transport channels and physical channels for downlink and uplink:
Figure 1 : Downlink Channel Mapping
Figure 2 : Uplink Channel Mapping