Logical Channels

As shown in Figure 6-8, information is passed
from the MAC layer to the physical layer in the form of transport channels.
That information, however, can begin higher in the protocol stack, in which case it is passed from the RLC layer to the MAC layer in the form of logical
channels. The logical channels are mapped to transport channels, which
in turn are mapped to physical channels.
As mentioned, RLC interfaces with MAC through a number of logical
channels. MAC maps those logical channels to the transport channels previously
described. Logical channels relate to the information being transmitted,
while transport channels relate largely to the manner in which the
information is transmitted. Basically, two groups of logical channels exist—
control channels and traffic channels. These are shown in Figure 6-12.
The Broadcast Control Channel (BCCH) is used for the downlink transmission
of system information.The Paging Control Channel (PCCH) is used for the paging of an MS across one or more cells. The Common Control
Channel (CCCH) is used in the uplink by terminals that want to access the
network but do not already have any connection with the network. The
CCCH can be used in the downlink to respond to such access attempts. The
Dedicated Control Channel (DCCH) is a bidirectional point-to-point control
channel between the MS and the network for sending control information.
WCDMA also defines the Shared Channel Control Channel, but that channel
is used only in TDD mode.
Two types of logical traffic channels are available. The Dedicated Traffic
Channel (DTCH) is a point-to-point channel, dedicated to one UE, for the
transfer of user data. DTCHs apply to the uplink and the downlink. The
Common Traffic Channel (CTCH) is point-to-multipoint unidirectional
channel for the transfer of user data to all UEs or just to a single UE. The
CTCH exists in the downlink only.
Numerous options are available for mapping between logical channels
and transport channels. This mapping depends on a range of criteria such as
the types of information to be sent, whether it is to be sent to multiple UEs
(in the downlink), and whether the UE has already an established connection
with the network. The possible mapping between logical channels and
transport channels for the FDD mode of operation is shown in Figure 6-13.