Types of Air Interface Channels

The foregoing description of the RF interface suggests that only traffic
channels and control channels exist. This is only partly correct. In fact,
there are traffic channels, numerous types of control channels, and a number
of other channels. To begin with, a number of broadcast channels are
available:
■ Frequency Correction Channel (FCCH) This is broadcast by the
BTS and used for frequency correction of the MS.

■ Synchronization Channel (SCH) This is broadcast by the BTS and
is used by a mobile station for frame synchronization. It addition to
frame synchronization information, it also contains the Base Station
Identity Code (BSIC).
■ Broadcast Control Channel (BCCH) This is used to broadcast
general information regarding the BTS and the network in general. It
is also used to indicate the configuration of the Common Control
Channels (CCCH) described in the following section.
The CCCH is a bidirectional control channel used primarily for functions
related to initial access by a mobile station. It has a number of components:
■ Paging Channel (PCH) This is used for the paging of mobile stations.
■ Random Access Channel (RACH) This is used only in the uplink
direction. It is used by a mobile station to request the allocation of a
Stand alone Dedicated Control Channel (SDCCH) described later.
■ Access Grant Channel (AGCH) This is used in the downlink in
response to an access request received on the RACH. It is used to
allocate an MS to an SDCCH or directly to a Traffic Channel (TCH).
■ Notification Channel (NCH) This is used with voice group call and
voice broadcast services to notify mobile stations regarding such calls.
A number of dedicated control channels exist. These are channels that
are used by one mobile station at a time, typically either during call establishment
or while a call is in progress. The dedicated control channels are
as follows:
■ Stand Alone Dedicated Control Channel (SDCCH) This is a
bidirectional channel used for communication with an MS when the
MS is not using a TCH. The SDCCH is used, for example, for Short
Message Service (SMS) when the MS is not in a call. It is also used for
call establishment signaling prior to the allocation of a TCH for a call.
■ Slow Associated Control Channel (SACCH) This is a
unidirectional or bidirectional channel, used when the MS is using a
TCH or SDCCH. For example, when an MS in engaged in a call on a
TCH, power control messages from a BTS to an MS are sent on the
SACCH. In the uplink, the MS sends measurement reports to the BTS
on the SACCH. These reports indicate how well the MS can receive
transmissions from other BTSs and the information is used in
determination of if or when a handover should occur. The SACCH is
also used for short message transfers when the MS is in on a TCH.
■ Fast Associated Control Channel (FACCH) This is associated
with a given TCH and thus is used when the mobile is involved in a
call. It is typically used to transmit non-voice information to and from
the MS. Such information would include, for example, handover
instructions from the network, commands from the MS for generation
of DTMF tones, supplementary service invocations, and so on.