GPRS Control Channels

Similar to GSM, GPRS requires a number of control channels. To begin
with, the Packet Common Control Channel (PCCCH), like the CCCH in
GSM, is comprised of a number of logical channels. The logical channels of
the PCCCH include
■ Packet Random Access Channel (PRACH) Applicable only in the
uplink, this is used by an MS to initiate a transfer of packet signaling
or data.
■ Packet Paging Channel (PPCH) Applicable only in the downlink,
this is used by the network to page an MS prior to a downlink packet
transfer.
■ Packet Access Grant Channel (PAGCH) Applicable only in the
downlink, this is used by the network to assign resources to the MS
prior to packet transfer.
■ Packet Notification Channel (PNCH) This is used for Point-to-
Multipoint Multicast (PTM-M) notifications to a group of MSs.
The PCCCH must be allocated to a different RF resource (that is, a different
timeslot) from the CCCH. The PCCCH, however, is optional. If it is
omitted, then the necessary GPRS-related functions are supported on the
CCCH.
Similar to the BCCH in GSM, GPRS includes a Packet Broadcast Control
Channel (PBCCH). This is used to broadcast GPRS-specific system information.
Note, however, the PBCCH is optional. If the PBCCH is omitted,
then the BCCH can be used to carry the necessary GPRS-related system
information. If the PBCCH is provisioned in a cell, then it is carried on the
same timeslot as the PCCCH in the same way that a CCCH and BCCH can
be carried on the same timeslot in GSM.
In the case where a given timeslot is used to carry control channels
(PBCCH or PCCCH), then radio block 0 is used to carry the PBCCH, with
up to three additional radio blocks allocated for PBCCH. The remaining radio blocks are allocated to the various PCCCH logical channels such as
PPCH or PAGCH.
Similar to GSM, GPRS supports some Dedicated Control Channels
(DCCHs). In GPRS, these DCCHs are the Packet Associated Control Channel
(PACCH) and the Packet Timing Control Channel (PTCCH). The
PTCCH is used for the control of the timing advance for MSs. The PACCH
is a bidirectional channel used to pass signaling and other information
between the MS and the network during packet transfer. It is associated
with a given Packet Data Traffic Channel (PDTCH) described in the following
section. The PACCH is not permanently assigned to any given resource.
Rather, when information needs to be sent on the PACCH, part of the user
packet data is pre-empted, in much the same manner as is done for the
FACCH in GSM.