1. Random access channel (RACH)
It is an uplink channel used for MS randomly access to
network by requesting for an SDCCH. The request includes a 3bit setup reason
(call request, paging response, location update request and short message
request etc.) and a 5bit random reference number for MS to differentiate the
access granted messages.
2. Stand-Alone Dedicated Control Channel (SDCCH)
It is a bi-directional dedicated channel used in
transmission of signaling messages concerned with connection setup, location
update message, short message, authentication message, encryption command,
channel allocation message and various kinds of additional services etc. It can
be divided into the Stand-Alone Dedicated Control Channel (SD/8) and the
Dedicated Control Channel in combination with CCCH (SD/4).
3. Slow associated control channel
(SACCH)
It is used together with the
traffic channel or SDCCH. It carries some specific information while
transmitting the subscriber information. At the uplink, it mainly transmits the
measurement report ; while at the downlink, it mainly transmits some system information.
These messages include the quality of communication, LAI, CELL ID, BCCH signal
strength of the adjacent cell, NCC permit, cell option, TA and power control
level etc.
4. Fast associated control channel
(FACCH)
It is used together with TCH for
providing signaling messages whose speed and timeliness are much higher than
the slow associated control channel (SACCH) for the system during the
transmission process. This channel uses frames borrowed from the traffic channel
for its connection and transmits such instruction messages as “handover”. For
the voice decoder can repeat the voice of the last 20ms, this kind of
interruption due to frame stealing will not be detected by the subscriber.
Besides the three types of control channels described above, there is a cell
broadcast control channel (CBCH). It is used at the downlink and carries the short message service cell
broadcast (SMSCB) information. This kind of control channel uses the same
physical channel as that used in SDCCH.
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