There are two types of channels in GSM:
a.) Physical Channels
b.) Logical Channels
A physical channel is combination of Time slot and Carrier freq. One RF channel supports eight
physical channels in time slots 0, 1, 2, ----7.
A logical channel carries information of specific type:
Traffic channel (TCH) carries digitally encoded user speech or data and have same function
in both forward and reverse link.
Control channel carries signaling and synchronizing commands between BS and MS. Certain
type of control channels defined for forward and reverse link.
TCH Traffic Channel Full rate and Half rate.
When transmitted as full rate user data is contained within 1 time slot per frame. 22.8 Kbits/ps.
When transmitted as half rate user data is mapped onto same time slot but in alternate frames.
11.4 Kbits/ps.
*Four types of control channels:-*
1. Broadcast Control Channels.
2. Associated control Channels.
3. Dedicated Control Channels.
4. Common Control Channels.
Broadcast Channels: - operates on forward link and transmit data on first time slot. It
Contains.
1. *SCH (Synchronization Channel)* it is used to identify the serving BS and allowing
each mobile to frame synchronize with the BS. The frame no. is sent with the BSIC
during SCH burst. And also 6 bit BSIC.
2. *FCCH (Frequency Correction Channel)* The FCCH allows each MS to
synchronize its internal freq. with exact freq. of the BS.
3. *BCCH (Broadcast Control Channel)* It carries information’s such as cell and
network identity. It also broadcast a list of channels that are currently in use within
a cell.
4. *CBCH (Cell Broadcast Channel)* Used to transmit short alphanumeric text msg. to
all MS within a cell.
Common Control Channels (CCCH): - CCCH helps to establish the call from the MS. Three
different types of CCCH are defined.
1. *The Paging Channel (PCH)*. It is used to alert the MS of an incoming call.
2. *The Random Access Channel (RACH).* Is used by MS to access the network.
3. *The Access Grant Channel (AGCH)*. Is used by the Base Station to inform the MS
that which channel it should use.
Dedicated Control Channels (DCCH): - These channels are used for message exchange
between several mobiles or a mobile and network. Two types of DCCH are there.
1. *Stand Alone Dedicated Control Channels (SDCCH)*. Authentication, Registration,
Location area updation, SMS etc. needed for setting up a TCH.
2. *Slow Associated Control Channels (SACCH).*
Associated Control Channels: - Associated with the TCH.
1. Slow Associated Control Channel (SACCH). Associated with TCH, Channel
quality, Signal power level.
2. *Fast Associated Control Channel (FACCH).* Uses time slots from TCH, Handover
info.
