4.4 GSM Air Interface Channels

GSM Chhaannells

GSM defines two fundamental channel types:

• Physical Channels:

    • the individual channels carried by a radio frequency carrier
    • Each carrier comprises 8 time-separated channels

• Logical Channels:

    • time-dependant virtual channels carried on a single physical
       channel
    • one physical channel may support one or multiple logical channels

GSM Physical Channels

• GSM employs both FDMA and TDMA at the Air Interface
• Each BTS may comprise a number of TRXs, with the carrier of each TRX
operating on a different frequency (FDM)
• Each GSM carrier supports 8 time-separated physical channels (TDMA)
• Each physical channel is allocated to a specific timeslot on the carrier
• A group of 8 timeslots on a carrier is known as a TDMA frame
One burst of data (0.577 ms or 156.25 bit periods) is a physical channel. This is used via
multiframe structures to provide all the logical channels required.

GSM Logical Channels

• Two types of logical channel are defined; traffic and control channels
• Each is further sub-divided as shown:

The naming of the GSM logical channels is as follows:
TCH Traffic Channels
TCH/F Traffic Channel (full rate) (U/D)
TCH/H Traffic Channel (half rate) (U/D)
BCH Broadcast Channels
FCCH Frequency Correction Channel (D)
SCH Synchronisation Channel (D)
BCCH Broadcast Control Channel (D)
CCCH Common Control Channels
PCH Paging Channel (D)
RACH Random Access Channel (U)
AGCH Access Grant Channel (D)
CBCH Cell Broadcast Channel (D)
NCH Notification Channel (D)
DCCH Dedicated Control Channels
SDCCH Stand alone Dedicated Control Channel (U/D)
SACCH Slow Associated Control Channel (U/D)
FACCH Fast Associated Control Channel (U/D)
U = Uplink D = Downlink

Traffic Channels (TCH)

• One physical channel (1 timeslot) can support:
• 1 TCH/F or 2 TCH/H
• TCH/F: 13 kb/s voice or 9.6 kb/s data
• TCH/H: 6.5 kb/s voice or 4.8 kb/s data
0 1 2 3 4 5 6 7
5 6 7 0 1 2 3 4
BTS transmits:
MS transmits:
Uplink / Downlink Synchronisation
The MS transmit burst is delayed by 3 timeslots after the BTS
burst.. This delay allows enables:
• Use of the same UL and DL timeslot number in TDMA frame
• Avoids simultaneous Tx/Rx requirement
• Allows for timing advance (TA)
• Allows time to switch between Tx and Rx

Full Rate traffic channels (TCH/F) and Enhanced Full Rate (EFR) traffic channels are allocated
dedicated physical channels in form of one timeslot allocated per TDMA frame.
If Half Rate Traffic Channels are implemented, two traffic channels are allocated to single
physical channel with a proportional reduction in available bit rate per traffic channel as
shown above.
A three timeslot delay exists between downlink and uplink TDMA frames. The purpose of
this delay is:
· To enable the same timeslot to be used for the both up and down link within the same
TDMA frame without the requirement for simultaneous transmit and receive.
· Allows Timing Advance to be implemented which can reduce the 3-timeslot delay.
· To provide time for the MS to switch between transmit and receive modes.

Broadcast Channels (BCH)

• FCCH: Frequency control channel sends the MS a burst of
all ‘0’ bits which acts as a beacon and allows MS to fine
tune to the downlink frequency and time-synchronise.
• SCH: Synchronisation channel enables TDMA-Frame
number synchronisation by sending the absolute value of
the frame number (FN), together with the BTS’s BSIC
• BCCH: Broadcast Control Channel sends network-specific
information such as radio resource management and control
messages, Location Area Code etc.
BCH channels are all downlink and are allocated to
timeslot zero. BCH channels include:

Common Control Channels (CCCH)

CCCH contains all point to multi-point downlink channels (BTS to
several MSs) and the uplink Random Access Channel:
• RACH: Random Access Channel is sent by the MS to request a
resources from the network e.g. an SDCCH channel for call setup.
• AGCH: Access Grant Channel is used to allocate a dedicated channel
(SDCCH) to the mobile.
• PCH: Paging Channel sends paging signal to inform mobile of a call.
• CBCH: Cell Broadcast Channel is an optional GSM Phase II
implementations for SMS broadcast messages, for example road
traffic reports or network engineering messages.
• NCH: Used for GSM Phase II voice services such as Voice Broadcast
Service (VBS) or Voice Group Calling Service (VGCS).

Dedicated Control Channels (DCCH)

• SDCCH: Standalone Dedicated Channel is used for call
set up, location updating and also SMS
• SACCH: Slow Associated Control Channel is used for link
measurements and signalling during a call
• FACCH: Fast Associated Control Channel is used (when
needed) for signalling during a call, mainly for delivering
handover messages and for acknowledgement when a
TCH is assigned