Channels in GSM Air interface
 Logical Channels : ARFCN –x, Timeslot – y,
Frame Number - N
Logical Channels
Control Channels Traffic Channels

Logical channel
 One or more logical channel can be
transmitted on a physical channel.
Different types of logical channel are –
 Traffic Channels (Speech & Data)
 Broad cast channel (Signalling)
 Common control channel (Signalling)
 Dedicated control channel (Signalling)

TRAFFIC CHANNEL

Traffic Channels
Traffic Channels
TCH - F
TCH - H

 Traffic Channel carries the voice data
 Two blocks of 57 bits contain voice data
 One TCH is allocated for every active call. While call
is in progress if there is degradation in quality of
current channel, BTS may shift the communication
to another TCH on a different Carrier and/or
Timeslot
 A full rate TCH carries 13 kbps voice data, and Half
rate TCH carries a 6.5 kbps voice data

Traffic Channels (TCH)
One physical channel (1 timeslot) can support:
Traffic channels are used to send Speech or Data
services.
1 TCH/F or 2 TCH/H
• TCH/F: 13 kb/s voice or 9.6 kb/s data (22.8 Kbps after channel
coding)
• TCH/H: 6.5 kb/s voice or 4.8 kb/s data (11.4 kbps after channel
coding)
Both can be used to carry signalling information.
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

Speech channels are supported by two different
methods of coding known as Full Rate (FR) and
Enhanced Full Rate (EFR). Enhanced Full Rate
coding provides a speech service that has
improved voice quality from the original
Full Rate speech coding, whilst using the same air
interface bandwidth. EFR employs a new speech
coding algorithm and additions to the full rate
channel coding algorithm to accomplish this
improved speech service,

CONTROL CHANNEL

Concepts of Channels in GSM
 A company vehicle is used for several purposes in
a day
 Similarly in GSM, the timeslots are used for
different purposes at different times

These control channels are used by the MS to
establish communication with
the network in the idle mode and also in initiating
calls to enter the dedicated
mode. Timeslot 0 is grouped into structures of 51
frames referred to as
Control Channel Multiframes
The control channels are grouped as Broadcast
Control Channels (BCCH) Common Control
Channels (CCCH) and Dedicated
Control Channel (DCCH).

Broadcast Channels
(BCCH)

BCH (Broadcast Channel)
 One ARFCN, On all the time, in every cell
 Uses Timeslot 0 on a channel, in Downlink
 FCH – Frequency correction channel (allows mobile to
tune to BTS frequency. This channel carries a 142 bit zero
sequence and repeats once in every 10 frames on BCH)
 SCH – Synchronization channel (carries the Frame
number and BSIC). A midamble of 64 bit helps mobiles to
synchronize. It also repeats once in 10 Frames

 BCCH – Broadcast Control Channel. It has
information encoded on it which identifies the
network (LAC). It also carries list of the
channels in use in the cell (CA tables)
BCH (Broadcast Channel)

The Broadcast Control Channel is transmitted by the BTS at all times. The RF carrier
used to transmit the BCCH is referred to as the BCCH carrier. The information carried
on the BCCH is monitored by the MS periodically (at least every 30 secs), when it is
switched on and not in a call.
Each cell has one carrier designated as a BCH carrier. The BCH carrier has all 8 timeslots
continuously on, either with traffic or dummy bursts. Timeslot 0 of the BCH carrier
contains logical control channels.
Broadcast Control Channel (BCCH) – Carries the following information (this is only a
partial list):
 Location Area Identity (LAI).
 List of neighbouring cells which should be monitored by the MS.
 List of frequencies used in the cell.
 Cell identity.
 Power control indicator.
 DTX permitted.
 Access control (for example, emergency calls, call barring).

Information distributed over Broadcast channel helps the MS to orient themselves
in the mobile radio N/W .BCH channels are all downlink Pt. to Multi pt. and are
allocated to timeslot zero.
FCCH: Frequency correction channel
This is transmitted frequently on the BCCH timeslot and allows the mobile to
synchronize its own frequency to that of the transmitting base site. The FCCH may
only be sent during timeslot 0 on the BCCH carrier frequency and therefore it acts
as a flag to the mobile to identify Timeslot 0.
SCH: (Synchronisation channel )
The SCH carries the information to enable the MS to synchronize to the TDMA
frame structure and know the timing of the individual timeslots. The following
parameters are sent:
– Frame number.
– Base Site Identity Code (BSIC).
The MS will monitor BCCH information from surrounding cells and store the
information from the best six cells. The SCH information on these cells is also
stored so that the MS may quickly resynchronize when it enters a new cell.

Common Control Channels
(CCCH)

Common Control Channel
 CCCH shares Timeslot 0 with BCH on a
Multiframe
 CCCH consists of PCH, RACH and AGCH
 PCH – Paging channel is used to alert mobiles on
incoming calls. PCH carries IMSI to page for
Mobiles in the cell. PCH is Downlink channel.
 RACH – Random Access Channel is a short burst
sent by mobile to BTS, to initiate a call request.
RACH uses timeslot 0 on reverse BCH channel on
uplink

Common Control Channels (CCCH)
CCCH contains all point to multi-point downlink channels (BTS to
several MSs) and the uplink Random Access Channel:
RACH: (UL) Random Access Channel is sent by the MS to request a
resources from the network e.g. an SDCCH channel for call setup.
• AGCH (DL) Access Grant Channel is used to allocate a dedicated
channel (SDCCH) to the mobile.
• PCH (DL) Paging Channel sends paging signal to inform mobile of a call.
Active MSs must frequently monitor both BCCH and CCCH. The CCCH will be
transmitted on the RF carrier with the BCCH.

Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink / downlink) point to
point control channels .Used for signalling between MS & BTS:
• SDCCH: Standalone Dedicated control Channel is used for call set
up, location updating and also SMS.It carries all signalling B/W BTS
& MS when no TCH is allocated. Which supports the transfer of Data to
and from the MS during call setup and validation.
Summary—Used for services request, SMS,Location update ,subscriber
Authentication,Equipment validation. (Afterwards TCH Comes into
picture)
• SACCH: Slow Associated Control Channel is used for link
measurements and signalling during a call .Used for non urgent
procedures.
Summary—transmission of Signalling Data ,Radio Link Supervision
measurement ,Transmit power control & Timing advance data

FACCH
• 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.Used
mainly during handovers when SACCH Signalling
is not fast enough.FACCH data is transmitted over
TCH, this is marked by Flag called “Stealling Flag”

RACH – Random Access Channel
8
start
bits
41
Synchronization
bits
36
Encrypted
bits
3
stop
bits
68.25
Extended
Guard Period
88 bits
 Used by mobile to get attention from Base Station
 Several mobiles might originate RACH simultaneously
 Mobile doesn’t know path delay
 So RACH has to be a special short burst
 Mobile sends normal burst after getting timing advance on
downlink SACCH

DCCH – Dedicated Control Channels
 SDCCH – Standalone Dedicated Common Control
Channel. It is used as a interim channel before final
assignment of TCH. SDCCH is used for signaling and
Authentication message transfers.
It can be called the stepping stone between BSC and TCH
The SDCCH, by using less of the cells resource of
physical channels, improves efficiency, and provides a
useful holding channel for the mobile until speech data
needs to be exchanged.

Dedicated Control Channels (cont..)
 FACCH – Fast Associated Control Channel
Interrupts TCH on uplink and downlink
Rapid message exchange for handovers
Control bits on either side of midamble – 0 indicates
TCH and 1 indicates FACCH
Both ways – uplink and downlink

SACCH – Slow associated Control
Channel
 Downlink (BTS – MS)
 Mobile Tx Power Commands
 Mobile Timing Advance
 Cell’s Channel Configuration
 Uplink (MS - BTS)
 Received signal quality report (RXQual)
 Received signal level report (RXLev)
 Adjacent BCH power measurement

Dedicated Control Channels (DCCH)
The DCCH is a single timeslot on an RF carrier which is used to
convey eight
Stand-alone Dedicated Control Channels (SDCCH). A SDCCH is
used by a single MS for call setup, authentication, location
updating and SMS point to point.
As we will see later, SDCCH can also be found on a BCCH/CCCH
timeslot, this configuration only allows four SDCCHs.
Associated Control Channels (ACCH)
These channels can be associated with either an SDCCH or a
TCH. They are used for carrying information associated with the
process being carried out on either the SDCCH or the TCH.

CONTINUE
Slow Associated Control Channel (SACCH)
Conveys power control and timing information in the downlink direction
(towards the MS) and Receive Signal Strength Indicator (RSSI), and link
quality reports in the uplink direction.
Fast Associated Control Channel (FACCH)
The FACCH is transmitted instead of a TCH. The FACCH ‘‘steals” the TCH
burst and inserts its own information. The FACCH is used to carry out user
authentication, handovers and immediate assignment.
All of the control channels are required for system operation, however, in the
same way that we allow different users to share the radio channel by using
different timeslots to carry the conversation data, the control channels share
timeslots on the radio channel at different times. This allows efficient passing
of control information without wasting capacity which could be used for call
traffic. To do this we must organize the timeslots between those which will be
used for traffic and those which will carry control signalling.

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