7.GSM Channels

GSM Channels

Number of channels required during call set-up (1)

Channel to transmit information to help the mobile station to tune into the network.

Channel to transmit synchronisation information.

Channel to transmit information about the network to help the mobile know about the frequencies being used in its cell as well as in surrounding cells.

Number of channels required during call set-up (2)

Channel to transmit mobile station’s request to initiate call set-up.

Channel to set up a call. Channel to transmit handover

information. Channel to page the called party. Channel to transmit measurements. Channel to transmit actual

conversations.

Channel Organization Each timeslot on a TDMA frame is called

a physical channel. Therefore, there are 8 physical channels

per carrier frequency in GSM. The paths used to carry information

between a Mobile Station and a BTS is known as the Physical Channel.

Physical channels can be used to transmit speech, data or signaling information.

A physical channel may carry different messages, depending on the information that is to be sent.

These different information carried on the Physical channel are classified as Logical Channels.

For example, on one of the physical channels used for traffic, the traffic itself is transmitted using a Traffic Channel (TCH) message, while a handover instruction is transmitted using a Fast Associated Control Channel (FACCH) message.

Logical Channels Many types of logical channels

exists each designed to carry a different message to or from an MS.

Logical Channels are divided into two groups

1) Control Channels2) Traffic Channels

Control Channels Broadcast Channels (BCH)

Frequency Correction Channel (FCCH) Synchronization Channel (SCH) Broadcast Control Channel (BCCH)

Common Control Channels (CCCH) Paging Channel (PCH) Random Access Channel (RACH) Access Grant Channel (AGCH)

Dedicated Control Channels (DCCH) Stand alone Dedicated Control Channel (SDCCH) Slow Associated Control Channel (SACCH) Fast Associated Control Channel (FACCH)

Control Channels

Control channels carry signaling information

used by the MS to locate a BTS, synchronize

itself with the BTS, and receive information

required to perform the call setup.There are three categories of Control

Channels:6) Broadcast Control7) Dedicated Control8) Common Control

Broadcast Channels

All Broadcast Channels are transmitted point to multi-point over the downlink

Frequency Correction Channel (FCCH):Provides the frequency correction information used by the MS The MS searches for this channels to

switch on. Downlink

FCCH contd..,

On FCCH, information containing only zeroes are transmitted.

This serves two purposes. First to make sure that this is the BCCH carrier, and second to allow the MS to read the frequency.

FCCH is transmitted downlink only.

Synchronization Channel (SCH): After locking to the frequency the MS

synchronises with the SCH. Contains the Base Station Identity

Code (BSIC) and the TDMA frame number used for synchronization of the MS to the new BTS

Broadcast Control Channel The MS must receive some general

information concerning the cell in order to start roaming, waiting for calls to arrive or making calls.

The needed information is broadcast on theBroadcast Control CHannel (BCCH) and includes

List of frequencies used in the cell Frequency hopping sequence List of neighbouring cells which should be

monitored by MS Location area Identity Max transmission power allowed in the cell

Using FCCH, SCH, and BCCH the MS tunes to a BTS and synchronized with the frame structure in that cell.

The BTSs are not synchronized to each other.

Therefore, every time the MS camps on another cell, it must listen to FCCH, SCH and BCCH in the new cell.

Common control channels

Random Access Channel:• Used by a MS to request access to the

system. • RACH information is transmitted over

the uplink• Used for e.g. mobile originated calls.

Paging Channel (PCH) Used to page the mobile station. At certain time intervals the MS listens to the

PCH to check if the network wants to make contact with the MS.

The reason why the network may want to contact the MS could be an incoming call or an incoming short message.

So, MS actively listen to PCH to check contact info within certain time.

PCH information is transmitted over the downlink.

The MS listens to the PCH to determine when it is being paged.

When the MS is paged, it replies on the RACH requesting a signaling channel.

RACH can also be used if the MS wants to contact the network.

For example, when setting up a mobile originating call.

Access Grant Channel (AGCH): Used by the BTS to assign a

dedicated control channel. (SDCCH) A downlink channel only.

Dedicated channels

Standalone Dedicated Control Channel (SDCCH):

• Carries signaling information between BTS and MS during call setup. ex: authentication

• The traffic channel (TCH) is assigned by using SDCCH.

• Bi-directional channel.

Dedicated channels contd..,

Slow Associated Control Channel (SACCH):

• Transmits call control data, measurement reports during connection.

• Carries control and measurement parameters along with routine data necessary to maintain a radio link between the MS and the BTS

• Bi-directional channel.

Dedicated channels contd..,

Fast Associated Control Channel (FACCH):

• Carries urgent signaling information• For quick control communication, e.g.

handover.• Physically replaces speech, “stealing

mode”

Dedicated channels contd..,

FACCH works in stealing mode meaning that one 20 ms segment of speech is exchanged for signaling information necessary for the handover.

Under normal conditions the subscriber does not notice the speech interruption because the speech coder repeats the previous speech block.

This channel is used only if required.

Traffic Channels Traffic Channels carry either speech or

data. There are three types of traffic channels:1) Full-Rate • Bi-directional channel.• Used for speech or data transmission.• User data bit rate 13 kbit/s

6) Half-Rate • Bi-directional channel.• Used for speech or data transmission.• User data bit rate 6-7 kbit/s.

3) Enhanced Full Rate (EFR)• Bi-directional channel.• Used for high quality speech

transmission.• User data bit rate 13 kbit/s.