How GSM Work – Part 2 (Channels in Mobile Networks)

Channels in GSM and Mobile Networks
What comes to your mind when you think of the world channels? Maybe you think of your favorite tv channels. Or you think of tuning to a channel on radio. In GSM, its not much different except that the tuning happens behind the scenes an you never get to know. The interesting thing about the GSM technology is that the complexity have been moved away from the normal users, so you never never gets to know how the system work. But I do think it would be ok to have a glimpse of what is happening between you mobile phone and other components of the GSM network, some of which you see everyday such as the communication mast. But in this lesson we would be looking at channels.

GSM Channels are divided into two broad categories:

  1. Logical Channels
  2. Physical Channels

This lesson discusses Logical Channels.

Also Logical Channel can be either:

  • Traffic Channel: Used to Carry voice or data
  • Control Channel: Used to carry only data

Control Channels: Control Channels are used to exchange management data between your mobile phone and the BTS(Base Transceiver Station) within a cell. Remember that only Control channels carry only data, no voice.
A specific frequency called the beacon frequency (read in up on …) is allocated for the control channel. Lets now examine the three types of control channels.

Types of Control Channels
There are three main categories of Control Channels, namely
1. Broadcast Control Channel (BCCH)
2. Common Control Channel (CCCH)
3. Dedicated Control Channel (DCCH)
Broadcast Control Channel(BCCH)
This is a downlink channel. This means that it carries data from the BTS to mobile phones and not the other way. So it is a unidirectional channel. It is called Broadcast Channel because it is used to broadcast data from the BTS to all mobile phones in the cell.
So is it a point-to multipoint channel. Try to understand the terms.
This channel is used for call set up(remember call set up- read it up on…) and synchronization between your mobile phone and the BTS.

Some of the Broadcast Control Channels are

1. Frequency Correction Channel: Carries a sequence of 148 zeroes transmitted by the BTS and enables the mobile phone to synchronize to the right frequency of the BTS.

2. Synchronization Channel: This is a downlink channel. This follows the FCCH and contains the BTS identification and location information.

3. Broadcast Control Channel: Don’t get confused! This is also under itself. It is not an error! This is a downlink channel. It carries information such as frequency allocation information used by cell phones to adjust their frequencies to that of the mobile network.

Data carried includes:
* Lis of frequencies used in a cell
* Cell identity information
* Location area Identity(LAI)
* List of neighbor cells
* Access and power control information etc
The BCCH is continuously being broadcast by the BTS.
Common Control Channels (CCCH)
This is both uplink and downlink, that is bidirectional. Just like BCCH, it is point-to-multipoint.
It carries signaling information for management and access control functions.

Some of the Common Control Channels are:

1. Paging Channel(PCH): This is a downlink channel used for paging purposes (read up paging..). The BTS uses this channel to inform the mobile phone about an incoming call. The mobile phone periodically monitors these channel from time to time.

2. Random Access Channel(RACH): This is an uplink channel. It is used by the cell phone to set up a call. The cell phone uses this channel to send a request to the BTS when a dedicated channel has not been assigned. This channel is used during call set up.

3. Access Grant Channel(ACGH): This is a downlink channel. After the mobile phone request for dedicated channel using the RACH, the BTS uses ACGH, to send acknowledgement back to the cell phone. This channel is also used during call set up.

4. Cell Broadcast Channel(CBCH): A downlink channel. Works like the SDCCH (don’t worry much about this)



Dedicated Control Channel (DCCH)
This is a point-to-point bi-directional channel. Used during call for managing calls. They are called dedicated because they are used to communicate with a single mobile phone unlike the broadcast channel. Dedicated Channels can be standalone or associated. Some dedicated control channels are:

Standalone Dedicated Control Channel (SDCCH): This channel is used during call set-up along with SACCH to send and receive messages and other signaling information between the phone and the BTS.

Associated Control Channel (ACCH): Used along with SDCCH or TCH. There are two of them namely: FACCH and SACCH

-Slow Associated Control Channel (SACCH): Carries information such as power control information form the BTS to the mobile. The mobile sends back acknowledgement back to the BTS using this channel.

-Fast Associated Control Channel (FACCH): This channel is used to transmit unscheduled and urgent messages between mobile and BTS. SACCH carry only 4 messages while FACCH carry up to 50 messages. This channels is also used for user authentication and handovers.


Summary of all the channels we’ve discussed so far is illustrated in the figure below.


Logical Channels in GSM and Mobile Networks