In telecommunication, all the communication happens over network using channels. Here I have provided the information about Logical channels in GSM. There are different types of channels and they perform specific functions. Mainly, we can see two types of logical channels – Traffic channels and Signalling channels.
Traffic Channel: This channel carries the encoded speech or user data. It has two types:
Full rate TCH: It carries the data at a gross rate of 22.8 K bits/sec.
Half rate TCH: It carries the data at a gross rate of 11.4 K bits/sec.
Signaling Channels: This helps to set up, control and terminate the calls. There are three types of signalling channels:
1. Broadcast channels(BCH)
2. Common control channels(CCCH)
3. Dedicated control channels(DCH)
Broadcast Channels: This channel is used by the BS’s (Base station) to provide the signaling information to the Mobile station (MS). BCH is downlink channel in GSM. This has been categorized into three types:
MS needs to synchronize to the time structure within the particular cell which is assigned by the BS. SCH provides the information to the MS about the Base station (like identifying it and synchronizing with it).
Frequency Correction Channel (FCCH): This has two functionalities. First to make sure that this is BCCH carrier and second it allows MS to synchronize to corresponding frequency.
Broadcast Control Channel: In order to start roaming, waiting for calls to arrive or making calls, the MS must receive some general information concerning the cell. This information is broadcast on BCCH and it includes LAI(Location Area Identity), maximum output power allowed in the cell and also BCCH carriers for the neighbouring cells on which the MS performs measurements.
Common Control Channel (CCCH) : This channel is used between the Base Transceiver Station and MS to request and give access to the network. This channel is subdivided into three types:
Paging Channel (PCH): This is downlink channel. At regular intervals of time, MS listen to PCH to check if the network wants to contact MS, to check if it has any incoming calls or incoming short messages. Data on the PCH is a paging message and it includes the information like IMSI or TMSI.
RACH (Random Access Channel): MS listens to the PCH to determine when it is being paged. When it get paged, it will replies via RACH requesting a signalling channel. Also RACH will be used when MS wants to make the contact with network.
AGCH (Access Grant channel): It is a downlink channel. Network assigns SDCCH to MS via AGCH.
Dedicated Control Channels (DCCH): This channel controls the other traffic channels in the GSM network.
Slow Associated Control Channel (SACCH): This is both uplink and downlink channel. On uplink, it will send across the information about signal strength and quality with the BTS. On the downlink, it receives the information about transmitting power to use and instructions on timing advance.
Cell Broadcast Channel (CBCH): It is used to carry SMS cell Broadcast. This is downlink channel....