Downlink Logical Channel in GSM
In 2G GSM Technology in forward direction Downlink (GSM BTS to GSM Mobile). There are mainly five Downlink Logical Channel in GSM.
- Frequency correction channel (FCCH)
- Synchronous channel (SCH)
- Broadcast control channel (BCCH)
- Paging channel (PCH)
- Access granted channel (AGCH)
Frequency correction channel (FCCH)
First start with FCCH which full form is frequency correction channel in Downlink logical channel in GSM. It carries the information for frequency correction for the mobile station. The MS can communicate with a cell and demodulate other information of the same cell just via FCCH. Moreover, the MS can also know whether the carrier is a BCCH carrier via FCCH in downlink.
Synchronous channel (SCH)
The Second channel is SCH channel which full form is Synchronous channel in downlink logical channel in GSM. After FCCH decoding, the MS will continue to decode the SCH channel message. This message includes the information for MS frame synchronization and BS identification: Base Station Identifying Code (BSIC). It seizes 6 bits, in which 3 bits are PLMN color codes ranging between 0~7; while the remaining 3 bits are Base Station Color Codes (BCC) ranging between 0~7.
Here one more thing about sch logical channel of gsm in note is – The simplified TDMA frame number (RFN) seizes 22 bits.
Broadcast control channel (BCCH)
Generally, there is always a BCCH channel (full form is Broadcast control channel) in every cell, which is responsible for broadcasting system information to the mobile station. This system information enables the MS to identify and access network at the idle mode. BCCH is main logical channel among all five gsm logical channel.
Paging channel (PCH)
There is always one channel required to page anything in general word call to mobile for call and any update to mobile which first direct using PCH logical channel in 2G GSM. This is a downlink channel which is used to page mobile stations. When the network is to set up communication with a certain MS, it will send paging messages via the PCH channel to all cells in the LAC area in which the certain MS has currently registered, and indicates TMSI or IMSI of the certain mobile.
Access granted channel (AGCH)
Now, as similar to PCH, mobile also required to ask for sms and call and that time AGCH in picture means when mobile try to access network using ACH GSM logical channel than BTS will replay to mobile using AGCH that your access is granted so don’t try more access I will give you traffic channel that you can use that for send real traffic(Voice). In Simple, AGCH GSM Logical channel is a downlink channel used in answering a network access request by the mobile station, i.e. allocation of an SDCCH or a TCH directly.