GSM CHANNEL

GSM Frequency band

•          900 Band :- Uplink 890-915MHz, Downlink 935-960MHz, Channel spacing 200 KHz, Carrier Frequency 124, Duplex Spacing 45 MHz
•         1800 Band:- Uplink 1710-1785 MHz, Downlink 1805-1855MHz, channel spacing 200 KHz, Carrier Frequency 374, Duplex Spacing 95 MHz
•     1900 Band:- Uplink 1850-1910 MHz, Downlink 1930-1990 MHz, Channel spacing 200 KHz, Carrier Frequency 299, Duplex Spacing 80 MHz

Uplink:-      The communication going from a satellite to ground.

Downlink:- The communication going from ground to satellite.

GSM bandwidth allocation 

GSM CHANNELS

CHANNEL:- Channel is the medium which through we receive and transmits signals or information from one place to another place.

TRAFFIC CHANNEL(TCH):- It carry digitally encoded user speech or user data and have identical function and formats on both the forward and reverse link.

•   When transmitted as full-rate, user data is contained within one Ts per frame.
•  When transmitted as half-rate, user data is mapped onto the same time slot, but is sent in       alternate frames.

CONTROL CHANNEL:- It carry signalling and synchronizing commands between the base station and the mobile station.

BROADCAST CHANNELS(BCH):- Broadcast channel operates on the forward link of a specific ARFCN within each cell, and transmits data only in the first timeslot(TS0) OF Certain GSM frames

1. BROADCAST CONTROL CHANNEL(BCCH):- The BCCH is a forward information such as cell and network identity and operating characteristics of the cell .  The BCCH also broadcast a list of channels that are currently in use within the cell.
2. FREQUENCY CORRECTION CHANNEL(FCCH):- The FCCH is a special data between which occupies TS0 for the very first GSM frame and is repeated every ten frames within a control channel multi-frames.  The FCCH allows each subscriber unit to synchronize its internal frequency standard (local oscillator) to the exact frequency of the base station.
3. SYNCHRONIZATION CHANNELS(SCH):- It is used to identify the serving base station while allowing each mobile to frame synchronize with the base station .  The SCH is transmitted once every ten frames within the control channel multi-frame.

COMMON CONTROL CHANNELS(CCCH):- CCCH are the most commonly used control channels and are used to page specific users and receive mobile request for service.

1. PAGING CHANNEL(PCH):- It is forward link channel . The PCH provides paging signals from the base station to all mobiles in the cell, and notifies a specific mobile of an incoming call which originates from the PSTN. The PCH transmit the IMSI of the target subscriber along with a request for acknowledgement from the mobile unit on the RACH.
2. RANDOM ACCESS CHANNEL(RACH):-It is a reverse link channel , used by a subscriber unit to acknowledge a page from the PCH, and is also used by mobile to originate a call.
3. ACCESS GRANT CHANNEL(AGCH):- The AGCH is used by the base station to provide forward link communication to the mobile and carrier data which instructs the mobile to operate in a particular physical channel with a particular dedicated control channel.

DEDICATED CONTROL CHANNEL(DCCH):- DCCH are bi-directional and have the same format and function on the both the forward and reverse link.

1. STAND-ALONE DEDICATED CONTROL CHANNEL(SDCCH):- The SDCCH carries signalling data following the connection of the mobile with the base station and just before a TCH assignment is issued by the base station. The SDCCH ensures that the mobile station and the base station remain connected while the base station and MSC verify the subscriber unit and allocate resources for the mobile. The SDCCH is used to send authentication and a list messages as the mobile synchronize itself with the frame structure and waits for  a TCH.
2. SLOW ASSOCIATED CONTROL CHANNEL(SACCH):- SACCH carries general information between the MS and BTS.  On the forward link, the SACCH IS used to send slow but regularly changing control information to the mobile , such as transmit power level instruction and specific timing advance instructions for each user on the ARFCN. On the reverse SACCH carries information about the received signal strength and quality of the TCH ,as well as BCH measurement results from neighbouring cells.
3. FAST ASSOCIATED CONTROL CHANNEL(FACCH):- FACCH carries urgent message and contains essentially the same type of information as the SDCCH.