GSM Channels:Traffic Channels and

 

GSM provides a number of different types of channels for user traffic and signalling.

These channels are divided into two classes:

1. traffic channel                      
2. signalling channels   

Traffic Channels:TCH falls into two categories namely TCH Half(TCH/H) and TCH Full(TCH/F).A GSM traffic channel (TCH) carries speech and data traffic. Of each 26-frame multiframe, 24 frames are used for traffic channels, one is used for the slow associated control channel (SACCH) and one is unused.

TCH/H: A GSM half-rate traffic channel (TCH/H) has a raw rate of 11.4 Kbps, with a useable rate of 4.8 Kbps for data.

TCH/F: A GSM full-rate traffic channel (TCH/F) has a raw rate of 22.8 Kbps, with a useable rate of 9.6 Kbps for data.

Signalling Channels: There are three types of GSM signaling channels namely broadcast channels, common control channels and dedicated control channels.GSM signalling channels are used for the control of the GSM network.

Broadcast Channel:The GSM broadcast channels are used to provide information to a mobile station about the network and timing information required for synchronization.

Common Control Channel:The GSM common control channels are used to inform mobile stations of incoming calls and to request and grant channels.

Dedicated Control Channel:GSM dedicated control channels carry data used during connection establishment and for handover decisions.

2G Spectrum:GSM Frequency Bands 900/1800 Mhz

GSM 900 MHz:GSM(Global Sytem for Mobile Communication)is used by over 3 billion subscribers across the world.India has about 160 million GSM-based mobile subscribers according to figures released by the Cellular Operators Association of India (COAI). The subscriber base represents a significant increase on the 95.7 million GSM customers reported on October 2008 and shows healthy monthly growth.

Bharti Airtel is the major GSM operator in India with 50 million subscribers followed by Vodafone at 38 million subscribers as on October 2008 as released by COAI.

900/1800 Mhz:Most 2G GSM networks operate in the 900 MHz or 1800 MHz bands. Some countries in the Americas (including Canada and the United States) use the 850 MHz and 1900 MHz .Most 3G GSM networks in Europe operate in the 2100 MHz frequency band.

GSM 900 Mhz has uplink frequecy 880-915 Mhz  to send information from mobile station to base station and downlink frequency 935-960 Mhz for sending information to other directions.There are 125 RF Channels(0 to 124) spaced at 200 KHz.Time division multiplexing is used to allow eight full-rate or sixteen half-rate speech channels per radio frequency channel. There are eight radio timeslots (giving eight burst periods) grouped into what is called a TDMA frame. Half rate channels use alternate frames in the same timeslot.The channel data rate for all 8 channels is 270.833 kbit/s, and the frame duration is 4.615 ms.

GSM 1800 Mhz:1800 Mhz networks emit on a frequency around 1800 Mhz.A high frequency has a couple of big advantages. The digital network isn't interrupted by the influences of the weather.  Another advantage is that you only need a small antenna and because of this the portability of your cellular phone is a lot better. Although, the higher the frequency the smaller the reach.

GSM-1800 uses 1710–1785 MHz to send information from the mobile station to the base tranceiver station (uplink) and 1805–1880 MHz for the other direction (downlink), providing 374 channels (channel numbers 512 to 885). Duplex spacing is 95 MHz.

Evolution of Telecom Companies

TELECOM NETWORK OVERVIEW

A telecommunications network is the network of lines, trunks and telephone switches operated by telephone companies, long distance companies and Competitive Local Exchange Carriers. 

When referring to our local telecommunications service such as we would have in our homes, we generally refer to the Public Switched Telephone Network (PSTN) which is a heirarchial network that is heavily regulated to provide highly reliable service. Calls are placed on this network via a multi-frequency tone or rotary dial signalling.