GSM (or Global System for Mobile Communications) was developed in 1990. The first GSM operator has subscribers in 1991, the beginning of 1994 the network based on the standard, already had 1.3 million subscribers, and the end of 1995 their number had increased to 10 million!
There were first generation mobile phones in the 70's, there are 2nd generation mobile phones in the 80's and 90's, and now there are 3rd gen phones which are about to enter the Indian market. GSM is called a 2nd generation, or 2G communications technology.
GSM uses a combination of TDMA (time devision multiplexing)and FDMA(frequency devision multiplexing). This means that users A and B are not only sharing the channel in time but also frequency. This means that user A is on the channel 890Mhz for 2 seconds, then jumps to 900Mhz channel for the next to seconds, then jumps to 910Mhz for the next 2 seconds and so on... Thus, each user is uses a different frequency at different time slots. This is called Frequency Hopping. there are gsm 900,1200,1800,2100etc these days.we will tell you about the gsm900.900 is the operational frequency of the gsm in mega hz. there are 125 up link channel and 125 down link. uplink channel means from mobile to base station and downlink means from base station to the mobile. 124 channel reserved for communication and 1 is used for safe guard(100 hz in each)which provide a separation bw the channel.