A basic GSM configuration is shown in Figure above.
It consists of the basic system blocks of the GSM network namely the MSC (Mobile Switching Center), TRAU (TRANSCODER), BSC (Base System Controller, BTS ( Base Transceiver Station) and MS (Mobile Station).
In Figure above, the mobile station talks with the BTS through the use of full duplex radio transmissions using a separate transmit and receive frequency to communicate with each other.
The BTS transmits on the frequency the mobile is tuned to and the mobile transmits to the frequency the BTS receiver is tuned to.
The BTS in a system connect to the BSC through a transmission system. It can be one of these: Terrestrial transmission, Fiber Optic Transmission, Satellite Transmission, Leased line Transmission, etc.
From BSC , the connection is going to TRAU (Trans-coder), and converting or adapting the bit rates from 16 kbps to 64 kbps (PCM-Standard) , and the end of connection is MSC.
The MS is a radiotelephone that may be used whenever "cell" coverage is provided. The term "mobile phone" has been used generically to include several forms of wireless communication. This term represents fully portable cellular and digital phones in addition to hand-held and hands-free car phones.
The BTS is a transceiver facility that provides the cell coverage. The cell provided by a cell site can be from one mile to twenty miles in diameter, depending on terrain and transmission power. Several coordinated cell sites are called a cellular network.
The BSC is the controller of BTS and manage the group of BTS for doing something like handover, radio resources management.
The TRAU is a unit which has function to convert the bit rate from 16 kbps (GSM bit rates Full rate) to 64 kbps (PCM Standard).
The MSC is the switching parts. These systems is doing the function like allocate the traffic channel, arranges handoffs, call routing, keeps track of billing information, etc.You may want to know GRPS Architecture klik here.