11-10-2012, 05:18 PM
GSM Tutorial
GSM Tutorial.docx (Size: 167.61 KB / Downloads: 44)
What is GSM?
If you are in Europe, Asia or Japan and using a mobile phone then most probably you must be using GSM technology in your mobile phone.
• GSM stands for Global System for Mobile Communication and is an open, digital cellular technology used for transmitting mobile voice and data services.
• The GSM emerged from the idea of cell-based mobile radio systems at Bell Laboratories in the early 1970s.
• The GSM is the name of a standardization group established in 1982 to create a common European mobile telephone standard.
• The GSM standard is the most widely accepted standard and is implemented globally.
• The GSM is a circuit-switched system that divides each 200kHz channel into eight 25kHz time-slots. GSM operates in the 900MHz and 1.8GHz bands in Europe and the 1.9GHz and 850MHz bands in the US.
• The GSM is owning a market share of more than 70 percent of the world's digital cellular subscribers.
• The GSM makes use of narrowband Time Division Multiple Access (TDMA) technique for transmitting signals.
• The GSM was developed using digital technology. It has an ability to carry 64 kbps to 120 Mbps of data rates.
• Presently GSM support more than one billion mobile subscribers in more than 210 countries throughout of the world.
• The GSM provides basic to advanced voice and data services including Roaming service. Roaming is the ability to use your GSM phone number in another GSM network.
Why GSM?
The GSM study group aimed to provide the followings through the GSM:
• Improved spectrum efficiency.
• International roaming.
• Low-cost mobile sets and base stations (BSs)
• High-quality speech
• Compatibility with Integrated Services Digital Network (ISDN) and other telephone company services.
• Support for new services.
GSM network areas:
In a GSM network, the following areas are defined:
• Cell: Cell is the basic service area: one BTS covers one cell. Each cell is given a Cell Global Identity (CGI), a number that uniquely identifies the cell.
• Location Area: A group of cells form a Location Area. This is the area that is paged when a subscriber gets an incoming call. Each Location Area is assigned a Location Area Identity (LAI). Each Location Area is served by one or more BSCs.
The GSM Specifications
Specifications for different Personal Communication Services (PCS) systems vary among the different PCS networks. The GSM specification is listed below with important characteristics.
Modulation:
Modulation is a form of change process where we change the input information into a suitable format for the transmission medium. We also changed the information by demodulating the signal at the receiving end.
The GSM uses Gaussian Minimum Shift Keying (GMSK) modulation method.
Access Methods:
Because radio spectrum is a limited resource shared by all users, a method must be devised to divide up the bandwidth among as many users as possible.
GSM chose a combination of TDMA/FDMA as its method. The FDMA part involves the division by frequency of the total 25 MHz bandwidth into 124 carrier frequencies of 200 kHz bandwidth.
One or more carrier frequencies are then assigned to each BS. Each of these carrier frequencies is then divided in time, using a TDMA scheme, into eight time slots. One time slot is used for transmission by the mobile and one for reception. They are separated in time so that the mobile unit does not receive and transmit at the same time.
• MSC/VLR Service Area: The area covered by one MSC is called the MSC/VLR service area.
• PLMN: The area covered by one network operator is called PLMN. A PLMN can contain one or more MSCs.