GSM Vs CDMA - A Comparative Study

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Services Provided

GSM technology allows users to send and receive data at rates up to 9600 bytes per second (bps). Since GSM is a digital network, a modem is not required between the user and the network. But an audio modem is required inside the GSM network to inter-work with POTS19 (Plain Old Telephone Service) since the latter is an analog service. Although GSM currently has a low data transfer rate, new developments like HSCSD20 (high-speed circuit switched data) and GPRS (general packet-switched radio) will improve this rate considerably in future. GSM is useful for offering roaming services. Phones using the roaming facility can be switched from one network of cells to another, thus allowing subscribers to use the phone virtually around the world. GSM systems also offer Short Message Service (SMS). SMS is a bidirectional service for short alphanumeric messages (up to 160 bytes). Using this service, a subscriber can send a message to another subscriber of the service and also receive an acknowledgment of the message. SMS can also be used in a cell-broadcast mode21 to send messages such as traffic updates and news updates. Messages can also be stored in a SIM card so that they can be retrieved later.

Several additional services are provided apart from the basic telecom services. The first phase of GSM specifications (released in 1990) provided facilities such as call forward (forwarding the call when the mobile subscriber cannot be reached by the network) and call barring of outgoing or incoming calls, for example when roaming in another country (restricting incoming calls and thus keeping a complete control of the mobile communications expense). Many more services were added in the second phase of GSM specifications (released around 1991) such as caller identification (displaying the number or name of the person calling), call waiting (holding on to a call while answering another call so that no call goes unanswered) and multi-party conversations (communicating with more than 2 people at a time).

Working of GSM

GSM uses a technique called Time Division Multiple Access (TDMA). Using this technique, it divides the frequency band or a part of the electromagnetic spectrum available with a particular phone company into tiny slots and allots each slot to a particular caller. The operators have to confine all their activities to these bands and tune their frequencies to it. This technology was adopted by many global players in 1995.

GSM divides its users by frequency channels and time slots. The available spectrum (900MHz²² – 1800MHz in India) is split into 200KHz frequency channels, which are then further divided into time slots. The users of a channel take turns in their allotted time slots. At a particular instant of time, only one person uses the channel, and that too for very short bursts of time (about 577 micro seconds).

In order to optimise the frequency of use and cover large areas, the total available channels are split into 7 reusable groups. The adjacent cells in GSM use different frequency channels and hence there is no interference at the boundaries of the cells.

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19] The original common analog telephone system, which is still in worldwide use today.
20] Provides data transmission at rates up to 38.4 kbps, four times faster than the data rates of GSM communication.
21] A feature which allows messages of up to 93 characters to be broadcast to all mobiles within a geographical area. Users can decide whether to accept or reject the messages.
22] Hertz-the international unit for measuring frequency. It is equivalent to the older unit of cycles per second.

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