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Medium Access Control Techniques

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Medium Access Control Techniques

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It is quite obvious that if N stations are providing relatively comparable steady flow of information (like digital audio stream), then it makes sense to divide the transmission medium into N Channels which can be allocated for transmission to each of the N stations. To achieve this the most popular techniques for sharing the medium are: 

1. FDMA (Frequency Division Multiple Access)

2. TDMA (Time Division Multiple Access)

3. CDMA (Code Division Multiple Access)

4. SDMA (Space Division Multiple Access)

5. Packet Mode methods

All the above techniques form the basis of cellular telephony.

Cellular telephone networks evolved from first generation systems (1G) in the form of AMPS (Advanced Mobile Phone System) which used FDMA for transmission between a base station and mobile stations.

Then came second generation systems (2G) like GSM (Global System for Mobile Communications) which gained worldwide acceptance and employed a hybrid of TDMA/CDMA for transmission. With the emergence of 3G, wireless access standards based on CDMA have become popular.

The other two techniques are also used in cellular telephony in parallel with others. For example, SDMA is used in GSM cellular networks to ensure a better battery life of Mobile Phones where power saving feature is enabled and the BTS (Base Transceiver Station) is aware of the mobile phone’s position by use of a technique called “timing advance”.

The use of Packet mode methods is widespread and particularly in GPRS (General Packet Radio Service): an enhancement of GSM. Here slotted ALOHA access (a packet mode method) is used by mobile station to transmit a reservation request in the packet random access channel.

We will take up all the above techniques in detail in subsequent articles.

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