Asynchronous Transfer Mode ATM

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Asynchronous Transfer Mode ATM is dedicated connection switching technology that organizes digital data into byte cell unit and through digital technology transmit them into physical medium. It is designed small and fixed size of cells which is used to transmit data from one to others.

ATM solve all telecommunication and networking problems by merging voice, data, cable television, telegraph and everything into a single integrated system that could do everything for everyone.

It is more successful than OSI, and it is now widely used deep within the telephone system, often for moving IP packets.

ATM Asynchronous Transfer Mode Virtual Circuits -:
It is connection oriented; sending data requires first sending a packet to set up the connection. As the set up packet wends its way through the subnet, all the routers in the path make an entry in their internal tables nothing the existence of the connection and reserving whatever resource are needed for it.

Once connection has been established, ATM transmits all information in small size of packets called cells.

The cells are 53 bytes long, of which 5 bytes are header and 48 bytes are payload.
Header -: is connection identifier, so that sending and receiving hosts and all the intermediate can tell which cell belongs to which connection. This information allows to router to know how to route each incoming cell and it is done at high speed.

All cells follow same route to the destination. Cell delivery is not guaranteed but their order is but it is up to higher protocol levels to recover lost cells.

The most common speeds for ATM networks are 155 Mbps and 622 Mbps, although higher speed is also supported by ATM.

ATM Reference Model -:
It consist of three layers viz.
1] The physical
2] ATM
3] ATM adaption layer
plus whatever user want to put on top of that.
Physical layers deals with physical medium: voltage, bit timing and other issues. It is divided into two sub layers one is Transmission convergence (TC) and other is Physical Medium Dependent Sub layer (PMD). PMD sub layer interfaces to the cable. It moves the bits on and off and handles the bit timing. When cells are transmitted, TC layer sends them as of bit to the PMD layer.

ATM layer deals with cell and cell transport. It manages cell including transport. It is mixture of the OSI data link and network layer. It also deals with establish and release virtual circuits and to locate congestion control.

ATM adaption layer is situated above to the ATM layer and allows sending packets larger than a cell. These packets transfer the cells individually, and reassemble them at the other end. It splits into Segmentation and Reassembly (SAR) and Convergence sub layer (CS). Lower sub layer breaks packets into cells on the transmission side and put them back together again at the destination. Upper sub layer makes it possible to have ATM system offer different kind of services to different application.

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