A.T.M.

A.T.M.

ATM is asynchronous transfer mode and also known as broadband ISDN. ATM works with cells so it is also termed as cell switching. It can support upto 622 Mbps so it can be use for transferring the data of very high data rate.

PROTOCOL STACK: The ATM network has three layers. And some of them have sub layers also.

The first layer of the ATM is physical layer. Physical layer mainly deals with the raw bit of data and physical medium.

The second layer is ATM layer and deals with frame or cells of some fixed sizes for different types of data. It also support error control and flow control. All the work of OSI datalink layer is done by this layer.

The third layer of ATM is atm adaptation layer and it deals with external network and supports different types of data for different purposes. It has two sub layers. First of them is CS(convergence sublayer) and second is SAR(segmentation and reassembly). This layer provides four major services.

1.Constant data rate: in this type of services the data rate is constant and not changing with time. Once the connection is made and communication starts the data rate will not change throughout the communication.

2.Variable bit rate: in this type data rate is changing for time to time.

3.Connection less packet services : in this services no connection is made before or release after the communication.

4.Connection oriented packet service : in this service we make connection first then data communication takes place and after the communication the connection has to be released.

For different data we have different frame formats.
The atm adaptation layer is divided in five groups. And they are AAL1, AAL2, AAL3, AAL4, AAL5.