ATM(Asynchronous Transfer Mode) Protocol Stack-AAL layer,ATM layer,Physical layer

This page ATM(Asynchronous Transfer Mode) tutorial covers ATM protocol stack. ATM protocol layers include AAL layer, ATM layer and ATM Physical layer.

ATM protocol layers

The figure-1 depicts ATM protocol layers at end host points and at ATM switch. As shown end systems i.e. host-A and host-B consists of PHY layer, ATM layer, AAL layer and upper layers. ATM switch consists of only two layers i.e. ATM layer and physical layer.

ATM protocol stack

The figure-2 depicts ATM protocol stack which consits of ATM physical layer (similar to OSI layer-1), ATM layer and AAL layer. Upper layers reside above AAL layer. Let us understand functions of all the ATM protocol layers including sublayers (if any).

The functionalities of all the ATM protocol layers are categorized into control plane, user plane and management plane.
➨User plane layers handle user information transfer and required associated controls e.g. error control and flow control.
➨Control plane takes care of call and connection related control signals.
➨Management plane is divided into plane and layer management. Plane management manages whole system functionality. Layer management takes care of managemement of all resources and parameters of the protocol entities.

ATM AAL layer function

AAL-ATM Adaptation Layer

AAL layer in ATM protocol stack consists of two sublayers viz. convergence sublayer and SAR (Segmentation and Reassembly) sublayer. ATM AAL layer does following functions:

• AAL does encapsulation of user data generated by upper layers.

• It does segmentation of data into small size ATM cells of size 48 bytes at transmit host. It does re-assembly of segmented data at the receive host.

ATM Layer

Following are the functions performed by ATM layer in ATM protocol layers:

• This layer incorporates header (of size 5 bytes) to segmented cells of size 48 bytes each. ATM layer does this header generation at transmit end and header extraction at receive end.

• It is responsible for generic flow control.

• It does VPI vs VCI translation.

• It takes care of cell multiplexing and demultiplexing i.e. multiplexing of logical channels to one physical channel and viceversa.

• ATM layer provides variety of services for ATM cells from ATM virtual connection.

ATM Physical Layer

ATM physical layer in protocol stack consists of Transmission Convergence sublayer and Physical Medium Dependent Sublayer. It performs following functions:

• Transmission convergence sublayer takes care of following:
-HEC header sequence generation as well as verification
-Cell Delineation
-Transmit frame generation and recovery

• Physical Medium Dependent sublayer takes care of following:
-Bit timing
-Physical medium related encoding and decoding of bits

Similar posts on ATM (Asynchronous Transfer Mode)

Refer Following links to subtopics on this ATM tutorial:
ATM Network Architecture & Interfaces   ATM Protocol Stack  ATM services  ATM Switch  ATM Call Flow  ATM vs STM  ATM vs TDM  ATM versus Frame Relay  ATM AAL Layer Types 

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