Explain ATM cell format. Also describe different function layers of ATM. And bring out the significance of AAL layer.

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Different ATM cell header has been defined for use in UNI and NNI. UNI is the interface between end user and private or public ATM switch. While NNI is the interface between two ATM switches in the same network. Fig. shows the 5-byte cell header for UNI.
- Generic Flow Control: GFC is 4 bytes long and was intended to provide flow control and shared medium access to several terminals at the UNI. It is currently unidentified and set to 0000. The GFC field has significance only at the UNI and is not carried end to end in the network. In NNI header GFC field is replaced with augmented VPI field.
- Virtual Path Identifier: The VPI field is 8 bit long so it allows to identify 256 virtual paths in given UNI link. Where virtual paths are bundle of virtual channels that are switched over as a unit over the sequence of network nodes that correspond to the path.
- Virtual Channel Identifier: The VCI field is 16 bit long, so it allows the definition of up to 658536 virtual channels per virtual path. The VPI/VCI field is local identifier foe a given connection in a given link, and the value of the field changes at every switch.
- Payload Type: This 3-bit field is used to define the type of payload an ATM cell is carrying. Out of these three-bit field,
  - First bit tells whether cell is data cell or the cell used for operation, administration or maintenance. If this bit is 0 it is data cell otherwise it is operation or maintenance or administration cell.
  - Second bit acts as explicit forward congestion identification. When set to 1 it indicates congestion experienced by the cell.
  - Last bit is used to indicate SDU used in AAL5 layer.
- Cell Loss Priority: This bit is similar to that of DE bit of Frame Relay. It provides priority to cell in case it is required to discard when experiencing congestion. When CLP bit is set to 0 it has highest priority and is not discarded when it experiences congestion. It can be set by terminal to indicate less important traffic or set by network to indicate lower priority.
- Header error control: IT is 8 bit CRC code which can correct single bit error and detect double bit errors. This bit provides protection against misdelivery of cells that may occur in transmit.

Significance of AAL:
- The ATM Adaptation Layer (AAL) is designed to support different types of applications and different types of traffic, such as voice, video, imagery, and data.
- Its basic function is the enhanced adaptation of services provided by the ATM layer to the requirements of the higher layer.
- It maps higher layer PDUs into the information field of the ATM cell.
- AAL is divided into the Convergence Sublayer (CS) and the Segmentation and Reassembly (SAR) Sublayer.
- The Convergence Sublayer is service dependent and provides the AAL services at the AAL-SAP.
- The functions of the SAR sublayer are segmentation of higher layer PDUs into a suitable size for the information field of the ATM cell (48 octets) at the transmitting end and reassembly of the information fields into higher layer PDUs at the receiving end.

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