3.3 Layer 2 Protocol Overview - WAN Protocols
3.3.12 ATM  cell format
ATM transfers information in fixed-size units called cells (analogous to frames). Each cell consists of 53 bytes. Cell relay is a flexible and responsive method for multiplexing all forms of digital traffic (data, voice, image, and video). Cell relay can handle rapid changes in the quantity and pattern of the traffic in a network. All traffic is placed in fixed-length packets of information (cells) and switched at high speeds. Cell relay is considered by many to be the best multiplexing technology for modern communication applications because it combines the strengths of time-division multiplexing (TDM) and conventional packet switching. Using cell-relay packet-handling techniques, a mixture of bursty and delay-sensitive traffic can be processed simultaneously, while at the same time providing the services required by each traffic type. Also, because cell-relay processing is based on the use of small packets, the process technology is adaptable to a wide range of interface speeds.

ATM is unique in its use of fixed-size cells. The cell format is shown in Figure . The fields are described below:

  • Generic flow control - This field does not have an end-to-end significance in the standard and is rarely used.
  • Virtual path identifier - This field indicates the virtual path (see Figure ) that this particular cell corresponds to.
  • Virtual connection identifier - This field indicates the virtual circuit within the virtual path that this particular cell should be associated with.
  • Payload type - This field identifies whether the data (payload) included with this cell is user data or cell relay management information intended from network components.
  • Cell loss priority - This bit can be set to indicate high versus low priority traffic. If congestion occurs within the network, lower priority cells are discarded first.
  • Header error check - This field provides a checksum to ensure that the information within the cell header is accurately received. It does not include checking for the information in the payload.
  • Payload - The payload is up to 48 bytes of user data or cell relay management information. If there is less than 48 bytes of user data to be sent, the cell receives padding to create 48 bytes. If there is more than 48 bytes of user data to be sent, the rest of the data is split off and put into the next cell corresponding to this particular call.

This curriculum will not explore ATM troubleshooting. That will be left for another course. An excellent reference for ATM is the recent Cisco Press book "Cisco ATM Solutions".