Asynchronous transfer mode (ATM), also known as cell relay, is similar in concept to frame relay. Both frame relay and ATM take advantage of the reliability and fidelity of modern digital facilities to provide faster packet switching than X.25. ATM is even more streamlined than frame relay in its functionality, and can support data rates several orders of magnitude greater than frame relay.

The “asynchronous” in ATM means ATM devices do not send and receive information at fixed speeds or using a timer, but instead negotiate transmission speeds based on hardware and information flow reliability. The “transfer mode” in ATM refers to the fixed-size cell structure used for packaging information.

ATM transfers information in fixed-size units called cells. Each cell consists of 53 octets or bytes as shown in Figure.

                                                                 Figure: ATM cell Format

  • Transmits all information in fixed-size blocks called cells.
  • Cells are transmitted asynchronously.
  • The network is connection-oriented.
  • Each cell is 53 bytes long – 5 bytes header and 48 bytes payload.
  • Making an ATM call requires first sending a message to set up a Subsequently all cells follow the same path to the destination.
  • ATM was envisioned as the technology for providing B-ISDN services.
  • It can handle both constant rate traffic and variable length Thus, it can carry multiple types of traffic with end-to-end quality of service.
  • ATM is independent of transmission It doesn't prescribe any particular rule.
  • They may be sent on a wire or Fiber by themselves or they may be also packaged inside the payload of the other carrier system.
  • Delivery of the system is not guaranteed but the order is.

When the virtual circuit is established, what really happens is that a route is chosen from source to destination. All the switches along the way make table entries for the virtual circuit and have the opportunity to reserve resources for the new circuit. The cells are sent from one switch to the next (stored and forwarded) until they reach the destination. When a cell comes along, the switch inspects its header to find out which virtual circuit it belongs to.


                                              Figure: Cells travelling from Host 1 to Host 5