Asynchronous Transfer Mode

  • The physical layer which is in turn divided into two sublayers,
    1. The PMD (Physical Medium dependent layer, which is responsible for bit timing and Physical network access, and the
    2. TC (transmission convergence) sublayer, which has the responsibility for retrieving the cells from the data stream.
  • The ATM layer which is responsible for flow control, header generation and extraction, virtual circuit and path management and multiplexing. This layer provides a connection oriented service that transmits the cells.
  • The AAL (ATM Adaptation layer) which again has two sublayers, and is responsible for providing interfaces to higher level protocols such as TCP/IP and X.25
    1. The SAR (Segmentation and Reassembly sublayer) which breaks up and reassembles packets to conform to the ATM size requirements, and
    2. The CV (convergence sublayer) which makes it possible for ATM systems to offer different kinds of services.

    Nodes in an ATM network have three distinct roles

    A virtual path is a bundle of virtual circuits associated with a physical path between tow nodes.

    Routing in ATM

    ATM tries to do routing at a much lower level of the protocol stack than is the case in the OSI model. Packets thus do not have to climb three layers and descend them again at each node. The first four bytes of the header contain identifiers for the VP and VC. Hardware switching is employed to route the packets so that except at the originating and terminating nodes the packet remains on the lowest possible layer. It is this ability to do switching in hardware that makes the data rates of ATM possible whilst at the same time requiring that the packet size remain small.


    More Information on ATM
    The ATM forum provides a web page that gives an overview of ATM.

    Return to Networks home page

    Last Changed: 10 February 1997