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
- The SAR (Segmentation and Reassembly sublayer) which breaks up and reassembles packets
to conform to the ATM size requirements, and
- 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
- Hosts that send and receive messages
- VP (virtual path) switches that maintain tables of routing information
- VP/VC (virtual circuit) switches.
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.
Networks home page
Last Changed: 10 February 1997