What does it do?
ATM relies on a cell-switching technology. ATM cells have a fixed length of 54 bytes. This small size allows for very fast switching. Basically the cells are like buckets or box-cars, which are arranged in a regular repeating pattern. ATM creates pathways between end nodes called virtual circuits that are identified by an identifier (VCI) on each cell.
Data from the virtual circuits is segmented into these cells, so that each circuit gets a time slice in which to transfer its data. Under conditions of balanced traffic the cells may repeat regularly, i.e. 1-2-3-4 : 1-2-3-4 : 1-2-3-4. Thus channels 1 through 4 each get an equal time slice. However, if channel 1 becomes a high-speed, dense data stream while channel 4 is relatively idle, the cell pattern may change to 1-1-2-3 : 1-2-3-4 : 1-1-2-3 : 1-2-3-4. In this way channel 1 gets some of channel 4s bandwidth. This capability is called statistical multiplexing, and is a unique advantage of an ATM network.

ATM consists of the lowest three layers of the OSI model. At the top is the ATM adaptation layer. This layer performs the conversion from the higher level protocols into the format required by ATM. For example, it will take IP packets, convert the address information into appropriate VCIs (virtual channel identifiers) and chop the IP packet into cells. It also provides the end-to-end controls to insure that the virtual circuit is established, and that it offers the level of service requested.

The ATM data link layer is responsible for the transmission of cells across the virtual circuit. In the course of reaching its destination, a cell may have to use several virtual circuits, in the same way that an IP packet may use several ethernet addresses as it goes from the end-node, to the ethernet port on a router, and finally to the destination nodes ethernet port.

The ATM physical layer is not strictly considered part of the ATM specification, but is recommended to be fiber-optic based SONET. SONET (synchronous optical network) has a set of specifications with different speeds: STS-3c at 155 Mb/sec, STS-12 at 622 Mb/sec, and STS-48 at a rather quick 2.4 gigabits/sec. The lowest speed envisioned for ATM is the STS-3c speed of 155 Mb/sec.

The interface to an ATM network is called a UNI (user network interface). It is typically expected that LANs will interface to the UNI using the LAN protocol frame type, and ATM cells will come out the other end.

How is it used?
There are a number of standards connected with this technology that have not been fully established, which inhibits the development of hardware and public services using ATM. Currently the telecom carriers have limited implementation of ATM switching in their networks, so that only private line ATM is possible at this time.

This is a technology which will become vitally important in the near future, and holds the promise of being able to transport very dense, real-time data streams such as full motion interactive video with simultaneous audio, graphics, and huge data files.

Where do I get more information?
Contact your local CBV Office.