Here are some words on ATM. There are many books on the topic. There are also many online resources. A tutorial is available as well as ATM FAQ.

Learn about our ATM and multimedia equipment and about the experience we gathered through ATM trials.

ATM stands for Asynchronous Transfer Mode. It is a new information transport technique defined by the ITU (International Telecommunication Union). ATM was envisioned to be the universal transport technique and, indeed, it was designed so as to be capable of supporting any kind of communication. Now, this may not seem to be a big deal to some, but one first has to consider at least two important facts from the history and present to get a glimps of the idea what an enormous task it is trying to merge all various kinds of communications networks and stuffing all sorts of information into this single network with the same information coming out at the other end.

First of all it is important to distinguish between two kinds of communications: traditional telecommunications and computer communications. Former started out in the days when old chaps Morse and Bell were stars. Then came radio and TV (it is communication, though unidirectional; but there is good news...), fax and finaly ISDN, which was the first attempt to integrate different communication services (voice and data). Computer networks, on the other hand, started out in the late sixties with military research and development and today's Internet originates from this early work. There is an important difference between the two. Traditional communications require very little signalling, which is also almost completely done prior to the transfer of information. Computer communications are a completely different matter, because of a high abstraction called process, which led to another high abstraction called layered communication model (or architecture). In order for information to be transferred it has to traverse this model downwards at the source and upwards at the destination. This traversal is only possible when every layer communicates with his upper and lower neighbours and, more importantly, with its counterpart at the other end (so-called peer). Thus every layer adds some additional protocol data to the actual information and we end up transferring a considerable amount of overhead data across the network during the connection. This overhead is essentially signalling. Layered approach was something new to the traditional communications but ATM has a layered protocol model since it aims to be universal.

Another thing that has to be considered is the diversity of communication services, the single most important reason why integrating those services is so difficult. This diversity is with regard to the necessary bandwidth, allowed delay, allowed variation of delay, and allowed loss of information. With bandwidth it is: either you got it or you don't. If you don't, you can kiss goodbye all that pretty, moving MM stuff (NO, it is not Marylin Monroe, it is MultiMedia). If you got the necessary bandwidth, then delay is not that critical either considering human perception (from network design point of view things can be very different, especially in broadband networks). Regarding jitter and loss, there are in general two categories of services. Some services are very sensitive to delay variations and less to loss of part of information. Others could not care less if packets of information arrive irregularly as long as they all do arrive. Examples of delay-sensitive services are voice and movie transfer, whereas all services that involve file transfer are loss-sensitive. What we are saying here is that a universal network must support a guarantee of adeqate features if a service requires those features. ATM manages to do both, support the diversity of services as well as guarantee a certain quality. Diversity is supported by four different service acces points, which effectively means it can act as four different networks. To banalize, ATM can disguise in four different costumes and a service can pick the one it likes most. At the connection setup a traffic contract is negotiated between a source and the ATM network and this contract explicitly defines the Quality of Service parameters which are to be guaranteed by ATM.

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