Asynchronous Transfer Mode (ATM)

In order to understand what ATM is all about, a brief introduction to STM is in order. ATM is the complement of STM which stands for "Synchronous Transfer Mode". STM is used by telecommunication backbone networks to transfer packetized voice and data across long distances. It is a circuit switched networking mechanism, where a connection is established between two end points before data transfer commences, and torn down when the two end points are done. Thus the end points allocate and reserve the connection bandwidth for the entire duration, even when they may not actually be transmitting the data. The way data is transported across an STM network is to divide the bandwidth of the STM links (familiar to most people as T1 and T3 links) into a fundamental unit of transmission called time-slots or buckets. These buckets are organized into a train containing a fixed number of buckets and are labeled from 1 to N. The train repeats periodically every T timeperiod, with the buckets in the train always in the same position with the same label. There can be up to M different trains labeled from 1 to M, all repeating with the time period T, and all arriving within the time period T. The parameters N, T, and M are determined by standards committees, and are different for Europe and America. For the trivia enthusiasts, the timeperiod T is a historic legacy of the classic Nyquist sampling criteria for information recovery. It is derived from sampling the traditional 4Khz bandwidth of analog voice signals over phone lines at twice its frequency or 8Khz, which translates to a timeperiod of 125 usec. This is the most fundamental unit in almost all of telecommunications today, and is likely to remain with us for a long time.

On a given STM link, a connection between two end points is assigned a fixed bucket number between 1 and N, on a fixed train between 1 and M, and data from that connection is always carried in that bucket number on the assigned train. If there are intermediate nodes, it is possible that a different bucket number on a different train is assigned on each STM link in the route for that connection. However, there is always one known bucket reserved a priori on each link throughout the route. In other words, once a time-slot is assigned to a connection, it generally remains allocated for that connections sole use throughout the life time of that connection.

To better understand this, imagine the same train arriving at a station every T timeperiod. Then if a connection has any data to transmit, it drops its data into its assigned bucket(time-slot) and the train departs. And if the connection does not have any data to transmit, that bucket in that train goes empty. No passengers waiting in line can get on that empty bucket. If there are a large number of trains, and a large number of total buckets are going empty most of the time (although during rush hours the trains may get quite full), this is a significant wastage of bandwidth, and limits the number of connections that can be supported simultaneously. Furthermore, the number of connections can never exceed the total number of buckets on all th2 different trains (N*M). And this is the raison-d'etre for ATM.

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Last Updated 24th January 1998