Introduction to Asynchronous Transfer Mode

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ATM (Asynchronous Transfer Mode) is a means of digital communications that is capable of very high speeds; suitable for transmission of images or voice or video as well as data; ATM is used for both LAN (Local Area Network) and WAN (Wide Area Network).  ATM is a standardized digital data transmission technology that is implemented as a network protocol and was first developed in the mid-1980s. It dynamically allocates bandwidth and uses a fixed-size data packet. Large files are broken into small standard sized units that are transmitted to the receiving computer where the packets are reassembled into a copy of the original file. The next sub-heading deals with the new applications that are enabled by ATM.


ATM has enough flexibility to provide transport for a wide range of services including audio, video and raw data. Not only may it accommodate any one of these, but also it may successfully accommodate a mixture of data types. Each type however has its own optimum transmission characteristics. Transfers such as video and audio transmissions are ideally served by a system offering low delays or more importantly, a low delay variation. One illustration of this is the transmission of human voice, which has a latency of around 250ms and the delay can be observed over a long distance satellite link. The human ear is also sensitive to the continuity of speech implying that for audio information, the ATM needs to provide a transmission as close to continuous as possible.
Due to the large cost of many ATM applications, it is ideally suited to large-scale transmissions such as long haul telephone calls. The initial investment of expensive switching equipment and optical fibres make ATM less feasible for a small scale communications solution which may be more ideally served by a high speed Local Area Network (LAN). With the rapid growth of telecommunications and the Internet, applications for video transmissions include services such as Movies on Demand (MoD), News on Demand (NoD), World Wide Web access, Video Telephony and Video Conferencing, all of which require transmission of at least one variety of data. ATM may also become crucial in the medical field where remote diagnosis will become feasible due to the ability to perform high-speed transfers of medical images between different geographical locations. All of these applications require different network characteristics in order to transfer the information optimally leading towards need to characterise and budget for the expected traffic types. ATM was chosen as the transport for B-ISDN as it promised to be flexible enough to accommodate all of the features mentioned above whilst also providing high speed access.


1.     Efficient transmission system.

Let us firstly consider the efficiency issue of a transmission system. In the transfer of data, whether it be voice, video or computer data, it is common to use some form of compression technique in order to use the transmission medium efficiently. These data compression techniques commonly include some form of Run Length Encoding (RLE) to remove redundancy within a signal and thus reduce the bandwidth consumed by the transmission. Although, data compression is performed at a much higher level, ATM achieves its efficiency by making use of small, compact packets with a small header field, maximising the user information to system overhead ratio leading to higher data rates than other transmission mechanisms.

2.     Connection

One reason ATM works well with disparate kinds of data is that it is connection-oriented. A sender and receiver on a network set up a fixed path between each other before sending data, and the information arrives in the order it was sent. Other protocols, such as TCP/IP, are connectionless. That is, they don’t have fixed connections, so individual data packets may go to different destinations and they may be delayed or arrive in the wrong order.

3.     Optimal for Real-Time Use

According to Lawrence Orans, an analyst at Gartner Group Inc., in Stamford, Conn, Fixed-route transmission is what makes ATM optimal for real-time communication like voice and video. With proper tuning, the quality of voice traffic will equal telephone quality, and video traffic will mirror cable television, says Orans. It’s also easier to track and bill network usage.

An ATM network transfers data in 53-byte cells. Cell size never varies, and cells with the same source, destination and class of service parameters always follow the same path, as long as that path meets performance criteria. To deal with congestion or network failure, there are also pre-established secondary routes. Real-time data takes precedence over non-real-time data on an ATM network. For example, if voice and e-mail traffic are sent simultaneously, the switch grabs the voice traffic before grabbing the e-mail. In addition, the small, constant cell size enables data to be forwarded through the network more efficiently. Frame relay and other connection-oriented protocols transfer data in packets of varying lengths, which makes for variable delays between packet transfers, Orans explains. With ATM, those delays are eliminated, because the switch is “always looking for 53-byte chunks,” Orans says.


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