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[08-23-2000] u99143 : I would like to know more about ATM, i.e., Asynchronous Transfer Mode. ?
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[08-26-2000] saintly :
ATM is a relaying protocol designed to offer both reliable and fast broadband (high-capacity) to packet data. Well... actually, ATM (Asynchronous Transfer Mode) is just the international standard for the class of packet-relaying technologies known as cell-relay. There are three other packet-relaying protocols; Frame Relay (very similar to ATM), SMDS and MAN. ATM is the standard for Broadband ISDN (BISDN) and shares ISDN's features and benefits.
The technology was invented in the late 1960s. What it boiled down to was that the current technology in use, TDM (Time-Division Multiplexing) worked by assigning everyone that wanted to use the same network cable a time slot to work in. This had the bonus for applications in that they were gauranteed the cable for that time period and could rely on their data to get there quickly. However, they also wasted tons of bandwidth doing nothing for most of the time. Cell Relay was supposed to work by breaking the data down into huge chunks and labeling each with a "virtual circuit" identifier. This required more hardware time, memory and resources - but it worked much better for computers that transmitted data in bursts. Unfortunately, the state of 1960s technology meant that the extra hardware overhead required made cell-relay infeasible.
The technology made cell-relay possible in the late 1980s and became more and more popular ever since. It breaks data down into huge (53 byte) blocks for transmission over very high-speed network cables (like fiber-optic cabling) for transmission and works well for both time-sensitive (data packets must get to the other end quickly) and loss-sensitive (data packets must get there reliably) applications. It comes with some handy tricks, like support for multiplexing (pretending that the same physical network connection is really a whole bunch of them) and switching (fast routing of data to the intended destination).
Between the 1960s and the 1980s the alternative technology was packet-switching. Unlike fixed-size cells, it attempted to attach routing labels to variable-length data (packets). The best example of this are the X.25 switches we all know and love. Packet routing tends to be much slower and has a higher lag time. After ATM was introduced, packet switching started looking worse and worse. Some people grumbled about switching (particularly IBM) but most people came around eventually.
ATM is usually implemented as the backbone network connecting lots of local LANs together. It provides point-to-point connections (whereas a LAN is a single shared resource that any connected device can use to send to any other connected device). Individual ATM stations or switches derive their address (equivalent to an ethernet MAC address) from a large hierarchy based on what it is attached to.
ATM compares well to Frame Relay. Frame Relay is more efficient at using the bandwidth, but ATM provides support for multiple services much better, and is easier to add to. Being newer, however, it requires newer hardware to implement and is probably more expensive. Unlike X.25, IP, Frame Relay and SMDS (a more complex cell-relaying protocol) which can only send data, ATM allows you to simultaneously transmit Voice, Data and Video all over the same line.
Does this help get you started? Other good pages to track down more info are:
http://set.gmd.de/~halfmann/komm/primer.html
A good primer on ATM with history and overview
http://shrike.depaul.edu/~jbao/ATM/atm.htm
Comparisons of ATM to other high-speed technologies
http://www.networking.ibm.com/atm/atmover.html
Hey look! It's IBM, selling ATM products! That's a switch <duck and run - bad joke, I know>!
http://www.fit.qut.edu.au/DataComms/itn540/gallery/a198/celeste/fratm.htm
Good comparison of Frame Relay to ATM
http://cell-relay.indiana.edu/cell-relay/FAQ/ATM-FAQ/FAQ.html
A lot of FAQs and links to other ATM technical references and more info
http://www.npac.syr.edu/users/mahesh/homepage/atm_tutorial/
An ok tutorial, assuming you already know lots about ATM.
Hope that helps! Let me know if you need more info...
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