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Cells and Cell Relay

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Cells are the basic unit for packaging and transmitting data in ATM. Cell relay is the process of moving cells through switching elements. ATM cells are 53 bytes in length. The first 5 bytes contain header information such as source and destination address; the remaining 48 bytes are reserved for data. In contrast, frames are the basic unit of data transport in local area networks and frame relay networks. The main difference is that frames vary in size and may be up to 8,000 bytes in length. The difference between cells and frames is illustrated here:

Fixed-size cells can be switched at very high speed and add predictability to data transmissions. In contrast, variable-length frames produce unpredictable traffic patterns. Imagine a traffic intersection with a four-way stop sign. If all the vehicles entering the intersection are Porsche 911s, delays should be minimal. But if the vehicles are a mix of cars, buses, and trucks with trailers, throughput is difficult to predict.

ATM is ideal for deliverying real-time traffic like voice and video because its predictable traffic flows support QoS. It is a simple matter to reserve cells for specific types of traffic like voice to guarantee that traffic will get through. MPLS can be used in frame-based networks to add traffic engineering, which is step toward providing QoS in those networks.

On the downside, the 5-byte header is excessive and cuts into the amount of data that can be transferred. The term "cell tax" is often used to describe the overhead imposed by ATM cells.

There has always been a rift between the telecommunication engineers who advocated cells and the data communication engineers who advocated packet switching. The outcome of this rift was summarized by Charles N. Judice writing in IEEE Communications Magazine, August 2000:

I submit that the communication industry lost it when the computer guys could not get their 1000-byte packets into ATM standards. While those of us with the "Bell Shaped Heads" thought we won a great compromise in establishing 53 bytes as the ATM packet size, what we really did was demonstrate to the computer industry that we had little understanding of their requirements or the implications of their design. So rather than design the next-generation network with us, they just kept making their datagram network work harder and faster.

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
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