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Traffic Management for the Available Bit Rate (ABR) Service in ...

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CHAPTER 3<br />

SWITCH SCHEME DESIGN ISSUES<br />

The most important part <strong>in</strong> <strong>ABR</strong> tra c management framework is <strong>the</strong> switch<br />

feedback calculation algorithm. The switch algorithm calculates <strong>the</strong> feedback to be<br />

given to <strong>the</strong> sources. We use <strong>the</strong> follow<strong>in</strong>g switch model <strong>for</strong> fur<strong>the</strong>r discussion:<br />

3.1 Switch Model<br />

A switch <strong>in</strong>terconnects multiple l<strong>in</strong>ks and supports multiple ports, typically an<br />

<strong>in</strong>put port or/and an output port per-l<strong>in</strong>k. Each port may have some bu ers as-<br />

sociated with it. It is possible to put <strong>the</strong> bu ers exclusively at <strong>the</strong> <strong>in</strong>put port (an<br />

<strong>in</strong>put-bu ered architecture), exclusively at <strong>the</strong> output port (an output-bu ered ar-<br />

chitecture), or at both <strong>the</strong> <strong>in</strong>put and output ports. Popular switch architectures tend<br />

towards be<strong>in</strong>g exclusively output bu ered [70] due to its superior per<strong>for</strong>mance when<br />

compared to <strong>in</strong>put bu ered switches. We choose to focus on output bu ered switch<br />

architectures.<br />

Bu ers may be logically partitioned <strong>in</strong>to queues, which are scheduled us<strong>in</strong>g a<br />

speci c discipl<strong>in</strong>e. Queu<strong>in</strong>g and schedul<strong>in</strong>g at <strong>the</strong> bu ers may be handled <strong>in</strong> a First In<br />

First Out (FIFO) manner where all <strong>the</strong> cells com<strong>in</strong>g to <strong>the</strong> port are put <strong>in</strong>to a common<br />

bu er (and later serviced) <strong>in</strong> <strong>the</strong> order <strong>the</strong>y arrived at <strong>the</strong> port. On <strong>the</strong> o<strong>the</strong>r hand, a<br />

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