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

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advantages of <strong>the</strong> mechanism justify <strong>the</strong> cost of implementation. The imple-<br />

mentation issues of VS/VD are fur<strong>the</strong>r discussed <strong>in</strong> section 9.<br />

6. Ano<strong>the</strong>r issue of importance to long-distance ATM service providers is how<br />

to price <strong>the</strong> <strong>ABR</strong> service <strong>for</strong> ISPs. This requires switch support <strong>in</strong> terms of<br />

management software <strong>for</strong> usage-based bill<strong>in</strong>g. The de nition of \usage" is \<strong>the</strong><br />

number of cells delivered to <strong>the</strong> dest<strong>in</strong>ation end-system." This de nition implies<br />

that <strong>the</strong> measurements of usage have to be made <strong>in</strong> <strong>the</strong> egress switches. This<br />

adds cost and complexity to <strong>the</strong> implementation of edge switches. Currently,<br />

no cost estimates exist <strong>for</strong> <strong>ABR</strong> service.<br />

7. There is a cost-per<strong>for</strong>mance tradeo <strong>in</strong> implement<strong>in</strong>g <strong>the</strong> various options of<br />

<strong>the</strong> <strong>ABR</strong> service. LAN switches are typically lower end, and <strong>the</strong> EFCI feedback<br />

mechanism provides su cient per<strong>for</strong>mance, s<strong>in</strong>ce <strong>the</strong> round trip times are small.<br />

It is anticipated that <strong>ABR</strong> will be <strong>the</strong> service of choice <strong>for</strong> WAN and satellite<br />

networks. This is because <strong>the</strong> <strong>ABR</strong> service (ER-based implementations) can<br />

provide throughput and delay per<strong>for</strong>mance, and is more scalable <strong>in</strong> terms of<br />

bu er requirements than <strong>the</strong> UBR service. WAN switches are expected to use<br />

ER-based <strong>ABR</strong> implementations. Complex alternatives like per-VC queue<strong>in</strong>g<br />

and schedul<strong>in</strong>g are required <strong>for</strong> <strong>ABR</strong> only at <strong>the</strong> edge switches. Interior network<br />

switches typically would use ER-based feedback, simple FIFO queue<strong>in</strong>g, allo-<br />

cate small amount of bu ers, and have simple bu er management and cell drop<br />

policies. The VS/VD alternative is required at a few edge switches of a very<br />

large delay-bandwidth product network. This option allows network managers<br />

to isolate <strong>the</strong> e ects of <strong>the</strong> large network on downstream small networks (see<br />

371

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