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

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active, <strong>the</strong> average rate of <strong>the</strong> source is di erent from <strong>the</strong> transmitted cell rate. This<br />

average is called <strong>the</strong> o ered average cell rate andisalso<strong>in</strong>cluded<strong>in</strong><strong>the</strong>cell.<br />

Normally <strong>the</strong> OCR should be less than <strong>the</strong> TCR, except when <strong>the</strong> TCR has just<br />

been reduced. In such cases, <strong>the</strong> switch will actually see a load correspond<strong>in</strong>g to <strong>the</strong><br />

previous TCR and so <strong>the</strong> feedback will correspond to <strong>the</strong> previous TCR. The OCR,<br />

<strong>in</strong> such cases, is closer to <strong>the</strong> previous TCR. Putt<strong>in</strong>g <strong>the</strong> maximum of current TCR<br />

and OCR <strong>in</strong> <strong>the</strong> TCR eld helps overcome unnecessary oscillations caused <strong>in</strong> such<br />

<strong>in</strong>stances. In o<strong>the</strong>r words,<br />

TCR <strong>in</strong> Cell maxfTCR, OCRg<br />

Dur<strong>in</strong>g an idle <strong>in</strong>terval, no control cells are sent. If <strong>the</strong> source measures <strong>the</strong> OCR<br />

to be zero, <strong>the</strong>n one control cell is sent, subsequent control cells are sent only after<br />

<strong>the</strong> rate becomes non-zero.<br />

Respond<strong>in</strong>g to Network Feedback<br />

The control cells returned from <strong>the</strong> network conta<strong>in</strong> a \load adjustment factor"<br />

along with <strong>the</strong> TCR. The current TCR may be di erent from that <strong>in</strong> <strong>the</strong> cell. The<br />

source computes a new TCR by divid<strong>in</strong>g <strong>the</strong> TCR <strong>in</strong> <strong>the</strong> cell by <strong>the</strong> load adjustment<br />

factor<strong>in</strong><strong>the</strong>cell:<br />

New TCR<br />

TCR <strong>in</strong> <strong>the</strong> Cell<br />

Load Adjustment Factor <strong>in</strong> <strong>the</strong> Cell<br />

If <strong>the</strong> load adjustment factor is more than one, <strong>the</strong> network is ask<strong>in</strong>g <strong>the</strong> source to<br />

decrease. If <strong>the</strong> new TCR is less than <strong>the</strong> current TCR, <strong>the</strong> source sets its TCR to<br />

<strong>the</strong>newTCRvalue. However, if <strong>the</strong> new TCR is more than current TCR, <strong>the</strong> source<br />

is already operat<strong>in</strong>g below <strong>the</strong> network's requested rate and <strong>the</strong>re is no need make<br />

any adjustments.<br />

99

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