Traffic Management for the Available Bit Rate (ABR) Service in ...
Traffic Management for the Available Bit Rate (ABR) Service in ... Traffic Management for the Available Bit Rate (ABR) Service in ...
Figure 5.2: Transmitted cell rate (controlled) and O ered Average Cell Rate (measured). only increase LAF. Increasing the LAF corresponds to decreasing the allowed source rate. Hence, successive switches only reduce the rate allowed to the source. Thus, the source receives the rate allowed by the bottleneck along the path 4. Averaging interval (AI): The OSU scheme primarily uses measured quantities instead of parameters for control. These quantities are measured at the source (eg., OCR) and the switch (eg., current load level z discussed in section 5.1.3). The measurements are done over intervals (called \averaging intervals") to smoothen out the variance in these quantities. To ensure coorelation of the measured quantities at the switch and at the source, we require the source av- eraging intervals to be the maximum of the averaging interval of the switches along the path. This maximum value is returned in the AI eld. The AI eld is initialized to zero at the source. 5. The direction of feedback (backward/forward) 6. Timestamp containing the time at which the control cell was generated at the source 97
The last two elds are used in the backward congestion noti cation option described in Section 5.2.8 and need not be present if that option is not used. 5.1.2 The Source Algorithm The source algorithm consists of three components: 1. How often to send control cells 2. How to measure the o ered average cell rate 3. How to respond to the feedback received from the network These three questions are answered in the next three subsections. Control-Cell Sending Algorithm The sources send a control cell into the network every T microseconds. The source initializes all the elds. The network reads only the OCR, LAF and AI elds and modi es only the LAF and AI elds. The TCR eld is used by the source to calculate the new TCR as discussed in the next section. LAF and AI are both initialized to zero as discussed earlier. The initialization of the OCR and TCR elds are discussed in the next section. Measuring O ered Average Load Unlike any other scheme proposed so far, each source also measures its own load. The measurement isdoneoverthesameaveraging interval that is used for sending the control cells. The transmission cell rate (TCR), as de ned, is the inverse of minimum inter-cell transmission time at the source. However, when the source is not always 98
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The last two elds are used <strong>in</strong> <strong>the</strong> backward congestion noti cation option described<br />
<strong>in</strong> Section 5.2.8 and need not be present if that option is not used.<br />
5.1.2 The Source Algorithm<br />
The source algorithm consists of three components:<br />
1. How often to send control cells<br />
2. How to measure <strong>the</strong> o ered average cell rate<br />
3. How to respond to <strong>the</strong> feedback received from <strong>the</strong> network<br />
These three questions are answered <strong>in</strong> <strong>the</strong> next three subsections.<br />
Control-Cell Send<strong>in</strong>g Algorithm<br />
The sources send a control cell <strong>in</strong>to <strong>the</strong> network every T microseconds. The source<br />
<strong>in</strong>itializes all <strong>the</strong> elds. The network reads only <strong>the</strong> OCR, LAF and AI elds and<br />
modi es only <strong>the</strong> LAF and AI elds. The TCR eld is used by <strong>the</strong> source to calculate<br />
<strong>the</strong> new TCR as discussed <strong>in</strong> <strong>the</strong> next section.<br />
LAF and AI are both <strong>in</strong>itialized to zero as discussed earlier. The <strong>in</strong>itialization of<br />
<strong>the</strong> OCR and TCR elds are discussed <strong>in</strong> <strong>the</strong> next section.<br />
Measur<strong>in</strong>g O ered Average Load<br />
Unlike any o<strong>the</strong>r scheme proposed so far, each source also measures its own load.<br />
The measurement isdoneover<strong>the</strong>sameaverag<strong>in</strong>g <strong>in</strong>terval that is used <strong>for</strong> send<strong>in</strong>g <strong>the</strong><br />
control cells. The transmission cell rate (TCR), as de ned, is <strong>the</strong> <strong>in</strong>verse of m<strong>in</strong>imum<br />
<strong>in</strong>ter-cell transmission time at <strong>the</strong> source. However, when <strong>the</strong> source is not always<br />
98
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