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 ...
incremented when the bit is changed from zero to one. This count variable and the bits are cleared at the end of the interval. Given the number of active sources, a fair share value is computed as follows: FairShare = Target Cell Rate Number of Active Sources Underloading sources are sources that are using bandwidth less than the FairShare and overloading sources are those that are using more than the FairShare. To achieve fairness, we treat underloading and overloading sources di erently. If the current load level is z, the underloading sources are treated as if the load level is z=(1+ )and the overloading sources are treated as if the load level is z=(1 ; ). If (OCR in cell < FairShare) LAF in cell Max(LAF in cell, else LAF in cell Max(LAF in cell, z (1 ; ) )g z (1 + ) )g We prove later in this chapter that this algorithm guarantees that the system, once in the TUB, remains in the TUB, and consistently moves towards fair operation. We note that all the switch steps are O(1) w.r.t. the number of VCs. If is small, as is usually the case, division by 1 + is approximately equivalent to a multiplication by 1 ; and vice versa. What Load Level Value to Use? The OCR in the control cell is correlated to z when the control cell enters the switch queue. This is because the queue state at arrival more accurately re ects the e ect of the TCR indicated in the control cell. The value of z may change before the control cell leaves the switch queue. The OCR in the cell at the time of leaving the queue is not necessarily co-related with z. As shown in Figure 5.4, the queue state at 103
the time of departure (instant marked \2" in the gure) depends upon the load that the source put after the control cell had left the source. This subsequent load may be very di erent from that indicated in the cell. Figure 5.4: Correlation of Instantaneous Queue States to TCR 5.1.4 The Destination Algorithm The destination simply returns all control cells back to the source. 5.1.5 Initialization Issues When a source rst starts, it may not have any idea of the averaging interval or what rate to use initially. There are two answers. First, since ATM networks are connection-oriented, the above information can be obtained during connection setup. For example, the averaging interval and the initial rate may be speci ed in the connection accept message. Second, it is possible to send a control cell (with TCR=OCR=0) and wait for it to return. This will give the averaging interval. Then 104
- Page 79 and 80: y giving only one feedback per meas
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<strong>the</strong> time of departure (<strong>in</strong>stant marked \2" <strong>in</strong> <strong>the</strong> gure) depends upon <strong>the</strong> load that<br />
<strong>the</strong> source put after <strong>the</strong> control cell had left <strong>the</strong> source. This subsequent load may<br />
be very di erent from that <strong>in</strong>dicated <strong>in</strong> <strong>the</strong> cell.<br />
Figure 5.4: Correlation of Instantaneous Queue States to TCR<br />
5.1.4 The Dest<strong>in</strong>ation Algorithm<br />
The dest<strong>in</strong>ation simply returns all control cells back to <strong>the</strong> source.<br />
5.1.5 Initialization Issues<br />
When a source rst starts, it may not have any idea of <strong>the</strong> averag<strong>in</strong>g <strong>in</strong>terval<br />
or what rate to use <strong>in</strong>itially. There are two answers. First, s<strong>in</strong>ce ATM networks<br />
are connection-oriented, <strong>the</strong> above <strong>in</strong><strong>for</strong>mation can be obta<strong>in</strong>ed dur<strong>in</strong>g connection<br />
setup. For example, <strong>the</strong> averag<strong>in</strong>g <strong>in</strong>terval and <strong>the</strong> <strong>in</strong>itial rate may be speci ed <strong>in</strong><br />
<strong>the</strong> connection accept message. Second, it is possible to send a control cell (with<br />
TCR=OCR=0) and wait <strong>for</strong> it to return. This will give <strong>the</strong> averag<strong>in</strong>g <strong>in</strong>terval. Then<br />
104
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