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| Committed Information Rate (CIR) |
| In traditional circuit switched networks, once a communication path is established, the bandwidth along that path is fixed and reserved for the duration of the call. In packet switched networks on the other hand, there is no explicit bandwidth reservation, and the packet flow makes use of whatever capacity is available on the link. These two paradigms were separate until a few years ago, when it was discovered that it is possible to establish a circuit switched like bandwidth reservation in packet switched networks, using special scheduling disciplines called Weighted Fair Queueing (WFQ). Aperto® Networks has adapted WFQ for use in broadband wireless systems, and used it to design the CIR service class. Active CIR flows are given a guaranteed minimum amount of bandwidth, which they can exceed, if extra bandwidth is available (in contrast to peak rate traffic shaping, which enforces a strict maximum). CIR flows have the following properties: |
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- CIR flows are subject to admission control and once a flow is admitted, it is guaranteed its bandwidth for the duration that it is active.
- CIR flows are buffered separately from each other and from flows in the CBR and BE classes.
- If a CIR flow is not using up its minimum guaranteed bandwidth, then the unused portion can be used by other CIR and BE flows on the link.
- CIR flows are allowed to burst to link capacity, or the peak rate allowed for the flow.
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| CIR flows are ideal for providing business grade services that emulate T1/E1 or fractional T1/E1 type services. From the operator’s viewpoint, they are more profitable than traditional circuit switched T1/E1 services since they do not tie down access bandwidth to subscribers, and allow sharing of bandwidth by whoever is active at a particular time, while providing rate guarantees. This allows the operator to over-subscribe the link and realize additional revenues. |
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| The dynamic nature of the bandwidth reservation in CIR flows is illustrated in the above figure. If a virtual SU in the CIR service class is idle, then its reserved bandwidth is available to other flows. When it becomes active, the scheduler reserves its CIR rate, from the total pool of bandwidth that has been set aside for the CIR service class. As long as the CIR flow is active, it is guaranteed its CIR rate. As soon as it becomes idle, the scheduler returns that bandwidth back to the CIR service class pool, and makes it available to other flows. The net benefit of this mechanism is that the operator can oversubscribe the bandwidth in the CIR pool, while being sure that the active CIR flows are getting their full guaranteed bandwidth. |
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