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Scheduling And Policing Mechanisms

Advanced Multimedia. Scheduling And Policing Mechanisms. University of Palestine Eng. Wisam Zaqoot November 2010. Ref: Computer Networking: A Top Down Approach, 4th ed., Kurose & Ross.

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Scheduling And Policing Mechanisms

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  1. Advanced Multimedia Scheduling And Policing Mechanisms • University of Palestine • Eng. Wisam Zaqoot • November 2010 Ref: Computer Networking: A Top Down Approach, 4th ed., Kurose & Ross

  2. In the last lecture, we identified the important underlying principles in providing Quality of Service (QoS) guarantees to networked multimedia applications. • Before we examine various mechanisms that are used to provides these QoS guarantees, we will study the scheduling and policing mechanisms first.

  3. Scheduling Mechanisms • scheduling: choose next packet to send on link • FIFO (first in first out) scheduling*: send in order of arrival to queue • real-world example? • discard policy: if packet arrives to full queue: who to discard? • Tail drop: drop arriving packet • priority: drop/remove on priority basis • random: drop/remove randomly

  4. Scheduling Policies: more Priority scheduling: transmit highest priority queued packet • multiple classes, with different priorities • class may depend on marking or other header info, e.g. TOS, IP source/destination, port numbers, etc.. • The choice among packets in the same priority class is typically done according to FIFO • Real world example?

  5. Scheduling Policies: more • In the following figure, we are following Priority scheduling. Note that while we are transmitting packet 2 (low priority), Packet 4 (a high priority packet) arrives. Under a so-called non-preemptive priority queuing, the transmission of packet 2 is not interrupted once it has begun, and packet 4 has to wait until the transmission of packet 2 is completed.

  6. Scheduling Policies: still more Round Robin scheduling: • multiple classes • cyclically scan class queues, serving one from each class (if available) • real world example?

  7. Scheduling Policies: still more Weighted Fair Queuing (WFQ): • generalized Round Robin • each class gets weighted amount of service in each cycle • real-world example?

  8. Weighted Fair Queuing (WFQ): • WFQ differs from round robin in that each class may receive a differential amount of service in any interval of time. • Specifically, let each class, i, is assigned a weight, wi. Under WFQ, during any interval of time during which there are class i packets to send, class i will then be guaranteed to receive a fraction of service equal to wi/Sum(wj), where the sum in the denominator is taken over all classes that also have packets queued for transmission.

  9. Weighted Fair Queuing (WFQ): • In the worst case, even if all classes have queued packets, class i will still be guaranteed to receive a fraction wi/Sum(wj), of the bandwidth. • Thus, for a link with transmission rate R, class i will always achieve a throughput of at least R. wi/Sum(wj). • As we will see in the following sections, WFQ plays a central role in QoS architectures. It is also widely available in today's router products

  10. Weighted Fair Queuing (WFQ): • Like round robin, WFQ is also a work-conserving queuing discipline. This means that this scheduling will immediately move on to the next class in the service sequence upon finding an empty class queue.

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