Quality of service in optical burst switched networks part 2
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Quality of service in optical burst switched networks part 2
4ABSOLUTE QOSDIFFERENTIATIONThe absolute Quality of Service (QoS) model ill Optical Burst Switching (OBS) aims to give worst-case quantitative loss gu Quality of service in optical burst switched networks part 2 uarantees to traffic classes. For example, if a traffic class is guaranteed to experience no more than 0.1% loss rate per hop. the loss rate of 0.1% is referred to as the absolute threshold of that class. This kind of QoS guarantee calls for different QoS differentiation mechanisms than those intend Quality of service in optical burst switched networks part 2 ed for the relative QoS model in the previous chapter. A common characteristic of these absolute QoS mechanisms is that they differentiate traffic basQuality of service in optical burst switched networks part 2
ed on the classes' absolute thresholds instead of their relative priorities. That is. traffic of a class will get increasingly favourable treatment as4ABSOLUTE QOSDIFFERENTIATIONThe absolute Quality of Service (QoS) model ill Optical Burst Switching (OBS) aims to give worst-case quantitative loss gu Quality of service in optical burst switched networks part 2 s the various absolute QoS mechanisms proposed in the literature. They include early dropping, preemption. virtual wavelength reservation and wavelength grouping mechanisms. Some of them such as early dropping and preemption are also used with the relative QoS model. However, the dropping and preemp Quality of service in optical burst switched networks part 2 tion criteria here arc different. The other mechanisms are unique to the absolute QoS model.4.1Early DroppingEarly dropping is first proposed in [1] tQuality of service in optical burst switched networks part 2
o implement proportional QoS differentiation in OBS. which is described in the previous924 ABSOLUTE QOS DIFFERENTIATIONchapter. In this section, its u4ABSOLUTE QOSDIFFERENTIATIONThe absolute Quality of Service (QoS) model ill Optical Burst Switching (OBS) aims to give worst-case quantitative loss gu Quality of service in optical burst switched networks part 2 rmance of all traffic classes against their required thresholds. When the performance of a particular class j(l < j < N) drops below the required level, the node selects another class i that has its QoS performance above the required level for dropping. Header packets of class i will be dropped befo Quality of service in optical burst switched networks part 2 re they reach the scheduler. Consequently, the offered load to the node is reduced and performance of other classes, including class j. will improve.TQuality of service in optical burst switched networks part 2
o decide which class whose header packets are to be dropped early, the node assigns class priorities based on how stringent their loss thresholds are.4ABSOLUTE QOSDIFFERENTIATIONThe absolute Quality of Service (QoS) model ill Optical Burst Switching (OBS) aims to give worst-case quantitative loss gu Quality of service in optical burst switched networks part 2 next higher priority class. An early dropping flag. et. is associated with each class i. e.ị is determined by generating a random number between 0 and 1. If the number is less than Pc?, e-i is set to 1. Otherwise, it is set to 0. Hence, e.ị is 1 with probability Pc? and is 0 with probability (1 - Pc Quality of service in optical burst switched networks part 2 ?)-Suppose class priorities are set. such that one with a higher index has a higher priority. An early dropping vector, EDi, is generated for the arriQuality of service in optical burst switched networks part 2
ving class i burst, where EDi = {e:, ei+i,..., e2\r-i }• The class i header packet is dropped if Ci VCt+1 V • • • V e.v-1 = 1. In other words, it is d4ABSOLUTE QOSDIFFERENTIATIONThe absolute Quality of Service (QoS) model ill Optical Burst Switching (OBS) aims to give worst-case quantitative loss gu Quality of service in optical burst switched networks part 2 bility (1 —- Pc?))- Note thatan element e/v for class Ar is not included in the formula because class ;V has the highest priority.Although the early dropping approach provides adequate differentiation among classes, it is not very efficient. This is because it may unnecessarily drop low priority bur Quality of service in optical burst switched networks part 2 sts that otherwise would not contend with any other bursts. This leads to under-utilization of wavelengths. Preemption, which will be described later,Quality of service in optical burst switched networks part 2
overcomes this drawback.4.2 Wavelength Grouping 934.1.2Calculation of the early dropping probabilityA key parameter of the early dropping scheme is t4ABSOLUTE QOSDIFFERENTIATIONThe absolute Quality of Service (QoS) model ill Optical Burst Switching (OBS) aims to give worst-case quantitative loss gu Quality of service in optical burst switched networks part 2 y Drop by Span (EDS).In the EDT method, all bursts of class i are early dropped when the loss probability of the next higher priority class (z + 1). PCi+n reaches the acceptable loss threshold • J die early dropping probability P^p of class i is given byTẼP =PCi —0.1.PCi+\Pci+Ỉ(4.1)where i > 1.Since Quality of service in optical burst switched networks part 2 this method has only one single trigger point, bursts of each class with lower priority than class (z + 1) suffer from high loss probability when pCiQuality of service in optical burst switched networks part 2
-1 exceeds •41A .To avoid the above negative side effect of the EDT method, the EDS method linearly increases Pc+i as a function of pci+1 over a span 4ABSOLUTE QOSDIFFERENTIATIONThe absolute Quality of Service (QoS) model ill Optical Burst Switching (OBS) aims to give worst-case quantitative loss gu Quality of service in optical burst switched networks part 2 Thus, the early dropping probabilityPc? of class i is given byinpMiN-pMIN*ci+l pMINGọi ngay
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