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Giới thiệu về IP và ATM - Thiết kế và hiệu suất P9

Burst-Scale Queueing information overload! ATM QUEUEING BEHAVIOUR We have seen in the previous chapter that queueing occurs with CBR traffic when two or more cells arrive during a time slot. If a particular source is CBR, we know that the next cell from it is going to arrive after a fixed duration given by the period, D, of the source, and this gives the ATM buffer some time to recover from multiple arrivals in any time slot when a number of sources are multiplexed together (hence the result that Poisson arrivals are a worst-case model for cell-scale queueing). . | Introduction to IP and ATM Design Performance With Applications Analysis Software Second Edition. J M Pitts J A Schormans Copyright 2000 John Wiley Sons Ltd ISBNs 0-471-49187-X Hardback 0-470-84166-4 Electronic 9 Burst-Scale Queueing information overload ATM QUEUEING BEHAVIOUR We have seen in the previous chapter that queueing occurs with CBR traffic when two or more cells arrive during a time slot. If a particular source is CBR we know that the next cell from it is going to arrive after a fixed duration given by the period D of the source and this gives the ATM buffer some time to recover from multiple arrivals in any time slot when a number of sources are multiplexed together hence the result that Poisson arrivals are a worst-case model for cell-scale queueing . Consider the arrivals from all the CBR sources as a rate of flow of cells. Over the time interval of a single slot the input rate varies in integer multiples of the cell slot rate 353 208 cell s according to the number of arrivals in the slot. But that input rate is very likely to change to a different value at the next cell slot and the value will often be zero. It makes more sense to define the input rate in terms of the cycle time D of the CBR sources i.e. 353208 D cell s. For the buffer to be able to recover from multiple arrivals in a slot the number of CBR sources N must be less than the inter-arrival time D so the total input rate 353 208 N D cell s is less than the cell slot rate. Cell-scale queueing analysis quantifies the effect of having simultaneous arrivals according to the relative phasing of the CBR streams so we define simultaneity as being within the period of one cell slot. Let s relax our definition of simultaneity so that the time duration is a number of cell slots somewhat larger than one. We will also alter our definition of an arrival from a single source no longer is it a single cell but a burst of cells during the defined period. Queueing occurs when the total number of cells .