TAILIEUCHUNG - Giới thiệu về IP và ATM - Thiết kế và hiệu suất P8

Cell-Scale Queueing dealing with the jitters In Chapter 4 we considered a situation in which a large collection of CBR voice sources all send their cells to a single buffer. We stated that it was reasonably accurate under certain circumstances (when the number of sources is large enough) to model the total cell-arrival process from all the voice sources as a Poisson process. Now a Poisson process is a single statistical model from which the detailed information about the behaviour of the individual sources has been lost, quite deliberately, in order to achieve simplicity | 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 8 Cell-Scale Queueing dealing with the jitters CELL-SCALE QUEUEING In Chapter 4 we considered a situation in which a large collection of CBR voice sources all send their cells to a single buffer. We stated that it was reasonably accurate under certain circumstances when the number of sources is large enough to model the total cell-arrival process from all the voice sources as a Poisson process. Now a Poisson process is a single statistical model from which the detailed information about the behaviour of the individual sources has been lost quite deliberately in order to achieve simplicity. The process features a random number a batch of arrivals per slot see Figure where this batch can vary as 0 1 2 . 1. So we could say that in for example slot n 4 the process has overloaded the queueing system because two cells have arrived - one more than the buffer can transmit. Again in slot n 5 the buffer has been overloaded by three cells in the slot. So the process provides short periods during which its instantaneous arrival rate is greater than the cell service rate indeed if this did not happen there would be no need for a buffer. But what does this mean for our N CBR sources Each source is at a constant rate of 167 cell s so the cell rate will never individually exceed the service rate of the buffer and provided N x 167 353 208 cell s the total cell rate will not do so either. The maximum number of sources is 353208 167 2115 or put another way each source produces one cell every 2115 time slots. However the sources are not necessarily arranged such that a cell from each one arrives in its own time slot indeed although the probability is not high all the sources could be accidentally synchronized such that all the cells arrive in the same slot. In fact for our

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