Evaluation of Jitter Introduced by ATM Switches and Workstations a Performance Evaluation

As ATM cells pass through an ATM network, they experience a variable amount of delay as a result of queuing and switching delays at the switches. Real-time applications need a low inter- cell arrival time variance or their real-time needs will not be meet. Therefore, in order to analyze t he end-to-end performance of real-time applications or to estimate the quality of service parameters (QoS) that an application must negotiate in-order to meet certain performance requirements, it is important t o form an accurate estimate of the end-to-end cell delay variation. We describe our approach to benchmarking the variation of inter-cell arrival ti mes or jitter in this paper. We present experimental results of actual jitter measurements on the FORE ASX200BX switch, using the HP OC3Port cell analyzer and a Pentium workstation running Linux. We also present a quantitative analysis of how this variation compares with the effects of the ATM network card driver being used at the workstation, which is running an application. From the data presented, we illustrate the amount of jitter that could be introduced into an ATM cell stream for various bit rate ATM streams. From our data obtained from the ATM network card driver at the workstation, we also illustrate that in several cases the cell-clumping that takes place at the ATM network card often negates any jitter an ATM switch on the ATM stream. The effects of ATM network card cell clumping have often been ignored in similar studies. Therefore we conclude that ther e are two different time scales at which jitter can e ffect end-to-end performance. If the end points include a workstation, then it is important to account for ce ll clumping introduced by the ATM network card driver as well as the jitter introduced by the switches in the path of the stream. However if the end points do n ot involve a workstation, as is the case in jitter ana lyses of traffic between switches, then only the jitter introduced by the switches is important. We also demonstrate that this jitter is very small and will only affect a very sensitive application.