Empirical Models TRAFFIC CHARACTERIZATION

88 CONGESTION CONTROL IN ATM NETWORKS ATM cloud Inter-departure gaps Sender cell i−1 Inter-arrival gaps Receiver cell i cell i+1 cell i−1 cell i cell i+1 t i−1 t i s i−1 s i Figure 4.7 Inter-departure and inter-arrival gaps. as a guidance to dimensioning ATM switches, and also a large number of call admission control algorithms were developed based on the cell loss rate. The jitter is an important QoS parameter for real-time applications, such as voice and video. In these applications, the inter-arrival gap between successive cells at the destination end device cannot be greater than a certain value, as this can cause the receiving play-out process to pause. In general, the inter-departure gaps between successive cells transmitted by the sender are not the same as the inter-arrival gaps at the receiver. Let us consider Figure 4.7. The gap between the end of the transmission of the ith cell and the beginning of the transmission of the i + 1st cells is t i . The gap between the end of the arrival of the ith cell and the beginning of the arrival of the i + 1st cell is s i . The inter-departure gap t i can be less, equal, or greater than s i . This is due to buffering and congestion delays in the ATM network. This variability of the inter-arrival times of cells at the destination is known as jitter. It is important that the service provided by an ATM network for a voice or a video connection is such that the jitter is bounded. If the inter-arrival gaps s i are less than the inter-departure gaps t i , then the play-out process will not run out of cells. If this persists for a long time, however, it might cause overflow problems. If the inter-arrival gaps are consistently greater than the inter-departure gaps, then the play-out process will run out of cells and will pause. This is not desirable, because the quality of the voice or video delivered to the user will be affected. Bounding jitter is not easy to accomplish. The cell transfer delay CTD is the time it takes to transfer a cell end-to-end; in other words, from the UNI of the transmitting end device to the UNI of the receiving end device. CTD is made up of a fixed component and a variable component. The fixed cell transfer delay is the sum of all fixed delays that a cell encounters from the transmitting end device to the receiving end device, such as propagation delay, fixed delays induced by transmission systems, and fixed switch processing times. The variable cell transfer delay, known as the peak-to-peak cell delay variation, is the sum of all variable delays that a cell encounters from the transmitting end device to the receiving end device. These delays are primarily due to queueing delays in the switches along the cell’s path. The peak-to-peak cell delay variation should not to be confused with the cell delay variation tolerance CDVT, which is used in the generic cell rate algorithm GCRA described in Section 4.7.1. The maximum cell transfer delay max CTD is another QoS parameter that defines an upper bound on the end-to-end cell transfer delay. This upper bound is not an absolute bound. Rather, it is a statistical upper bound, which means that the actual end-to-end cell transfer delay might occasionally exceed the max CTD. That is, the sum of the fixed QUALITY OF SERVICE QOS PARAMETERS 89 max CTD Fixed CTD pdf 1 of the total area peak-to-peak cell delay variation cells delivered late Figure 4.8 Cell transfer delay. cell transfer delay and the peak-to-peak cell delay variation might exceed max CTD see Figure 4.8. For example, let us assume that the max CTD is set to 20 msec and the fixed CTD is equal to 12 msec. Then, there is no guarantee that the peak-to-peak cell delay variation will always be less than 8 msec. The max CTD can be obtained as a percentile of the end-to-end cell transfer delay, so that the end-to-end cell transfer delay exceeds it only a small percent of the time. For instance, if it is set to the 99th percentile, then 99 of the time the end-to-end cell transfer delay will be less than the max CTD and 1 of the time it will be greater. Of the QoS parameters described above, the CLR, the peak-to-peak cell delay variation, and the max CTD were standardized by the ATM Forum and can be signaled at call setup time. That is, at call setup time, the calling party can specify values for these parameters. These values are the upper bounds, and represent the highest acceptable and consequently the least desired values. The values for the peak-to-peak cell delay variation and for the max CTD are expressed in msec. As an example, the calling party can request that the CLR is less or equal than 10 − 6 , the peak-to-peak cell delay variation is less or equal than 3 msec, and the max CTD is less or equal than 20 msec. The network will accept the connection, if it can guarantee the requested QoS values. If it cannot guarantee these values then it will reject the connection. Also, the network and the calling party might negotiate new values for the QoS parameters. As will be seen in the following section, the number of QoS parameters signaled at call setup time depends on the type of ATM service requested by the calling party. Three additional QoS parameters are used: the cell error rate CER, the severely errored cell block ratio SECBR, and the cell misinsertion rate CMR. These three parameters are not used by the calling party at call set-up. They are only monitored by the network. The CER of a connection is the ratio of the number of errored cells i.e., the cells delivered to the destination with erroneous payload to the total number of cells transmitted by the source. The severely errored cell block ratio SECBR is the ratio of the total number of severely errored cell blocks divided by the total number of transmitted cell blocks. A cell block is a sequence of cells transmitted consecutively on a given connection. A severely errored cell block occurs when more than a predefined number of errored cells, lost cells, or misinserted cells are observed in a received cell block.