The Traffic Contract ATM Traffic Management: Traffic Enforcement and Traffic

Misinserted cells are those which are not sent by the transmitting entity. These cells may arrive at the destination user either due to an undetected cell header or a configuration error. Cell Transfer Capacity is the maximum number of successfully delivered cells occurring over a specified ATM connection during a unit of time. Cell Transfer Delay CTD is defined as the maximum end-to-end cell transit time. The CTD is a function of the transmission delays and the ATM switch queuing delays. It is the total time from the first bit sent from the transmitting entity until the last bit arrives at the destination. Finally, the CLR is defined as the number of lost cells divided by the number of total transmitted cells. Figure 8-3 illustrates the probability density function of the cell transfer delay for real-time service categories. Its relation to peak-to-peak cell delay variation and maximum cell transfer delay maxCTD is important, since these two parameters are negotiated with the user. Cell Delay Variation refers to the fact that some cells will be switched very rapidly through the ATM network while others may take longer due to nodal congestion, etc. In other words, CDV is the amount of time between the maximum end-to-end cell transit time and the minimum end-to-end cell transit time. CDV is a function of multiplexing many connections onto a single physical link and the variability in ATM switching queuing delays. Since the processing delay will vary over time, the end-to-end cell transfer delay will vary over time as well. It is currently defined as a measure of how much more closely the cells are spaced than the normal interval. It can be computed at a single point against the nominal intercell spacing, or from an entry to an exit point. It is inevitable that some variability would occur in the rate of delivery of cells due to both effects within the ATM network and at the source. In case of ATM networks, cell delay variation due to events within the network are likely to be minimal since the ATM protocol is designed to minimize processing and transmission overhead internal to the ATM network so that very fast cell switching and routing is possible. Thus the major factor for causing the variation in cell transfer delay within the ATM network is congestion. Once congestion begins, cells must be discarded; otherwise there will be a buildup of queuing delays at the affected switches. Figure 8-4 illustrates the potential sources of cell delay variation at the source user network interface. An application may generate data at the constant bit rate for transmission across the ATM network, but processing at the three layers of the ATM model may introduce delays. At the AAL delays may be introduced during the interleaving process. At the ATM layer delay may occur while interleaving the OAM cells with the user cells. Figure 8-3 Probability density function for cell transfer delay. Variations in delays during cell transmission cause problems in constant-bit-rate CBR services such as voice and video. For example, human perception is highly attuned to the correct corelation of audio and video. This is apparent in foreign language films that are dubbed in English. Differential delays could cause similar effects in case of the ATM network if differential delays occur causing loss of correlation between image and voice. Another example is that of applications related to file transfer. Variations in delay can result in retransmission and consequent reduction in usable throughput. In the case of voice related applications if cells arrive at larger and larger intervals or shorter and shorter intervals, the voice will sound distorted. Therefore, a constant delay accross the ATM network is absolutely crucial for providing acceptable voice services. The mechanism for providing such consistent delay on a variable delay ATM network is known as ATM network conditioning. For maintaining QoS it is crucial to determine the source traffic characteristics: what exactly is required of the ATM network? The source traffic characterized by four parameters. The current ATM Forum Previous Table of Contents Next Copyr ight © CRC Pr ess LLC by Abhijit S. Pandya; Ercan Sen CRC Press, CRC Press LLC ISBN: 0849331390 Pub Date: 110198 Previous Table of Contents Next Peak Cell Rate PCR places an upper bound on the amount of traffic that can be submitted for transmission over a given ATM connection. Cell Delay Variation CDV At a given measurement point, it is the maximum variability observed in the pattern of cell arrivals with respect to the peak cell rate. Consider the following example, where ATM cells are represented by small automobiles. Figure 8-4 a Traffic with constant spacing of vehicles; b effect of a traffic signal on the traffic pattern. Constant bit-rate traffic is shown in Figure 8-4a, where the timespacing between cells is constant. The traffic light, or ATM switch, can cause the cells to bunch. Left uncorrected, the cells will arrive at the destination in bunches and will thus show variations in their individual journey times. This is called CDV, cell delay variation. This problem can be corrected, by doing active traffic shaping discussed in detail in Section 8-6, after the last switch, to restore the original cell pacing, as illustrated in Figure 8-5. This is sometimes achieved for guaranteeing the required QoS at the cost of increasing the total delay CTD --cell transmission delay. Figure 8-5 Traffic pattern after traffic shaping. Sustainable Cell Rate SCR For a given duration of the connection, it defines the upperbound on the average rate at which cells are transferred through an ATM connection. Maximum Burst Size MBS At a given measurement point, it is the maximum variability observed in the pattern of cell arrivals with respect to the sustainable cell rate. The first two are relevant for the constant-bit-rate CBR sources, while all four may be used for variable-bit-rate VBR sources. QoS parameters are defined at measurement points which coincide with interfaces shown in Figure 8-6a. As you can see, the switch congestion management and queuing algorithms play a critical role in the performance an ATM network can deliver. Figure 8-6b graphically depicts the three negotiated QoS parameters: the Cell Transfer Delay CTD, the Cell Delay Variation CDV, and the Cell Loss Ratio CLR. Figure 8-6 a QoS parameters are defined at measurement points which coincide with interfaces.