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Network latency for Clos networks

Network latency is defined as the delay from the transmission of the packet header at the source to the reception of the end-of-packet at the destination. Figure 4 shows the latency of four different size Clos networks under random traffic as a function of the aggregate network throughput. The packet length is 64 bytes. The results are produced by varying the network load and measuring the corresponding throughput and latency values. It can be seen that the average latency increases rapidly as the network throughput approaches saturation.

 

 
Figure 4: : Latency versus throughput for Clos networks under random traffic with 64 byte packets 

Some applications, e.g. multimedia traffic, may require statistical bounds on the maximum latency values occurring. This information can be obtained from figure 5 which shows the probability that a packet will be delayed by more than a given latency value for various network loads. The results have been obtained on a 256 node Clos network. Again the traffic pattern is random, with a packet length of 64 bytes. From figure 4 it can be seen that this network saturates at 1.35 Gbytes/s which corresponds to a throughput of 57%.

 

 
Figure 5: : Cumulative latency distribution for a 256 node Clos network under random traffic with 64 byte packets 

For 10% load the cumulative latency distribution is narrow and only a small percentage of the packets (0.01%) are delayed by more than 3 times the average latency value of about 11 s. As the network load increases, the tail of the latency distribution get wider and near the saturation throughput a significant fraction of the packets experience a latency many times the average value, e.g. at 50% load 0.7% of the packets are delayed by more than five times the average latency of 21 s. To reduce the probability of large latency values the network load must therefore be kept well below the saturation throughput.


next up previous
Next: Effect of grouped adaptive Up: Results Previous: Comparison of network topologies

Roger Heeley
Fri Sep 26 17:00:08 MET DST 1997