• United States
Neal Weinberg
Contributing writer, Foundry

Filtered routers

Aug 05, 20033 mins

* The Reviewmiester tested ImageStream Rebel and Lucent's Access Point 1500 to determine how well they would perform when filtering was introduced

Last week, we reviewed routers from Riverstone and Tasman to determine how well they performed when filtering was introduced. This week, we run ImageStream Rebel and Lucent’s Access Point 1500 through their paces.

We measured the performance effect of filtering with three metrics: throughput, average latency and maximum latency. To determine routers’ ability to recover from failure, we also measured reboot times under load for each device.

When it comes to throughput, there’s only one right result: line rate. Both ImageStream and Lucent achieved that result, no matter what the frame size, filter count or number of routes involved.

There might not have been major differences among products in throughput, but there certainly were when it came to latency.

Even at relatively slow T-1 rates, latency for 64-byte packets can theoretically be 500 microsec or less. The Riverstone 3000 and the Tasman 1004 stuck pretty close to that theoretical mark when we looked at average latency but ImageStream’s Rebel registered average latency scores that were nearly double that.

There’s no one good answer as to how much latency is acceptable. Humans perceive degraded video quality with delays of as little as 10,000 microsec, and degraded audio quality with delays of 50,000 to 200,000 microsec. For data applications, the threshold might be higher (sometimes much higher).

Not all vendors exhibited big differences between average and maximum latency. ImageStream’s Rebel fared the best in this regard; across all tests, it had the smallest difference between average and maximum latency.

Lucent’s Access Point 1500 presented a special case. According to the router testing RFC, latency is supposed to be measured at the throughput level. However, the Lucent Access Point 1500’s buffers are so large that we exceeded line rate when measuring throughput – so much so that the routers continued to forward packets for up to 17 seconds after the test was stopped. This led to absurdly high latency readings. Lucent reduced the size of its buffers for this test but even so recorded higher maximum-latency readings compared with most other routers. Big buffers can be helpful, especially if most traffic is bulk data transfer, but users looking to deploy the Lucent routers for delay-sensitive applications might want to reduce buffer size. It definitely helped in our test case: Average latency measurements hovered around 2,000 microsec, third lowest in our 256-byte test case.

With 256-byte frames, average latency differences among products were somewhat less pronounced. The ImageStream Rebel exhibited the least variation among different test cases, suggesting that latency won’t be affected no matter how many filters or routes are involved.

When it comes to maximum latency, Lucent’s Access Point 1500 also showed relatively high maximum latencies in a couple of the large-routing-table test cases. As a rule, however, most routers delayed packets by the same amount regardless of the amount of filtering or routing going on with the large frames. Lucent’s routers might have struggled a bit with our huge routing tables, but, in general, the delay they introduced was not enough to degrade performance of any application.

When it came to re-boot times, Lucent’s Access Point 1500 recovered the fastest, at just 23.6 seconds. The closest competitor, ImageStream’s Rebel, came back to life after 64.5 seconds of downtime. For the full report, go to