Switch jitters

While low latency is critical for delay-sensitive voice or video, it's not the only requirement. Delay also should remain constant from frame to frame. Some voice and video applications are even more sensitive to variations in latency than latency itself.

To see how well Enterasys Networks' Matrix E1 handles latency variation, we measured the delay added for every single frame we offered (more than 2 billion frames in some tests) and noted the difference between minimum and maximum measurements per stream.

In nearly all cases, latency variation was about the same or greater than the latency measurement.

Latency for 1,518-byte frames with 250 logical hosts per interface was around 47 microseconds. But maximum latency averaged about 93 microseconds per stream, and minimum latency averaged about 28 microseconds per stream. The result is an average latency range of about 65 microseconds, 28% higher than average latency.

The variations aren't enough to upset most applications by themselves; performance degradation really begins to show up in the millisecond range. However, two factors are worth bearing in mind. First, we ran our tests on a congestion-free network. In a congested network queuing certainly will add to latency, and latency variation will only make matters worse.

Second, latency and latency variation are cumulative, so a network comprised of many Enterasys devices might begin to show significant latency variation. Given the low-microsecond delay numbers posted by the Matrix E1, this second factor probably won't be a concern for any but the largest enterprise networks.