Sprint was the only provider to install the Global Positioning System receivers necessary for precise measurements of one-way delay - the time it takes for each packet to move across Sprint’s network. For many network designers, delay is even more important than throughput in predicting application performance.

Sprint’s delay measurements were generally low and consistent, suggesting Sprint’s network won’t have any adverse ramifications on application performance. However, we did observe one 30-minute period in which something in the provider’s New York point of presence went seriously wrong. During this one bad patch -- the only one during the entire four-week test - delays approached the 1-second mark.

That kind of delay is large enough to have serious ill effects on application performance. For TCP traffic, such high delay can lead to retransmissions and loss of TCP sessions, as cited by several public studies (For more, click here).

Initially, we asked all ISPs to install GPS receivers at all sites so we could measure one-way delay. GPS is necessary to ensure precise synchronization between sender and receiver. Absent synchronization, measurements can be so far off that it’s possible to record "negative delay," where the receiver’s timestamp will indicate that it gets a packet before the transmitter has sent it.

All carriers except Sprint balked at the GPS idea, with most citing cost and logistical difficulties of GPS installation. Many ISP POPs are located in basements, far from the view of the sky, which a GPS receiver requires. Further, not all ISPs own the buildings that house their POPs. Getting permission to put up an antenna can be difficult.

As a fallback plan, we offered to measure other ISPs’ delay by synchronizing transmitters and receivers using the network time protocol (NTP). Unfortunately, this method didn’t work out. The issue that killed NTP as our back-up plan was jitter -- variations in timing -- between the clock sources and the cNode measurement instruments.

Despite our best efforts to reduce jitter (including use of multiple NTP sources), we couldn’t reduce it below our goal of 2 msec. Rather than use imprecise clocks, we simply dropped one-way delay measurements for all providers not using GPS.