Christopher, The factors influencing latency are propagation delay (Pd), transmission delay (Td), and queueing or processing delay (Qd). For simplicity, assume Qd is negligible, then L = Pd + Td. Pd, as suggested by others, is: distance/.66c Transmission delay is the time it takes to transmit X bits over a link of bandwidth Y, without acknowledgement. For the single packet case, this reduces to a worst case scenario of the MTU/Bandwidth. For TCP applications, assuming a fully realized window WIN = (min{cwnd, rwnd}), this reduces to WIN/BW. Since it is simpler to dicuss the MTU case, you get the folowwing formula: L = ((MTU/BW) + (dist/(.66c))) So, given a T1 that traverses 4000 meters and an MTU of 1500 bytes (8000 bits), you get a one-way Latency: L = ((8000/1.536E6) + (4e6/(.66*3E8)) = 25.4 msec. For an OC-48 L = ((8000/2.448E9) + (4e6/(.66*3E8)) = 20.2 msec For 2900 mile circuit, you get L = ((8000/1.536E6) + (2900*1.6e3/(.66*3E8)) = 28.2 msec Note that distance begins to dominate the delay as distance increases. That is, a short OC-48 transmits 1500 bytes much faster than a T1, but an OC-48 to Jupiter transmits the data at nearly the same rate as a T1 (both take a really long time!) chris

-----Original Message----- From: Christopher Wolff [mailto:chris@bblabs.com] Sent: Sunday, September 30, 2001 10:09 PM To: nanog@merit.edu Subject: Long haul latency calculation?

Dear Nanog:

I was wondering if there is a benchmark for long-haul circuit latency... For example if I had a T1 circuit with 2900 miles between the two end-points (and assuming the provider is best case scenario) can I do something like (miles*latencyfactor) = 5 ms for 2900 miles?

Thanks, Christopher