# RE: Long haul latency calculation?

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