Long haul latency calculation?

Tue Oct 2 01:13:56 UTC 2001

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 at bblabs.com]
> Sent: Sunday, September 30, 2001 10:09 PM
> To: nanog at 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
> 