Accordign to my knowledge latency is 'the time measured from when packet enter into router to the time it out from router'. Is it right? I am confused so How to check or calculate it for leased lines like t1 and ds3.
Thanks and Regards
You can use the ping command to determine the round trip latency. This will be impacted by queueing and serialization delay, but will be pretty accurate across a lightly loaded T1 or faster link.
You can calculate the propagation time at 5 microseconds per kilometer for WAN links.
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Latency is the time it takes for a packet to travel from point A to point B. The total latency in a packet network will consist of propagation time plus handling time by routers. Propagation time will be a factor of the speed of light. Speed though optical fiber will be about .5 c, meaning 50% of the 300,000,000 meters per second that light travels at. This is where the 5 microseconds per kilometer is derived from.
Each router that handles a packet on the path from point A to point B will take some time to do it's job. With no load (light traffic level) that is usually 1-2 milliseconds, but will quickly rise if the link becomes full and the router has to start buffering packets before sending. So a packet that crosses the country through 15-20 routers will often have a round trip latency of over 200 milliseconds even though the propagation component is only 40-50 milliseconds.
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Thanks for this now query is clear.
Now my question is that is there any spacified latency for DS3 link and E1 link.
This can be measure by ping responce time ?
Yes, the ping will measure the time it takes for the ICMP echo packet to travel to the far end, and the ICMP echo reply to come back. If you ping from a router on one end of the link to the router on the other end you will be measuring round trip packet latency, including router queueing time. The result will vary depending upon distance, loading and QOS configuration.