Why does the delay in sending a packet from source to destination doesn't depend upon the transmission rate and the packet length? If we look at the delay formula then it is, L/R + D/V.
It's a component, but in most WAN situations it's the smallest contributor.
Assume a 100 Mbit/s link speed and a 1500 byte packet. Serialisation delay is 1500 bytes x 8 bits/byte / 100,000,000 bits per second = .00012s.
Many servers do process switching 1000 times per second. This means a processing delay of 1ms (0.001s) is common, or eight times as long as the delay from packet length.
Speed of light in fibre is approximately 200,000 km/s, so the packet transmission delay matches 24km of length, and the processing delay is equivalent to 200 km. Unless you're talking to a data centre in your local city, speed of light delay is typically the largest factor.
There are several delays:
- propagation delay = distance / wave propagation velocity (D/V)
- serialization delay = packet length / bitrate (L/R)
- processing delay = device dependent
- access delay (only for shared/multiple-access media) = depends on access method (CSMA/CD, CSMA/CA) and channel load
Add them together for total delay.