Bandwidth-Delay Product

Question

Its given that the bandwidth-delay product defines the number of bits that can fill the link. The sender should send a burst of data of (2*bandwidth*delay) bits. I am not getting why the term bandwidth*delay multiplied by 2.Please Explain the reason???

Answer 1

It depends what you mean by "delay". If delay is the round trip time (RTT) then you wouldn't multiply it by two. Presumably, in the formula you are looking at, the delay is the unidirectional transmiasion time, so you multiply it by 2 to estimate the RTT.

One RTT is the earliest time you could get an acknowledgement back for the first bit you transmitted, so that's why your window should be that big in order to fill the pipe.

Answer 2

Delay in your case is the propagation delay which is the time taken by the signal(message) to propagate from sender to receiver.

It is multiplied by 2 because the link is bidirectional i.e sender and receiver can both send the data at the same time i.e in order to completely fill the link you need to multiply the propagation delay by 2 and this term is known as round trip time(RTT).

bandwidth-delay product = RTT * bandwidth
bandwidth-delay product = 2 * propagation delay * bandwidth

where

RTT = 2 * propagation delay