Transimission Delay versum Propagation Delay

Transmission delay
Assuming that packets are transmitted in a first-come-first-serve manner, as is
common in the internet, it can be transmitted once all the packets that have
arrived before it have been transmitted. If we denote the length of the packet by
L bits and the transmission rate of the link from router A to router B by R
bits/second, the transmission delay (also called the store and forward delay) is
L/R. This is the amount of time required to transmit all of the packet’s bits onto
the link. Transmission delays are typically of the order of microseconds or less in
practice. R is determined by the transmission rate of the link to router B.
(Examples of R would be 10 Mbps for a 10-Mbps Ethernet link).

Propagation delay
Once a bit is pushed onto the link, it needs to propagate to router B. The time
required to propagate from the beginning of the link to router B is the
propagation delay of the network. The bit propagates at the propagation speed of
the link. The propagation speed depends on the physical medium of the link.
(e.g. multimode fiber, twisted-pair copper wire) and is in the range of :

2 ×10^8 meters / sec to 3×10^8 meters / sec .
This is equal to or a little less than the speed of light. The propagation delay is
thus the distance between the routers divided by the propagation speed. That is
the propagation delay is d/s, where d is the distance between the routers and s is
the propagation speed. Once the last bit of the packet propagates to B, it and all
its preceding bits are stored in B. B then performs the whole process of
forwarding. In wide area networks, the propagation delay is of the order of
milliseconds.

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