@ manu00x
This question is based on " pipelining in Packet Switching "
Read 2nd benefit of packet switching here :
http://www.erg.abdn.ac.uk/users/gorry/course/intro-pages/ps.html
Transmission delay refers to the time it takes to transmit a whole packet over a link ( all the bits ) from one end to other end and is given by L/R, where L is the amount of data and R is the rate.
Propagation delay refers to the time it takes for a single bit, once on the link, to reach the destination.
That means it is the time to transmit one bit over a link from one end to another end .
for first packet shouldn't we consider Tp twice, once from A to S and then from S to B?
Yes , two time Tp is to be consider , because we consider propagation time only for first packet not for first link ..so from A to S one time Tp is consider then again from S to B another time Tp need to be consider.
At the time packet 1 is sent from B to C, packet 2 is sent from A to B; packet 1 is sent from C to D while packet 2 is sent from B to C, and packet 3 is sent from A to B, and so forth.
This simultaneous use of communications links represents a gain in efficiency, the total delay for transmission across a packet network may be considerably less than for message switching .
So, this concept is used here .