Round Trip Time calculation in Bandwidth Delay product

Question

As far as I know, the "Delay" in BD product corresponds to RTT.

In some cases RTT is considered as 2*Tp, specially when calculating min, frame size in CSMA/CD

In some cases RTT is considered as Tx+2*Tp, specially when calculating the Optimal Window Size(w=1+2a)

Why is this the case? I agree with the 2nd case. But for first case, why 2*Tp is considered

Comments

This might help 

Computer Networks: Round Trip Time

Answer 1

When the receiver only sends acknowledgements, the transmissioon time for acknowledgements is considered negligible. So,

total RTT = 1 T_p  from sender to receiver + 0 T_transmission from receiver + 1 T_p  from receiver to sender

But when the acknowledgements are piggybacked, the receiver is sending acknowledgements piggybacked on its data. So, its basically its sending a complete packet.

So, Rtt = 1 T_p  from sender to receiver + 1 T_transmission from receiver + 1 T_p  from receiver to sender