circuit-switch

Question

Suppose a digital network implemented by circuit-switch technology which has a data rate of 1 Mbps. Assume that 2 machines are connected to this network and they are separated by 5000 Km with 2 routers in the path. If 1000 bits has to exchange between these 2 machines, What is the delay if the propagation speed of signal is 2 X 10⁸ m/s? Assume that the setup phase is a two-way communication and the teardown phase is a one-way communication.

1.   104 ms
2.    102 ms
3.    98 ms
4.    92 ms

Answer 1

Setup phase two way communication hence two propagation delays needed.

Teardown phase one way communication so 1 propagation delay needed.

For setup and teardown we need minimum size packet to be exchanged between A to B. Here something missing in the question that haven't specified packet size.So let's assume it is of 1000 bits.

More to transfer actual data 1 propagation and 1 transmission delay needed same way for setup and teardown phases.

So ans : 4[propagation delay + Transmission Delay] =104ms

Plz correct !

Comments

Question source gave the below explanation :

We assume that the setup phase is a two-way communication and the teardown
phase is a one-way communication. These two phases are common for all three
cases. The delay for these two phases can be calculated as three propagation delays
and three transmission delays or
3 [(5000 km)/ (2 ×10⁸ m/s)]+ 3 [(1000 bits/1 Mbps)] = 75 ms + 3 ms = 78 ms
We assume that the data transfer is in one direction; 
the total delay is then
delay for setup and teardown + propagation delay + transmission delay
78 + 25 + 1 = 104 ms

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could you please put some light on the below sentence I am unaware of teardown phase

"Setup phase two way communication hence two propagation delays needed.

Teardown phase one way communication so 1 propagation delay needed."

how is the exact communication happening between A & B