Consider a source computer (S) transmitting a file of size 10^6 bits to a…
20122025
Consider a source computer (S) transmitting a file of size 10^6 bits to a destination computer (D) over a network of two routers (R1 and R2) and three links (L1, L2, and L3). L1 connects S to R1; L2 connects R1 to R2; and L3 connects R2 to D. Let each link be of length 100 km. Assume signals travel over each link at a speed of 10^8 meters per second. Assume that the link bandwidth on each link is 1Mbps. Let the file be broken down into 1000 packets each of size 1000 bits. Find the total sum of transmission and propagation delays in transmitting the file from S to D?
- A.
1005 ms
- B.
1010 ms
- C.
3000 ms
- D.
3003 ms
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Correct answer: A
Key idea: packets are sent in 1000 pieces and routers forward packets as they arrive, so transmissions are pipelined.
Packet transmission time t_tx = 1000 bits / 1,000,000 bps = 0.001 s = 1 ms.
Propagation per link t_prop = 100 km = 100,000 m; speed = 10^8 m/s ⇒ t_prop = 100,000 / 10^8 s = 0.001 s = 1 ms.
For N = 1000 packets and L = 3 links, the end-to-end time with packet-level store-and-forward pipelining is (N + L - 1)·t_tx + L·t_prop.
Compute: (1000 + 3 - 1)·1 ms + 3·1 ms = 1002 ms + 3 ms = 1005 ms.
Therefore the total sum of transmission and propagation delays to deliver the file from S to D is 1005 ms.
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