We are sending a 30 Mbit MP3 file from a source host to a destination host. All links in the path between source and destination have a transmission rate of 10 Mbps. Assume that the propagation speed is 2 * 108 meters/sec, and the distance between source and destination is 10,000 km. 
a.Suppose there is only one link between source and destination. Also suppose that message switching is used, with the message consisting of the entire MP3 file. What is the transmission delay? 
Answer: 
dtran = 30Mbits/10Mbps = 3 second 
b.Refer to the question above, calculate the end-to-end delay (transmission delay plus propagation delay).
Answer: 
d = dtran + dprop in which 
dprop = 10,000km/2 * 108 meters/sec = 0.05 seconds
d = dtran + dprop = 3 + 0.05 = 3.05 seconds
c.Refer to the above question, how many bits will the source have transmitted when the first bit arrives at the destination?
Answer: 
This is when the 1st bit arrives at the destination, i.e., propagation delay.
10Mbps * dprop = 10Mps * 0.05 = 500,000 bits
d.Now assume there are two links between source and destination, with one router connecting the two links. Each link is 5,000 km long. Again suppose the MP3 file is sent as one message (message switching). Assume that there is no congestion (i.e., no queuing delay) and negligible router processing delay, so that the message is transmitted onto the second link as soon as the router receives the entire message. What is the end-to-end total delay?
Answer: 
For each link,
dtran = 30Mbits/10Mbps = 3 second
dprop = 5,000km/2 * 108 meters/sec = 0.025 seconds
Total delay = 2 * (dtran + dprop ) = 6.05 seconds
e.Now suppose that the MP3 file is broken into 3 packets, each of 10 Mbits. Ignore headers that may be added to these packets. Also ignore router processing delays. Assuming store and forward packet switching at the router, calculate the total delay.

Answer: 
For each link,
dtran = 10Mbits/10Mbps = 1 second
dprop = 5,000km/2 * 108 meters/sec = 0.025 seconds
Total delay = (3+2-1) * dtran + 2 * dprop  = 4.05 seconds
Where 3 is the no. of packets, 2 is the no. of links.