From CS 312 Fall 2011

CS312-F11: Quiz 1 - Propagation and Transmission Delays

Tuesday, Sep 27, 2011

This is an actual question from the 2010 mid-term exam.

Consider a network with a single 5 Mbps (megabits per second) link and a 15 ms (millisecond) propagation delay. The sender transmits a 3000-byte packet to a receiver.

Give all answers in units of milliseconds. Show your work. Circle your final answer.

1. What is the transmission delay for the packet?
2. Assuming transmission begins at time 0, when will the first bit of the packet arrive at the receiver?
3. What is the total end-to-end delay for the packet, i.e., at what time does the entire packet reach the receiver?
4. What is the total end-to-end delay for a 6000-byte packet?

Solutions

1) 4.8 ms

 dtrans = L/R

 L = 3000 B = 24,000 b

 24,000 b * 1000 ms        24
           ----------   =  --- ms = 4.8 ms
           5,000,000 b      5 

2) 15 ms (this is the definition of propagation delay, which is given in the problem statement)

3) 19.8 ms

 de2e = dtrans  + dprop 
 de2e  = 4.8 ms + 15 ms = 19.8 ms

4) 24.6 ms

 de2e = dtrans  + dprop 

 dtrans = L/R

 L = 6000 B = 48,000 b

 48,000 b * 1000 ms        48
           ----------   =  --- ms = 9.6 ms
           5,000,000 b      5 

 de2e = dtrans  + dprop 
 de2e = 9.6 ms + 15 ms = 24.6 ms

Note: d_trans for the 6000-byte packet could also be calculated by doubling d_trans for the 3000-byte packet.