Summary: Course No.___CS 456/CS 378________ Roll No._______________________
Computer Networks, Spring 2006: Mid-semester Test,
Wednesday, Feb. 22nd
, 2006. Marks: 25, Weight: 25 %
1. (6 marks) Assume you wish to transfer an nbyte file along a path composed of the source,
destination, and 3 switches (4 links). Suppose each link has propagation delay 2 ms, bitrate 4Mbps,
and that the switches support both packet switching and circuit switching. Thus, you can either
break up the file into 1KB packets or set up a circuit through the switches and send the file as one
contiguous bit stream. Suppose packets have 24 bytes header, and 1000 bytes data, packet
processing delay at switches is 1 ms after receiving the packet, that packets can be sent without
waiting for acks. Suppose the circuit setup requires a 1 KB message from source to destination and
another 1 KB message back from destination to source, incurring the same 1 ms switch processing
delay. Assume further, that there is no such processing delay when transferring data over an
established circuit. Also assume file size is a multiple of 1000 bytes.
a) For what file size n bytes is the total number of bytes sent across the network less for circuits
than for packets?
b)For what file size n bytes is the total latency incurred for the entire file to arrive at the
destination less for circuits than for packets?
(a) Packet Switching: 2 marks
No. of bytes transferred = no. of packets * packet size