CS 416/516 Computer Communications and NetworksFall 1993Midterm Exam (November 2)Prof. Thomas Narten

NAME: SCORE:

This is a closed book (and notes) examination; however, you may consult your summary. Answer all questions on the exam itself. Justify your answers! You must explain your reasoning to receive full credit on short answer questions.

All questions have equal weight. However, some questions are harder than others; budget your time accordingly.

Students enrolled in CSI 516 must answer all questions. Students enrolled in CSI 416 may omit any two questions. However, CSI 416 students must clearly mark those questions that are not to be graded, or I will assume you wish to omit the final two questions on the exam.

1. In which of the OSI reference model layers would you place the following tasks:
   1. Insuring that packets correctly reach the destination machine on which the peer application resides, even if retransmissions are required?

   2. Performing bit-stuffing to signal the end of a frame.

   3. Accepting requests from remote machines to perform a remote login service, through which users can invoke commands, run programs, etc.

   4. Specifying the use of a carrier signal with frequency-shift modulation to denote a 0 or 1 bit.

   5. Converting data into a standard form so that (for instance) ASCII and EBCDIC machines can communicate.

   6. Splitting up large packets into smaller ones when a packet is too large to be transported in one piece across one hop of a multi-hop path.

2. How wide would the frequency spectrum of a channel need to be in order to carry 56kbps of data if the channel had signal-to-noise ratio of 30db?

3. What is the checksum for the message ``1101'' using a CRC generator polynomial of ``11''?

   Is this checksum any better at detecting errors than a simple parity bit scheme? Explain.

4. Consider a sliding window protocol across a channel in which the one-way propagation delay is 60ms. If the channel has a transmission rate of 56kbps, is there any performance benefit in using a send window larger than 10 frames in size? Assume frames are 1000 bits in size.

5. In the ``old'' Arpanet routing algorithm, updates were generated at fixed intervals (e.g., every 10 seconds). Consider the following alternative: Upon receipt of a routing update that changes the local routing table, immediately send a copy of the new routing table to all neighbors. Does this scheme have any benefits? Disadvantages?

6. Describe (in detail!) the exponential backoff algorithm used by the Ethernet.

   Why does the Ethernet use an exponential backoff as opposed to linear backoff? In linear backoff, a station waits for a time after a collision before retransmitting, where n is the number of transmission attempts and C is a constant.

7. Consider the use of repeaters vs. (spanning tree) bridges for interconnecting Ethernet segments. Which approach is preferable (and why):
   1. if the primary concerns is preventing the failure of one LAN from partitioning the network into isolated segments,

   2. if the primary concern is keeping traffic between hosts attached to the same cable local to that cable,

   3. if the primary concern is minimizing delay.

8. In part II of the client-server project, a call to WaitForEvent was inserted in the application layer. Why was this call needed?

9. Consider a token ring network. How long does it take for a station to ``seize the token'' in the worst case? Assume that there are 150 nodes on the ring.

   How much time is a token delayed at each station, if the station does not seize the token (e.g., it has no data to send?)

About this document ...

This document was generated using the LaTeX2HTML translator Version 0.6.4 (Tues Aug 30 1994) Copyright © 1993, 1994, Nikos Drakos, Computer Based Learning Unit, University of Leeds.

The command line arguments were:
latex2html -split 0 mid.tex.

The translation was initiated by Thomas Narten on Sat Oct 12 13:56:11 EDT 1996