CS176A - Introduction to Computer Communication Networks
Homework Assignment #2
Due BEFORE (2:00pm) Class on Thursday, October 23rd, 2003

Assignment Overview

The purpose of this assignment is to make sure you grasp the concepts related to the OSI stack model in general, along with a focus on the physical, data link and network layers.


Some of the questions in this assignment are basic, while others will require some extra reading/searching (in the book or using Google) so that you could build on the basics that you know.


Use as much space as needed to answer the following questions. If you use Google search engine, list the URL's used. Include your name and e-mail address as the first line of the homework.


  1. Why do we have layers in the OSI model and protocols in general? (6 points)


  1. Mention the seven layers of the OSI stack, and describe the basic function of each layer in one sentence. (6 points)


  1. What are the differences between the circuit switched and packet switched networks? Give examples of each. (6 points)


  1. What is the difference between hubs, switches, bridges, routers, repeaters and amplifiers? (10 points)


  1. What is the minimum and maximum length for an Ethernet frame? Why do we have those minimum and maximum lengths? (8 points)


  1. What would the string 0111101111101111110 be after bit-stuffing (flag is 01111110)? (4 points)


  1. Could there be collisions on a switch? Justify your answer. (6 points)


  1. Rank the following LANs from most secure to least secure: Switched LAN, Wireless LAN and Shared LAN. Explain your reasoning. (6 points)


  1. What are the advantages and disadvantages of Manchester encoding when compared with Binary encoding? Are there any other encoding schemes that exist? If yes, give at least one example and briefly explain how it works. (10 points)


  1. What would happen if two machines on the same LAN have the same MAC address? (6 points)


  1. We know that all our computers (and most routers) nowadays are electronic based, i.e. they operate with voltages between them, how do we have optical networks work with such computers or routers since all the optical signals are based on light (photons) and not voltages? Why is this a challenge for building faster networks? What is one way this problem could be solved? (10 points)


  1. What would happen if two computers in the world had the same IP address? Justify your answer. (6 points)


  1. If you saw the following entries in a routing table: 54.210/16 and 54.211/16. Is there a way to combine them? If yes, what would the alternative entry or entries look like? (6 points)


  1. Assume each packet has typical TCP and IP headers each 20bytes long. If we have three computers, A, B and C. The link between A and B has an MTU of 3000 bytes, while the link between B and C has an MTU of 1000 bytes. Consider the case where a packet needs to be sent from A to C that has a size of 3000 bytes (including headers). How many fragments will we have from B to C, and how much data will be in each fragment (i.e. excluding headers)? (Note: assume the network connecting A, B and C is Ethernet)(10 points)


Assignment Turnin

This assignment will be turned in using the CS Department turnin program. From any computer in CSIL type the following command to turnin your answers to this homework:

csil-machine>turnin hw2@cs176a hw2.ps

Be sure to use exactly hw2@cs176a for this assignment. However the last argument is the name of the file containing your homework answers and can be any name you choose.

This assignment should be turned in a suitable format. Suitable formats include HTML, PostScript, PDF and plain text. Unfortunately, Microsoft Word documents are not suitable, they are too prone to viruses. However, that does not preclude you from using Microsoft Word to type out your answers. Simply save the Word document as an HTML file or a PostScript file if you know how. If you have any questions on a format ask the TA's.

Note: See the syllabus for the late turn-in policy. (No late assignments will be accepted.)