CSCI 278 : COMPUTER NETWORKS

Instructor : Seung Bae Im

MIDTERM EXAM ----- Fall 2000

 

 

Closed book, closed notes. Print your name on each page of the answer sheets.  Do not write on the back of a page on the answer sheets(they will not be graded). If necessary, state all assumptions clearly.

 

                                                            Name:___________________________________

 

1.(25 points) Describe the concept of OSI Reference Model. In the description use the following terms: Layer’s services, Service Provider, Service User, Service Access Points, Peer-to-peer virtual communication, Data encapsulation/decapsulation, Nth layer protocol.

 

2.(10 points) Local Area Networks have been evolving through 3 generations(4th in sight) so far. Describe each generation: Their speeds, MAC protocols, Standards. Include the terms: 1st Gen, 2nd Gen, 3rd Gen, also include switches. State the reasons why Ethernet dominates in LAN technologies, even through all generations of LANs.

 

3.(15 points) Describe

(a) There are two modes of synchronization in communication: Asynchronous mode and synchronous mode. Point out the difference between them.

(b) Describe how Ethernets achieve the frame synchronization. Is it an Asynchronous mode or a Synchronous mode?

 

4.(20 Points) Answer the questions.

(a) The bandwidth of a human voice ranges between 20Hz and 20KHz but we usually limit it to a narrower range(typically between 300Hz and 3400Hz). Why?

(b) Z1 carrier(a new digital carrier that you are designing) is designed to carry 64 channels(62 voice channels plus one signaling channel and one synchronization channel). Each sample of a voice will be represented with 8 bits and the voice bandwidth is limited to 8Khz. What is the total data rate of your digital carrier system?

(c) For the implementation of the above system, you are given a channel with 4MHz bandwidth. Calculate S/N(Signal to Noise Ratio) using Shannon’s formula          ( C= H log2 ( 1 + S/N ) ) bps  where H is the bandwidth of the channel). Exact value is not required.

(d) Shannon’s formula is not considered to be accurate. Why?

 

5.(10 points) Describe

(a) Why do we twist wires in UTP?

(b)   What was the original intention of the OSI Model? How is it used now?

 

6.(20 points) Discuss/describe

(a)    Draw the protocol diagram(showing layers) for the LAN protocols—Show IEEE 802.1, 802.2, 802.3, 802.5…..and name them. Also show the MAC and LLC layers.

(b)   List and briefly describe the 3 Media sharing mechanisms(MAC) for LANs.