CSCI 278 : COMPUTER NETWORKS

Instructor : Seung Bae Im

MIDTERM EXAM ----- Fall 2001

 

 

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

 

Name : _____________________________

 

1.(15 points) For the OSI 7 layers, describe the following terms: (a) Peer-to-peer virtual communications, (b) Data encapsulation/decapsulation, (c) Service Access Points,

 

2.(15 points) (a) There have been three generations of LANs(1st, 2nd, 3rd). Describe the evolution of LANs including the following terms: Ethernets, Token rings, 10BaseT, Ethernet Switches, 100BaseVG LAN (b) Over the generations of LANs, Ethernets dominate. State the reasons.

 

3.(10 points) Describe

(a) Describe how Ethernets achieve the synchronizations; (1) Bit synchronization (2) Frame synchronization.

(b) There are two modes of synchronizations in communication: Asynchronous(start-stop) mode and synchronous mode(bit-oriented or character-oriented, …). Point out the difference between them.

 

4.(20 Points) Answer the questions.

      (a) Describe how “multibit modulation” works.

(b) State how 4B/5B codes operate and also state why 4B/5B code is more efficient than Manchester code.

(c) Show(in a diagram or in calculation) how the data rate of T1 carrier, 1.544Mbps is achieved.

(d) Describe how the Ethernet Switches operate.

 

5.(10 points) Describe

(a)    Describe how a “Ethernet Bridge” operates including the terms:learning bridge, table(in bridge), port numbers, MAC addresses.

(b)   Describe why the 10BaseT(also 100BaseT) is most popular among the Ethernet physical layer choices.

 

6.(30 points) Discuss/describe

(a)    Describe the three ARQ modes(Stop-and-wait, Go-back-N, Selective-Repeat).

(b)   Draw the protocol diagram(showing layers) for the LAN protocols—Show IEEE 802.2, 802.3, 802.5.  Also clearly show the Physical, MAC and LLC layers on the diagram.

(c)    Ethernets have two frame formats(Ethernet II & IEEE802.3). The main difference is on their 2 byte fields: On Ethernet II, these 2 byte field is called “Type” field. On IEEE802.3, they are called as “Length” field. Describe how those two different frame formats can co-exist on LANs.

(d)   State why TCP/IP protocols dominate among many existing protocols.

(e)    Given the following diagram for IP packet format, describe “Fragmentation of IP packets: (a) Why fragmentation is necessary? (b) Where the fragmentation occurs? (c) How and Where the fragmented IP datagrams are reassembled?

In your answers, include the terms: “Identification”, “Fragmentation Offset”, “MTU(Maximum Transfer Unit)”

 

0                   1                   2                   3   
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version|  IHL  |Type of Service|          Total Length         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Identification        |Flags|      Fragment Offset    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Time to Live |    Protocol   |         Header Checksum       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       Source Address                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Destination Address                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Options                    |    Padding    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+