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    TC 635 Local Area Networks

Homework Assignments:


Read Chapter 15 in Stallings. Do problems: 15.2, 15.4, 15.7


  1. What percentage of an ATM link's total bandwidth is consumed by the ATM cell headers? What percentage of the total bandwidth is consumed by all nonpayload bits in AAL3/4 and AAL5, when the user data is 512 bytes long?
  2. The IP datagram for a TCP ACK message is 40 bytes long: it contains 20 bytes of TCP header and 20 bytes of IP header. Assume that this ACK is traversing an ATM network that uses AAL5 to encapsulate IP packets. How many ATM packets will it take to carry the ACK? Why if AAL 3/4 is used instead?
  3. The CS-PDU for AAL5 contains up to 47 bytes of padding, while the AAL3/4 CS-PDU only contains up to 3 bytes of padding. Explain why the effective bandwidth of AAL5 is always the same as, or higher than, that of AAL3/4, given a PDU of a particular size.


1. Consider the arrangement of learning bridges shown in Figure 7.1. Assuming all are initially empty, give the forwarding tables for each of the bridges B1-B4 after the following transmissions:

Identify ports with the unique neighbor reached directly from that port; that is, the ports for B1 are to be labeled "A" and "B2".

2. Consider hosts X, Y,Z, W and learning bridges B1, B2, B3 with initially empty forwarding tables, as in Figure 7.2.

  1. Support X sends to Z. Which bridges learn where X is? Does Y's network interface see this packet?
  2. Support Z now sends to X. Which bridges learn where Z is? Does Y's network interface see this packet?
  3. Suppose Y now sends to X. Which bridges learn where Y is? Does Z's network interface see this packet?
  4. Finally, support Z sends to Y. Which bridges learn where Z is? Does W's network interface see this packet?

3. Given the extended LAN shown in Figure 7.3, indicate which ports are not selected by the spanning tree algorithm. Read about Spanning Tree algorithm in the text book Chapter 14, page 490.


Some of the material covered in last class is also available in PPT slides from this site. Look at the slides for chapter 3.

1. Using the example given in figure 1, give the VC tables for all the switches after each of the following connections is established. Assume that the sequence of connections is cumulative; that is, the first connection is still up when the second connection is established, and so on. Also assume that the VCI assignment always picks the lowest unused VCI on each link, starting with 0.

  1. Host A connects to host B
  2. Host C connects to host G
  3. Host E connects to host I
  4. Host D connects to host B
  5. Host F connects to host J
  6. Host H connects to host A

2. For the network given in figure 2, give the datagram forwarding table for each node. The links are labeled with relative costs; your tables should forward each packet via the lowest cost path to its destination.

3. Give an example of a working virtual circuit whose path traverses some link twice. Datagrams sent along this path should not, however, circulate indefinitely.

4. Propose a mechanism that virtual circuit switches might use so that if one switch loses all its state regarding connections, then a sender of packets along a path through that switch is informed of the failure.

HW5:        1 & 2. from page http:// answer questions 16 and 17 ( "In the IEEE 802.11 specification...")

3 & 4. Chapter 11, Problem 11.4, 11.5

HW4:        1. An IEEE 802.5 token ring has five stations and a total wire length of 230 m. How many bits of delay must the monitor insert into the ring? Do this for both 4 Mbps and 16 Mbps; use a propagation rate of 2.3 x 10^8 m/s.

2. For a 100-Mbps token ring network with a token rotation time of 200 Ás and that allows each station to transmit one 1-KB packet each time it possesses the token, calculate the maximum effective throughput rate than any one host can achieve. Do this problem for IEEE 802.5 ring , and FDDI.

3. Chapter 6, problem 6.14

4. Chapter 8, problem 8.2

HW3:        1. The 1982 Ethernet specification allowed between any two stations up 1500 m of coaxial cable, 1000 m of other point-to-point link cable, and two repeaters. Each station or repeater connects to a the coaxial cable via up to 50 m of "drop cable". Typical delays associated with each device are given below (where c=speed of light in a vacuum=3 x 10^8 m/s). What is the worst-case round-trip propagation delay, measured in bits due to the sources listed?

Item Delay
Coaxial Cable propagation speed .77c
Link/drop cable propagation speed .65c
Repeaters approximately 0.6 microseconds each
Trasceivers approximately 0.2 microseconds each

2. Suppose the round-trip propagation delay for Ethernet is 46.4 Ás. This yoields a minimum packet size of 512 bits (464 bits corresponding to propagation delay + 48 bits of jam signal).

a) What happens to the minimum packet size if the delay timeis held constant, and the signaling rate rises to 100 Mbps?

b) What are the drawbacks to so large a minimum packet size?

3. From do Problem 15 in the PROBLEM section of the page (Suppose node A and B...).

4. What is the baud rate of the standard 10-Mbps 802.3 LAN?

HW 2: 1. Show NRZ, Manchester, and NRZI encodiings for the bit patter 10011111000100001. Assume that the NRZI              signal starts out low.

        2. Show 4B/5B encoding and the resulting NRZI signal for the following bit sequence

            1110 0101 0000 0011

        3. From the text book Chapter 2, problem 2.2

        4. From the text book Chapter 2, problem   2.3.

        For problems 3 and 4  calculate the efficiency of Miller and E-NRZ encodings.

HW 1 (Review of TC 535):
   1. Television channels are 6MHz wide. How many bits/sec can be sent if four-level digital siganls are used?          Assume  a noiseless channel.
   2. Recall that with the CSMA/CD protocol, the adapter waits K*512 bit times after a collision, where K is        drawn randomly. For K=100, how long does the adapter wait until returning to sensing the channel.
      Review ( for a 10 Mbps Ethernet? For a 100
      Mbps Ethernet?
   3. From page do problem 3 in the middle of
      the page ("Consider the network shown below...."). Review            
   4. Consider a 1-Gbps network with the average source and destination 20 km apart. What is a  
       round-trip delay for a 1-KB packet? Assume propagation velocity of 200 m/Ásec.
You can expect to learn about architecture, implementation and performance of various Local Area Networks and Wide Area Networks.
I expect that you are familiar with basic calculus and have taken the TC 535 Introduction to Computer Networks or equivalent.  I expect that know about OSI 7-layer protocol reference model (PRM), protocol layering, functions of the Physical, Datalink and Network Layers, TCP/IP, and basic routing algorithms.

        There is a lot of material I present that is not covered in the required text but is available on the Web or in           some of the books below. Class participation and attendance is very encouraged. You will never get an A in          this class if you don't attend lectures on the regular basis.

Required Text: W. Stallings, "Local & Metropolitan Area Networks" Fifth Edition, Prentice Hall 1997 ; Errata File

Here are some of the references I use to prepare for my lectures (listed in the order of preference):
ATM Bookmarks and Tutorials
Long list of ATM related sites
Raj Jain's web site
References to ATM and FDDI papers and books by a leading researcher
Introduction to Distributed Queue Data Bus
Brief description of DQDB technology
DQDB Physical Layer
High-level description of DQDB's Physical Layer
Simulation study of DQDB
Research Project of the Design and Implementation of a Distributed Queue Dual Bus Network
Digital Technical Journal: FDDI
A number of articles about FDDI from DTJ: vol 3. no. 2
Fibre Channel Association
  1. Introduction and background  (ch 1,2,3)
  2. Transmission Media (2,3)
  3. Protocol Architecture  (2,4)
  4. Physical and MAC layers of traditional LAN ch 6
  5. Logical Link Control ch 5
  6. High-Speed Ethernet-like LANs ch 7
  7. Fiber Distributed Data Interface ch 8
  8. Fiber Channel & Gigabit Ethernet ch 9 (See IPJ Article for good description of Gigabit Ethernet)
  9. Frame Relay
  10. ATM
  11. Cable Networks
  12. Wireless Networks
  13. Other things we might talk about: Routing, Switching, Performance, Security

Old Lecture Notes