2001: Homework assignment 1 Model Answer

Material

• The coursebook
• Collection of old home assignments
• "Satellite-Based Internet: A Tutorial" in course handouts.

Important information for all: Copyright basics for Web authors and users by Jukka Korpela

Assignment: The basics of Internet

1. Briefly tell about those people - who they are/were and what are their crowning achievements in telecommunications? (Each 0.5 point)

1. Alexander Graham Bell
   The inventor of the telephone
2. Emile Baudot
   The inventor of five level code for telegrapher
3. Samuel Morse
   The inventor of Morse Code
4. Harry Nyquist
   The inventor if Nyquist Theory
5. William Fothergill Cooke
   The inventor of the first practical electric telegraph system with Wheatstone
6. Claude Shannon
   The inventor of Shannon Theory, First to use bit and the founder of Information theory
7. Agner Erlang
   The first person to study the problem of telephone network, and ElangĄ¯s formula
8. Bob Metcalfe
   The inventor of Ethernet
9. Charles Wheatstone
   The inventor of the first practical electric telegraph system with Cooke

2. Transmission media (11.5 Points)

1. List the main properties for each of the following transmission media: bandwidth, cost, purpose of use and approximated maximum wiring distance (without repeaters). Are there any special problems occuring when using each media? (Each character is 0.25 point)

   Cable Kind	Bandwidth	Cost	Purpose	Distance(s)	Problem
   Twisted Pair (telephone)	1Mbps	Low	Telephone	1-5 km	Easily interrupted Easily interfered
   Twisted Pair (network)	100-1000 Mbps	Low	Computer networks	100m	Easily interrupted Easily interfered
   Coaxial cable	10Mbps	Low	LAN	500m	Terminator needed Thick cable
   Coaxial cable	1-2 Gbps	Low	Backbone	1 km	Easy to eavesdrop
   Optical fiber	10 Gbps	High	Connecting Network	Tens of Kms	Ring Topology Expensive Easily damaged Difficult to repair
   Radio wave	Up to 10Mbps, depending on frequency band	Low	Mobile	X*10Km	Easily influenced by weather
   Microwave	Up to 100 Mbps, depending on frequency band	Low	Mobile, WLAN, Bluetooth	about 80 kms with 100 m towers	Easily influenced by weather, requires line of sight

2. About 10 years ago, the soviet occupation ended in Estonia. There is a story about building a LAN those days. A company had bought enough of good cable for cabling the company's new network. A clever technician thought he could save some money for the company, and instead of using the new expensive cable, he made a "new" cable of pieces of telephone wires and other cables mixed up and used it in cabling the LAN. The boss of the technician was very angry to him after finding out what had happended. Why? (3 Points)
   (Truthness of this legend is unconfirmed)
   

   When we use different kinds of cable connecting together, due to the different impedance, the electrical signal will be reflected back at the interface of two different media. Then the transmission will fail. The bandwidth of the low quality telephone cables is significantly lower than proper LAN cables.

3. Choose the best alternative for each question.(Each 0.5 point)
   1. Providing satellite connection on Maldives is possible using
      1. GEO satellites
      2. LEO satellites
      3. both GEO and LEO satellites
      4. none of the above
      C

   2. An antenna size needed for satellite transmission depends of
      1. transmission distance
      2. wavelength
      3. none of the above
      A and B were both accepted

   3. Resource wasting is a problem of
      1. DAMA
      2. USAT
      3. TDMA
      C

   4. Which of these protocols will be probably leaved out in satellite-based Internet?
      1. TCP
      2. UDP
      3. both
      4. none of the above
      D

   5. DT-DVTR works by
      1. making static routing look like dynamic routing for each time period
      2. making dynamic routing look like static routing for each time period
      3. using static and dynamic routing in different parts of a network
      B

3. TCP/IP model (6.25 points)

1. Link these terms to correct TCP/IP layers. Explain your choise. (Each item 0.25 point)
   1. error correction, Host-to-network layer and transport layer
   2. routing, Internet layer
   3. Ethernet, Host-to-network layer
   4. FTP, Application layer
   5. bridge, Host-to-network layer
   6. repeater, Host-to-network layer
   7. NNTP, Application layer
   8. UDP, Transport layer

2. Define the terms connectionless and connection-oriented protocol. Connection-oriented: ( Each 1 point)
   
   The user first establishes a connection, uses the connection, then release the connection. The receiver takes bits out in the same order as sender.
   Connectionless:
   
   Each message has is destination address and is routed through the system independent of all the others; messages can arrive in an order different than they were sent.

3. Why are layered models used for protocols? (Each 0.5 point, at least give 4 items)
   1. Easier to understand
   2. Planning protocols is easier
   3. Developing new things is faster (possible to develop only one layer and not the whole system)
   4. Easier to implement
   5. More competition
   6. Modularity

4. Calculations (3 points)

1. Bandwidth of fictional network is 4000 Hz and the number of signal levels is 4.
   1. Calculate maximum data rate in a noiseless channel.

      According to Nyquist's theorem: R_MAX=2Hlog(2)V bits/sec=2*4000*log(2)4=16kbps
      

   2. Signal/noise ratio of the network is 10 dB. Calculate maximum data rate of the channel.

      According to Shannon's theorem: R_max=hlog(2)(1+S/N) bit/sec=4000*log(2)(1+10)=13.8kbps
      

2. Transmission channel between two terminals is made up of two sections. In the first section attentuation is 4.6 dB and in the second the attentuation is 5.2 dB. In a repeater between those sections the signal is amplified 3.8 dB. What should the power level of the signal in the beginning be if the receiver can't resolve signal below 200 mW?

   Total Attenuation=4.6dB-3.8dB+5.2dB=6dB
   Total Attenuation=10log(2)(P1/P2)
   P1=p2*10^(6dB/10)=796mW
   

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