Your Name ______

COPPIN STATE COLLEGE

COSC 406

Test 1 - Spring 2005

(Class Time)

1. (6pts) Write the full names of the following acronyms (no explanations are required):

A. RFC ______Request for Comments______

B. ARPA ______Advanced Research Projects Agency______

C. DTE ______Data Terminal Equipment______

D. DWDM ______Dense Wavelength Division Multiplexing______

E. CRC ______Cyclic Redundancy Checks______

F. ITU ______International Telecommunications Union______

2. (4pts) One of the probing tools used to probe the Internet is: ping. However, ping often fails to serve its purpose. Write at least two reasons of ping's failures that are discussed in your textbook:

__Destination computer node is disconnected ______

__firewall, inactive intermediate nodes, or traffic congestion_____

3. (2pts) Review the following ping result of an address over the Internet. What stands for TTL and the unit of TTL? ___Time-to-live______

(2pts) Explain briefly the term TTL: __When sender node sends a packet (in this case 32 bytes of data), it also sets a clock with a specific time (in this case 239 ms) within which if the sender does not get any acknowledgement for receiving of the packet from the receiver node, the sender assumes a loss of that packet and retransmit the packet again._____

H:\>ping www.sears.com

Pinging sears.com [129.33.131.133] with 32 bytes of data:

Reply from 129.33.131.133: bytes=32 time=35ms TTL=239

Reply from 129.33.131.133: bytes=32 time=36ms TTL=239

Reply from 129.33.131.133: bytes=32 time=35ms TTL=239

Reply from 129.33.131.133: bytes=32 time=36ms TTL=239

Ping statistics for 129.33.131.133:

Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),

Approximate round trip times in milli-seconds:

Minimum = 35ms, Maximum = 36ms, Average = 35ms

4. (2pts) Two of the probing tools used to probe the Internet are: ping and traceroute.

Which one you will select most to measure the round-trip time between your computer and the destination computer? ___ping______

Which one will you select if you want to know the status of an intermediate node between your computer and the destination computer? ____traceroute__.

5. (6pts)

A. Write the name of three conducted (wire) physical medias that are used mostly for data transmission:

_____Unshielded Twisted Pair – CAT 5______

_____Coaxial (Thin)______

_____Fiber Optics ______

B. Write the name of three radiated (wireless) physical medias that are used for data transmission:

_____Microwave______

_____Satellite______

_____Infrared______

6. (5pts) List the five most important factors in choosing cable for building a network:

______Electromagnetic Interference (EMI)______

______Bandwidth______

______Signal Attenuation______

______Cost______

______Expansion Issue______

7. (2pts) Write two advantages of UTP-CAT 5 cable have over thin coaxial cable.

___less susceptible to EMI____, __less expensive______

8. (2pts) With which type of basic networks would you use thin coax cable __peer-to-peer or small room environment______

and UTP- CAT 5 cable? ____LAN______

9. (1pt) How many pairs of wires that an UTP-CAT 5 cable has? __4 pairs __

10. (2pts) Explain why all pairs of cables of an UTP-CAT 5 cable are twisted.

______to minimize the EMI that generates due to the magnetic field developed from live wire and hence the signal can transmit longer path without signal distortion (attenuation) as compared to that of straight pair of wire._____

11. (1pt) The maximum distance limit is _100____ meters before the signal attenuation is occurred in an UTP CAT 5 cable.

12. (1pt) The maximum distance limit is __185___ meters before the signal attenuation is occurred in a thin coax cable.

13. (1pt) The maximum distance limit is _> 2000__ meters before the signal attenuation is occurred in a glass fiber or optical fiber cable.

14. (1pt) The maximum bandwidth is limited to __10___ Mbps for a thin coax cable.

15. (1pt) The maximum bandwidth is limited to __> 100___ Mbps for an UTP CAT5 cable.

16. (1pt) The maximum bandwidth is limited to ___2000 _ Mbps for a fiber optic cable.

17. (2pts) Explain briefly your understanding about EMI. __The interference of electrical signals across a cable by the outside electrical or magnetic devices______

18. (3pts) Rank the three cables (twisted-pair, coax, and fiber optic) from lowest to highest EMI problem. a. __fiber optic____, b. __twisted pair_, c. __coax______

19. (1pt) Name one major disadvantage of twisted pair transmission media. _____if it is unshielded one, susceptibility to signal distortion when they are deployed over longer distance compared to others.

1.  20. (1pt) Name one major advantage of twisted pair transmission media __ ability to support high speeds (bandwidth)at low cost.__

21. (1pt) Name one major disadvantage of fiber optic cable transmission media. ______most expensive and inability to add new nodes while other nodes are active______

22. (1pt) Name one major advantage of fiber optic cable transmission media __ very low error rates over long distance transmission and immunity to environmental interference __.

23. (1pt) Name one major disadvantage of microwave transmission media. ___ suceptible to interference from other radio signal, bad weather, physical obstruction.______

24. (1pt) Name one major advantage of microwave transmission media ___ cost effective in respect to speed and volume, ease to implement in the situation when wired transmission is not feasible or cost effective.__

25. (1pt) What is the signal type for communications through twisted pair cable? __electrical current_ (electric current, laser or optical, radio or electromagnetic wave)

26. (1pt) What is the signal type for communications through coax cable? _electrical current___ (electric current, laser or optical, radio or electromagnetic wave)

27. (1pt) What is the signal type for communications through fiber optic cable? __laser or optical___ (electric current, laser or optical, radio or electromagnetic wave)

28. (1pt) What is the signal type of microwave communications? __electromagnetic wave___ (electric current, laser or optical, radio or electromagnetic wave)

29. (1pt) What is the signal type of satellite communications? __radio or electromagnetic wave_ (electric current, laser or optical, radio or electromagnetic wave)

30. (2pts) A twisted pair cable can be of shielded type or unshielded type. If you have to select wire between the two types for a network pass near a high-voltage transmission tower, which one you would select? Justify your answer. ____Shielded type, additional insulator will protect its signal from interference by the signal generated by the high-voltage transmission tower____

31. (3pts) Rank wires, coaxial cable and fiber optics with respect to speed, cost and resistance to noise.

Which is fastest? _____fiber optics______

Which is least expensive? ____wires or twisted pair______

Which is least error prone? ____fiber optics___

32. (1pt) Which of the following is propagation delay usually a matter of concern?

a. telephone wires b. microwave c. coaxial cable

c. fiber optics d. satellite e. none of the above

33. (1pt) Baseband transmission

a.  transmit only one signal b. can transmit several signals on one cable

c.  is more expensive than broadband transmission

d.  used frequency division multiplexing

34. (1pt.) Modem is an acronym for

a.  MODel Emulation

b. Mmory Output DEcoding Mode

c. MOdulation and DEModulation

d. Message Output Definition and Exchange Mode

35. (2pts) Explain what you understand by a geosynchronous satellite.

Geosynchronous satellite is placed in an orbit that is exactly synchronized with the rotation of the earth. Hence, a geosynchronous satellite appears to be stationary from any location on the earth.

36. (2pts) We can place geosynchronous satellites in the space as many as we want. TRUE or FALSE? ______Justify your answer.

To avoid frequency interference from each other one satellite is to be separated by 4 to 8 degrees from another. So, numbers are limited

37. (2pts) For cellular telephone communications, we may use another kind of satellite setup called Low Earth Orbit (LEO) satellite. What’s the key disadvantage of such satellite? ____ It changes position with a particular location on earth. So the signal should pass through number of satellites not just one.______

38. (2pts) What’s the key scheme practiced to overcome the LEO satellite’s disadvantage? ______key to the scheme lies on arranging number of satellites in such a way that there should be at least one satellite available at any time in any particular location.______

39. (5pts) A digital signal flows through a wired transmission media in bit-form. Draw a waveform diagram to send the character ‘Go’ (ASCII codes for ‘G’ and ‘o’ are 47 and 6F in hex) in RS-232 standard. Show the MARK state and SPACE state on the diagram.

40. (5pts) If you want to establish a communication within a room at home between two computers, show the simplest connection diagram for full-duplex RS-232 communication. Label the pin numbers on the diagram for transmission, receive, and ground.

41. (3pts) Explain how the terms baud rate, bit rate, and bandwidth are used to describe the speed of communications link.

Baud rate: Electrical terms used to signify modem speed by number of signal change per second

Bit rate: the term used to signify modem speed by number of bits could be send per second

Bandwidth: the term specifies the capacity of a transmission media by frequencies.

42. (5pts)Why our modem speed was limited to less than 33.3Kbs (33300 kilobits per sec) in transmitting data through telephone line communications? Can you justify mathematically? Use Shannon’s Theorem: C = B log2(1 + S/N) measures? Assume the values of B, S and N with proper units.

43. (3pts) A single career waveform cannot transmit any data. To transmit the data a career should be modulated. What are the three common modulation techniques?

______Amplitude Modulation______

______Frequency Modulation______

______Phase Shift Modulation______

44. (2pts) What a Multiplexer used for? ______allows multiple sources to share single channel to send their messages to multiple destinations ______

45. (2pts) Explain the operation of a DWDM._____Used with fiber optic cables. It operates by sending multiple light waves across a single optical fiber. Receiver end uses prism to separate out different frequencies of light messages.______

46. (5pts) Suppose one sent 5KB (kilobytes) 7-bit characters across RS-232 connections that operated on 28,800 bps (bits per second) modem. How long would the transmission require? Show your work.

ANS: 1.56 sec

47. (3pts) While transmitting data in computer networks transmission errors are occurred frequently and write the three common problems that are called transmission errors:

______loss of packet______

______corrupted data______

______unwanted data generated______

48. (5pts) Suppose you want to send a sequence of six ASCII characters whose equivalent representation in hexadecimal numbering system are CD, F8, 65, DF, 9A and 2E. Calculate the carry and checksum in 8-bit. Show your computation.

Carry ____3______Checksum ___D4______

49. (5pts) To detect error in transmission data suppose both sender and receiver agree to transmit error detection scheme in odd parity. Suppose the sender sends following five ASCII characters in 7-bit. Show the parity bit and transmitted bits of each character.

ASCII Parity Bit Transmitted Bits

1000101 0 10001010

1111101 1 11111011

0111101 0 01111010

1001101 1 10011011

0001111 1 00011111

50. (4pts) Write two reasons a CRC can detect more errors than a simple checksum.

_____A CRC system uses two hardware: XORs and shift registers. Reason 1: As a single bit is shifted through a series of (three or more) shift registers, a single bit changes its final form from its original identity. Reason 2: the leftmost single bit from the last register is used for feedback to each XOR unit, which changes again its form in polynomial cycle.

51. (4pts) Draw a RS-232 frame and a long distance communication frame with error detecting code.