MSIS 8040/01 Data Communication Theory and Practice, Summer 2005

Assignment 5 (Don’t turn in.  Similar questions may appear in quizzes.)

 

 

  1. It turns out that the depth in the ocean to which airborne electromagnetic signals can be detected grows with the wavelength.  Therefore, the military got the idea of using very long wavelengths, corresponding to about 30 Hz, to communicate with submarines throughout the world.  It is desirable to have an antenna that is about one-half wavelength long.  How long would that be? (The velocity of the wave is 3 ´ 108 m/s.)

 

  1. The audio power of the human voice is concentrated at about 400 Hz.  Antenna of the appropriate size for this frequency are impracticably large, so that to send voice by radio the voice signal must be used to modulate a higher (carrier) frequency for which the natural antenna size is smaller.

 

    1. What is the length of an antenna one-half wavelength long for sending radio at 300 Hz?
    2. Suppose we would like a half-wave antenna to have a length of 1 m.  What carrier frequency would we use?

 

  1. Stories abound of people who receive radio signals in fillings in their teeth.  Suppose you have one filling that is 2.5 mm (0.0025 m) long that acts as a radio antenna.  That is that it is equal in length to one-half the wavelength.  What frequency do you receive?

 

  1. Using Manchester encoding, encode the bit pattern 0100.

 

  1. Suppose a file of 10,000 bytes is to be sent over a line at 2400 bps.

 

    1. Calculate the overhead in bits and time in using asynchronous communication.  Assume one start bit and a stop element of length one bit, and 8 bits to send the byte itself for each character.  The 8-bit character consists of all data bits, with no parity bit.
    2. Calculate the overhead in bits and time using synchronous communication.  Assume that the data are sent in frames.  Each frame consists of 1000 characters = 8000 bits and an overhead of 48 control bits per frame.
    3. What would the answers to parts (a) and (b) be for a file of 100,000 characters?
    4. What would the answers to parts (a) and (b) be for the original file of 10,000 characters except at a data rate of 9600 bps?