Lecture Notes

 

Types of encoding:

  1. Analog: change in signal is proportional to the value sent.
  2. Digital: values sent are encoded in 0’s and 1’s.

 

Digital signal is more tolerant to noise because a 0 and 1 can be distinguished in the presence of noise. Analog signals and noise cannot be distinguished.

 

If you put the repeaters in between, you can get a better quality.

 

Standard Organizations

ITU – International Telecommunications Union

EIA -  Electronic Industries Association

IEEE – Institute of Electrical Electronic Engineers

 

Example of a network standards:

  1. RS 232C (serial line) à very inexpensive

·        Used to connect dumb terminals to computers.

·        Now it is used to connect mouse or keyboard to computers.

·        Very simple standard.

·        Specifies the transmission of character.

·        It is called serial because it sends 1 bit after the other.

·        Needs 3 cables at least for full duplex communication.

 

·        Receiver does not know when a character will arrive (asynchronous communication).

·        Need a start bit and a stop bit (required).

·        Uses +15 for 0 and –15 for 1.

·        Limited to ~50 feet.

·        When idle, the level is always –15 V.

·        It uses 1 “start bit” to signal start of transmission.

 

·        The sender and the receiver have to agree on # of bits per character.

·        There is going to be a start bit and a stop bit for every character.

·        The original ASCII character can be represented by 7 bits.

·        Modem connected to a serial line.

·        Disagreement between transmitter and receiver results in “framing error”.

·        One way =  half duplex. (It can be two ways, but it is not possible that one computer can send and receive at the same time).

·        Full duplex: a computer can send and receive at the same time (uses 3 cables).

 

 

09/05/03

Electrical Transmission

RS 232 uses electrical transmission.

 

Problems with sending electricity:

  • Resistance of the cable increases with the length. It means the intensity of the signal decreases with the length of the cable.
  • The capacitance and inductance of the cable distorts the signal.
  • Magnetic and electronic interference distorts the signal.
  • Distortion results in loss of information.

 

RS 232 deals with these problems by:

  • Tolerate a lower voltage than +15/-15.

 

  • Takes multiple samples at different times.
  • Direct current (two levels) cannot travel long distances, therefore an oscilating signal (modulated) will be used instead for long distances.

 

  • For long distances, a sine wave (oscilating wave) is used (called carrier wave) and data is encoded by changing one of the properties of the sine wave.

 

Characteristics of a carrier( usually a sine wave)

 

Types of modulation:

  • Amplitude Modulation (AM)

AM is very susceptible to noise because noise is added into the signal and cannot be separated from the input signal. AM uses less bandwidth than FM, so lower frequency can be used (540 – 1600 kHz).

  • Frequency Modulation (FM)

FM is less susceptible to noise because noise usually does not change the frequency of the carrier. It uses more bandwidth than AM, so higher frequencies are needed (84 – 108 MHz).

  • Phase shift Modulation (PM)

Changes from 1 to 0 or 0 to 1 cause a shift in the signal. It is as tolerant to noise as FM. It uses more bandwidth.

 

Modem

Hardware device used for long distance communication.

 

DSL modem is more expensive than cable modem.

  • One modem at each end.
  • You use separate channels for signals in each direction.