September 3, 2003

Types of Encoding

Analog: Change in signal is proportional to value sent.
Digital: Values sent are encoded in 0's and 1's.

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

Analog Signals
 Analog Signals
Digital Signals

Digital Signal
Standards Organization

ITU - International telecommunication Union
EIA - Electronic Industries Association
IEEE - Institute of Electrical Electronic Engineers

Example of a network standard RS232C (Serial Line)
- used to connect dumb terminals to computers
- now it is used to connect mouse/keyboard to computers
- very simple standard
- specifies the transmission of characters
- it is called "Serial" because it sends on bit after other
- needs 3 cables at least for full duplex communications
Full Duplex
- receiver does not know when a character will arrive (asynchronous communication)
- A start bit and a stop bit is required
- uses     +15 V   - 0
              - 15 V   - 1
- limited to ~ 50 feet
- when idle, the level is always - 15 V
- it uses one "start bit" to signal start of transmission
RS232
- the sender and the receiver have to agree on:
    - #bits per character 7 or 8
    - duration of each bit
       (the duration affects the speed of transmission
    - tupical speeds
        9.6 Kbps
        14.4 Kbps
        28.8 Kbps
        56 Kbps
- disagreement between transmitter and receiver results in "framing error"
- onway = half duplex (it can be two ways but it is not possible that one computer can send and receive at the same time)
                                    (uses two cables)
- full duplex (uses three cables)
  A computer can send and receive at the same time

September 5, 2003

Electrical Transmission

RS232 uses electrical transmission

Problems:
- resistance of the cable increases with the length
- the intensity of the signal decreases with the length
- the capacitance and inductance of the cable distorts the signal
- magnetic and electronic interference distorts the signal
- distortion results in loss of information
- RS232 deals with these problems by tolerates a lower voltage than +15V -15V
RS232 take multiple samples
- takes multiple samples
- direct current (two levels +/-) cannot travel long distances, therefore an oscilating signal (modulated) will be used instead
Osilation
- for long distance a sine wave (oscillating 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)
Characteristics of a carrier
Types of modulation

Amplitude modulation
Modulation
- Amplitude modulation is very susceptible to noise because noise is added into the signal and cannot be seperated from the input signal.
- Uses less bandwith than FM so lower carrier frequencies can be used 540 khz - 1600 khz

Frequency Modulation
Frequency modulation
- FM is less susceptible to noise because noise usually does not change the frequency of the carrier
- Uses more bandwith than AM so higher frequencies are needed 84 Mhz - 108 Mhz

Phase Shift Modulation
Phase shift modulation
- 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

Modems
Modems
- hardware device used for long-distance communication
- one modem at each end
- use seperate channels for signals in each direction