Data Communications

1
Data Communications

• Introduction and Review

2
Transmission Media

• Copper Wires
• Low resistance. Electrical signal produces
  miniature radio station interference.
• When one wire encounters another
  electromagnetic wave occurs.
• Interference has to be minimized in order for
  communications to occur.
• Twisted Pair
• Coaxial Cable

3
Transmission Media

• Glass Fibres
• Transmitter at one end uses a light emitting
  diode (LED)
• Receiver at other end uses light sensitive
  transistor to detect the pulse.

Advantages No interference Reflect Inward Light
encodes more Travel over a single fibre
Disadvantages Special equipment to polish
ends Finding breaks is difficult
4
Transmission Media

• Radio Waves
• Radio frequency or RF
• No direct connection
• Cannot bend around the earth
• Can be combined with satellites
• Microwaves
• Electromagnetic radio wave beyond the frequency
  used by Radio and Television
• Do not broadcast in all directions but can be
  directed.
• Carry more information than RF transmission
• Cannot penetrate metal structures

5
Transmission Media

• Infrared
• Limited to small area
• Transmitter pointed toward receiver
• Inexpensive no antenna
• TV Remote
• Laser
• Similar to Microwave ie. line of sight

6
Asynchronous Communication

• Sender and receiver not synchronized

Parity Bit Stop Bit Parity Bit Stop
Bit
0 01000111 0 1 ?? 0 01001010 1 1
???????
Electrical signal transmitted does not contain
information the receiver can use to determine
where individual bits begin and end.
Start Bit ASCII G Start Bit ASCII J
7
Asynchronous Communication

• Using electric current to send bits
• Negative voltage could be used to represent 1
  while a positive voltage could be used to
  represent 0

8
Asynchronous Communication
Problem there is no set interval that a bit is
transmitted

Waveform Diagram

time
0
-
1
0
1
0
0
1
9
Asynchronous Communication

• Standards for communication
• How long should the sender hold the voltage on
  the wire for a single bit?
• What is the maximum rate at which hardware can
  change the voltage?
• How can users be sure that different vendors
  hardware will be compatible?

10
Asynchronous Communication

• Standards for communication contd
• International Telecommunications Union ITU
• Electronic Industries Association EIA
• Institute for Electrical Electronics Engineers
  - IEEE

11
Asynchronous Character Transmission with RS-232

• Standard by Electronic Industries Association
  (EIA)
• Connection less than 50 feet long
• Voltage range -15 to 15 volts
• Specifies transmission of characters
• Each character uses 7 bits
• Defines serial asynchronous communication
• Never leaves 0 volts on the wire

12
Asynchronous Character Transmission with RS-232
Sending and receiving hardware agree on the
length of time for transmission of a bit
15
time
0
-15
stop idle
Idle start
1 0 1 1 0 1 0
13
Asynchronous Character Transmission with RS-232

• Baud rate number of changes in the cycle per
  second
• bps
• Manufacturers make hardware to operate at various
  rates
• Framing errors can occur
• If voltages do not agree or the stop bit does not
  occur at the time expected.

14
Asynchronous Character Transmission with RS-232

• Communication is full duplex.

T R G
T R G
A modem transmits on pin 2 and receives on pin
3, while a computer transmits on pin 3 and
receives on pin 2. The ground is pin 7.
The cable that connects a computer to a modem has
a wire from pin 2 to pin 2, pin 3 to pin 3, and
pin 7 to pin 7 for the ground.
15
Asynchronous Character Transmission with RS-232

• Real hardware is limited.

• Bandwidth and bit transmission.
• Measured in cycles per second or Hertz (Hz) .

Real
Ideal
16
The End.