Data Communications
Introduction and Review
Transmission Media

Copper Wires

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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
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
Disadvantages
Light encodes more
Travel over
a single
Special
equipment
to fibre
polish ends
Finding breaks is difficult
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
Transmission Media

Infrared





Limited to small area
Transmitter pointed toward receiver
Inexpensive – no antenna
TV Remote
Laser

Similar to Microwave ie. line of sight
Asynchronous Communication

Sender and receiver not synchronized
Parity Bit Stop Bit
0 01000111 0 1
Start Bit ASCII “G”

Parity Bit Stop Bit
0 01001010 1 1

Electrical
signal transmitted
Start Bit ASCII
“J”
does not contain information
the receiver can use to
determine where individual
bits begin and end.
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’
Asynchronous Communication
Problem:
there is no set
interval that a
bit is
transmitted
Waveform Diagram
+
time
0
1
0
1
0
0
1
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 vendor’s
hardware will be compatible?
Asynchronous Communication

Standards for communication cont’d



International Telecommunications Union – ITU
Electronic Industries Association – EIA
Institute for Electrical & Electronics Engineers - IEEE
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
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
Idle
start 1
0
1
1 0
1
0 stop idle
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.
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,
The cable that connects a
while a computer transmits
computer to a modem has a
on pin 3 and receives on pin 2.
wire from pin 2 to pin 2, pin 3 to
The ground is pin 7.
pin 3, and pin 7 to pin 7 for the
ground.
Asynchronous Character
Transmission with RS-232

Real hardware is limited.
Real

Ideal
Bandwidth and bit transmission.

Measured in cycles per second or Hertz (Hz) .
The End.