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Transmission Medium

Data is encoded in form of energy and transmitted over a medium called transmission medium. This can be air, wire etc.

Copper Wire

These show very low resistance to electric current.
Example: Twisted wire pair.

Coaxial Cable

Single wire surrounded by an insulator and a metallic mesh to shield against interference.

Types of Transmission Medium

Guided: Where the path is physically defined, such as a wire.
Unguided: Where the path is air.

Optical Fiber

It uses light for carrying data.

Radio Waves

These are used for TV and public radio etc.

Satellites

Geosync Satellites

These are placed about 35785 KMs above the surface of the earth and appear stationary as the earth revolves.

Low Orbit Satellites

They are about 200 or 400 miles above the earth and do not appear stationary as the earth revolves.

Microwaves

This uses higher frequency as compared to radio waves and can be uni-directed.

Infrared

These are used for TVs etc.
They have very limited range.

Local Async Communication

The sender and receiver do not coordinate with each other and receiver should be able to interpret signals anytime it receives them.

The negative voltage represents 1 while 0 is represented by positive voltage.

The standards of communication define the timings and voltage specifications regarding the signals.

The RS-232-C standard is used for modems and keyboards
The RS-232 standard is used as serial for async communication.

RS-232-C

  • Supports wire length of 50 feet.
  • -15V for 1 and +15 for 0.
  • One character is 7 or 8 bits.
  • No delay between character bits.
  • When idle, it puts 1 on the bus.
  • Each character starts with start bit 0 and ends with stop bit 1.

Baud Rate

It is defined to be number of changes in the signal per second.

Frame Errors

These occur when the end bits do not occur at the expected times.

Full-Duplex Async Communication

It uses 2 wires in which 1 carries the data signal and other one carries the return path called ground.
The 2 directional communication at same time is called full-duplex communicaion.

Long Distance Communication

Modulation

The original signal is slightly modified by the transmitter by using a carrier. This process is called modulation.

Types of Modulation

  1. Amplitude Modulation
  2. Frequency Modulation
  3. Phase Modulation

Modem

A modem is a device which combines both the modulator and demodulator together.

Multiplexing

It is the process of passing multiple signals over the same wire by using multiple frequencies.

Types of Multiplexing

  1. Frequency Division
  2. Wave Division
  3. Time Division

Frequency Division

Different frequencies are passed onto the same medium

Wave Division

Same concept but applied to optical fibers.

Time Division

In this type, the sources take turns to send their data.
Meaning they do not send data all at once, rather, they wait for their turns.