Chapter 4

Transmission Medium

Transmission Medium: Guided and Unguided

The transmission medium is the pathway networked entities use to connect each other. So it is a communication channel that carries the information from the sender to the receiver. There are two ways of transmission medium described below:

Guided Media

It transmits signals by sending electricity or light over a cable or wire. The data signals are bounded by the cabling system, so it is also called bounded media.
There are 3 major types of Guided media:
1. Twisted Pair Cable:
And, twisted pair cable is divided into two types:

  • Shielded Twisted Pair ( STP )
  • Unshielded Twisted Pair ( UTP )

2. Coaxial Cable:
It has an outer plastic covering containing 2 parallel conductors each having a separate insulated protection cover.
Advantage:

  • High Bandwidth
  • Better noise Immunity
  • Easy to install and expand
  • Inexpensive

3. Fiber Optic Cable
A cable which is used to transmit the data through fibers ( threads ) or plastic ( glass ) is known as optical fiber cable.
Advantages:

  • Increased capacity and bandwidth
  • Light weight
  • Less signal attenuation
  • Immunity to electromagnetic interference
  • Resistance to corrosive materials

Disadvantages:

  • Difficult to install and maintain
  • High cost
  • Fragile
  • unidirectional

Unguided Media

It consists of a means for the data signals to travel but nothing to guide them along a specific path. Unbounded media transmit data through the open air.
Types of Unguided communication media:
1. Radio Waves
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. The transmitter takes some message, encodes it and then transmits it with radio wave. The receiver on the other hand receives the radio waves and decodes it.
Advantages:

  • It can travel through long distance in all directions.
  • It can easily pass through obstacles like a building.
  • It is easy to communicate through radio waves.

Disadvantages:

  • It is susceptible to weather effects like rain, thunderstorm etc.
  • Data transmitted through radio waves is not secure.

2. Microwaves
This type of transmission uses high frequency radio signals to transmit data through space. A transmitter and receiver of microwave system are mounted on very high towers and both should be visible to each other ( line of sight ).
Advantages:

  • It doesn’t require the expense of laying cables.
  • It can carry 25000 voice channels at the same time.

Disadvantages:

  • The transmission is affected by weather conditions like rain, thunderstorms etc.
  • Installation and maintenance of microwave links turns out be a very expensive affair.

3. Infrared Waves
Infrared ( IR ) is a wireless mobile technology used for device communication over short ranges. IR communication has major limitations because it requires line-of-sight, has a short transmission range and is unable to penetrate walls. e.g. Night vision camera, Remote control.

Transmission impairments terminology

Impairments may be called as noise or anything else that might cause an error in data transmission.

Some of the transmission impairements terminologies are:

  1. Jitter-
    The variation in latency of packets carrying (voice or video) data over a communication channel is called jitter. Most commonly, network congestion causes routes to change and latency to vary.

  2. Attenuation-
    It is the loss in the strength of a transmission signal cables or connections. The amplifier is used to increase the signal strength which gives the original signal back and compensates for this loss. It is measured in decibels (dB).

  3. Singing and Echo-
    A signal reflected to the speaker’s end of the circuit is called talker echo, and at the listener’s end is called listener’s echo. Singing occurs when the returning signal is repeatedly coupled back into the same channel to produce oscillations. An echo coming 0.5 milliseconds after the speech does not have much effect. The echoes with a round trip delay of more than 45 milliseconds cannot be tolerated.

  4. Crosstalk-
    It is unwanted signals in a communication channel caused by the transference of energy from another circuit. It is caused by undesired capacitive, inductive, or conductive coupling from channel to channel.

  5. Distortion-
    The change in the form of the shape of the original signal or waveform is known as distortion. It is generally seen in frequencies.

  6. Noise-
    The random or unwanted signal that mixes up with the original signal is called noise. There are several types of noise that may corrupt the signal, such as crosstalk, thermal, induced, and impulse noise.

  7. Bandwidth-
    The maximum data transfer rate of a network or Internet connection in a given amount of time is known as bandwidth.

  8. Number of receivers-
    Every connected node in the network introduces some attenuation and distortion. So, when the number of the receiver increases, the transmission distance and/or data rate are limited.