Communication Transmission Media

INTRODUCTION TP TRANSMISSION MEDIA

Transmission media provide the physical path through which electrons flow. At the physical layer of OSI model of networking, electrons represent network data as binary Os and 1s. Transmission media provides these electrons with a bound or unbound communications path. Telephone lines are well proven and commonly used communication media.

Basically, there are two methods by which the communication takes place. They are:

1. Wired Transmission Media or Bound Transmission Media and
2. Wireless Transmission Media or Unbound Transmission Media

Bound Transmission Media

As said earlier, bound transmission media consists of a central conductor surrounded by a physical jacket. This property offers advantages in security, reliability and speed. Bound media are ideal for LANs but distance limitations can be the problem for WANs. The four types of bound media are:

i. Unshielded Twisted Pair (UTP)
ii. Shielded Twisted Pair (STP)
iii. Coaxial Cable
iv. Optical Fiber

i. Unshielded Twisted Pair (UTP)

UTP is the most common type of telecommunication media these days. It is most suited for data and voice communication. It consists of two metal conductors (usually copper) hat which are insulated separately with their own colored plastic insulation. It can transmit up to 96000 bps. UTP usually is intended for analog communications not digital.

ii. Shielded Twisted Pair (STP)

STP cable has a metal foil or braided mesh covering that covers each pair of insulated conductors. The metal foil is used to prevent infiltration of electromagnetic noise. This shield also helps to eliminate crosstalk during the telephone conversation. STP provides shielded protection against EMI (Electromagnetic interference) whereas UTP doesn’t have.

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iii. Coaxial Cable

Coaxial cabling is commonly used for television transmissions. It provides higher bandwidth and better reliability. Unlike twisted pairs that have two wires, coaxial cables have the single central conductor, which is made up of solid wire. This conductor is surrounded by an outer jacket made up of PVC. A coaxial cable is capable of transmitting data at the rate of 10 Mbps.

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iv. Optical Fiber

Fiber optics plays by a completely different set of rules. You won’t find any electricity here. Instead, fiber-optic cabling uses pulses of light (photos) for network communications i.e. it relies on photonics instead of electronics. As a result, fiber optics is completely immune to EMI (Electromagnetic Interference) and is extremely fast.

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Analog and Digital Signal

The major role of the physical medium is to move information from one communicating device to another. However, information to be transmitted should be first transformed into electromagnetic signals. Information over any medium is transmitted by two methods.

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They are:

i. Analog Signal
ii. Digital Signal

i. Analog Signal

An analog signal is a continuous waveform that changes smoothly over time. The analog signals represent the continuous variation in the data. When data are plotted on the Y-axis with time, we get curves called analog signal. During data transmission, the signals are modulated or changed in analog form.

ii. Digital Signal

Digital Signal data is the data stored in the form of 0s and 1s. When the signal is at a high point, its value is 1 and when it is low, its value is 0. A signal in digital format has precise voltages that are not affected by noise or attenuation compared to analog signals, which are very prone to noise. Digital data represent two states either 0 or 1.

Unbound Transmission Media or Wireless Communication

Unbound transmission media extend beyond the limiting confines of cabling. They provide an excellent communication alternative for WANs. The lack of physical restrictions provides larger bandwidth as well as wide-area capabilities. Unbound media typically operate at very high frequencies. The four types of unbound media are:

1. Radio Wave
2. Microwave
3. Infrared
4. Satellite Communication

1. Radio Wave

Although radio waves are prevalent and well understood, we are just beginning to realize their enormous potential as a networking medium. Radio waves can operate on single or multiple frequency bands. In this case, the signals are carried over carrier waves which have frequencies in the range of radio frequency spectrum.

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There are three types of RF (radio frequency) propagation, namely, ground wave, ionospheric and line of sight. During the time of radio wave propagation, it should be modulated. The following are the different types of modulation.

1. Amplitude Modulation (AM)
2. Frequency Modulation (FM)
3. Phase Modulation (PM)

2. Microwave

Microwave, in contrast, have been used in data communications for a long time. They have a higher frequency than radio wave and therefore, they can handle larger amounts of data. There are, of course, problems with microwaves attenuation and environmental interference.

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Microwave transmission is the line of sight transmission. The transmit station must be in visible contact with the receive station. This sets the limit on the distance between stations depending on the local geography.

3. Infrared

Infrared offers a great unbound photonic solution. Like fiber-optic cabling, infrared communications use light. So, they are not bound by the limitations of electricity.

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4. Satellite Communication

Satellite communication is also kind of line of sight transmission. Satellites are set in geostationary orbits directly over the equator, which rotates in synchronization to the earth and hence looks stationary from any point on the earth. These geostationary orbits are placed 36,000km above the earth’s surface.

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The communication is carried through uplinks and downlinks. Uplinks and downlinks are also called earth stations because they are located on the earth. The area shadowed by the satellite in which the information or data can be transmitted and received is called footprints. There are many satellites in the space for communication.

Computing Models

It is better to appreciate the importance of network models from the evolution of computing. In the beginning, electronic computers were huge machines that used thousands of vacuum tubes. This was the dark ages of computing before transistors and silicon chips. Computing, in its earliest form, was centralized in the mainframes. Mainframe computers stored, organized and processed information.

Data were fed into these centralized machines through remote terminals and keyboards. All data were stored on the mainframe and processing was performed there as well.

In a distributed computing model, 95% of the processing is handled by distributing computers. The central file server handles housekeeping tasks only, such as security and printer sharing. A relatively new trend called collaborative computing has emerged largely because of distributed computing and the capabilities of personal computer networks.

Unlike the distributed computing model (in which individual PCs carry out independent tasks), collaborative computing involves two or more computers working together synergistically.

Today’s networks integrate personal computers, mainframes and a host of other computing and other communication services. To classify this complexity, you need network models that transcend these definitions.

References:

Khanal, R.C. Khanal, R.C. Computer Concept for XII. Pashupatigriha Marga, Thapathali, Kathmandu, Nepal: Ekta Books Distributors Pvt. Ltd., 2010. -13.

Adhikari, Deepak Kumar.,et.al., Computer Science XII,Asia Publication Pvt.Ltd