4. Transmission Media – Express Learning: Data Communications and Computer Networks

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Transmission Media

1. What are transmission media? What are the different categories of transmission media?

Ans: Transmission media refer to the media through which data can be carried from a source to a destination. Data is transmitted from one device to another through electromagnetic signals. Transmission media are located under and controlled by the physical layer as shown in Figure 4.1.

Figure 4.1 Transmission Media and Physical Layer

The different categories of transmission media include guided (or wired) and unguided (or wireless) media as shown in Figure 4.2. Guided transmission media use a cabling system that guides the data signals along a specific path. It consists of a cable composed of metals such as copper, tin or silver. The data signal in guided medium is bound by the cabling system; hence, the guided medium is also known as bound medium. There are three basic types of guided transmission media: twisted-pair cable, coaxial cable and fibre-optic cable.

Figure 4.2 Categories of Transmission Media

Unguided transmission media facilitate data transmission without the use of a physical conduit. The electromagnetic signals are transmitted through earth's atmosphere (air, water or vacuum) at a much faster rate covering a wide area. The electromagnetic waves are not guided or bound to a fixed channel to follow. There are basically four types of unguided transmission media including radio waves, microwaves, satellite transmission and infrared waves.

2. Differentiate guided and unguided transmission media.

Ans. In order to transmit a message or data from a source to a destination, we need a transmission medium. Transmission medium can be broadly classified into two categories, which are guided and unguided media. The differences between these two transmission media are listed in Table 4.1.

Table 4.1 Differences Between Guided and Unguided Media

  Guided media   Unguided media
  • Signal is transmitted by establishing a physical path between the source and destination.
  • No physical path is established between the source and destination; signals are propagated through air.
  • Signals propagate in the form of current or voltage.
  • Signals propagate in the form of electromagnetic waves.
  • Guided media are well suited for point-to-point communication.
  • Unguided media are well suited for broadcast communication.
  • Examples of guided media are twisted-pair cables, coaxial cables and fibre-optic cables.
  • Examples of unguided media are microwave satellites, infrared waves and radio waves.

3. Write in short the design factors for a data transmission system.

Ans: The major concerns during the design of a data transmission system are achieving a higher data rate and covering maximum transmission distance. The key design factors that affect the data rate and transmission distance are as follows:

Bandwidth: The greater the bandwidth of the signal is, the higher will be the data rate.
Transmission Impairments: Impairments such as attenuation, limit the transmission distance and hence, they are undesirable.
Interference: Different signals in overlapping frequency bands result in interference which can distort or cancel out a signal. Interference is a major issue with unguided media but it also affects the guided media.
Number of Receivers: In a guided medium, as the number of receivers on a shared link increases, more attenuation and distortion are introduced which limit the distance and/or data rate.

4. Explain in detail the various types of guided transmission media.

Ans: There are three basic types of guided media—twisted-pair cable, coaxial cable and fibre-optic cable.

Twisted-pair Cable

It is one of the most common and least expensive transmission media. A twisted-pair cable consists of two insulated copper conductors that are twisted together forming a spiral pattern. A number of such pairs are bundled together into a cable by wrapping them in a protective shield. One of the wires in each twisted pair is used for receiving data signal and another for transmitting data signal. Twisted pairs are used in short-distance communication (less than 100 metres). The biggest network in the world, the telephone network, originally used only twisted-pair cabling and still does for most local connections. A twisted-pair cable has the capability of passing a wide range of frequencies. However, with the increase in frequency, attenuation also increases sharply. As a result, the performance of a twisted-pair cable decreases with the increase in frequency. A twisted-pair cable comes in two forms: unshielded and shielded with a metal sheath or braid around it. Accordingly, they are known as unshielded twisted-pair (UTP) and shielded twisted-pair (STP) cables.

UTP Cable: This cable has four pairs of wires inside the jacket (Figure 4.3). Each pair is twisted with a different number of twists per inch to help eliminate interference from adjacent pairs and other electrical devices. The tighter the twisting is, the higher will be the supported transmission rate and greater will be the cost per foot. Each twisted pair consists of two metal conductors (usually copper) that are insulated separately with their own coloured plastic insulation. UTP cables are well suited for both data and voice transmissions; hence, they are commonly used in telephone systems. They are also widely used in DSL lines, 10Base-T and 100Base-T local area networks.

Figure 4.3 UTP Cable

STP Cable: This cable has a metal foil or braided-mesh covering that covers each pair of insulated conductors (Figure 4.4). The metal foil is used to prevent infiltration of electromagnetic noise. This shield also helps to eliminate crosstalk. An advantage of STP cables over UTP cables is that they are suitable for the environments with electrical interference. In addition, they provide better performance at higher data rates. However, the extra shielding makes the STP cables quite bulky and more expensive than UTP cables.

Figure 4.4 STP Cable

Coaxial Cable

Coaxial cables (or coax) have a single central conductor, which is made up of solid wire (usually, copper) (Figure 4.5). This conductor is surrounded by an insulator over which a sleeve of metal mesh is woven to block any outside interference. This metal mesh is again shielded by an outer covering of a thick material (usually PVC) known as jacket.

Figure 4.5 Coaxial Cable

Although coaxial cabling is difficult to install, it is highly resistant to signal interference. It can support greater cable lengths between network devices and can offer greater bandwidth than twisted-pair cable. However, attenuation in coaxial cables is much higher as compared to twisted-pair cables due to which the signal weakens rapidly. As a result, repeaters are to be used frequently to boost up the signals. Coaxial cables are capable of transmitting data at a fast rate of 10Mbps. Some of the applications that use coaxial cables include analog and digital telephone networks, cable TV networks, Ethernet LANs, and short range connections.

Fibre-optic Cable

Fibre-optic cable or optical fibre consists of thin glass fibres that can carry information in the form of visible light. The typical optical fibre consists of a very narrow strand of glass or plastic called the core. Around the core is a concentric layer of less dense glass or plastic called the cladding. The core diameter is in the range of 8–50 microns (1 micron = 10−6 metres) while cladding generally has a diameter of 125 microns. The cladding is covered by a protective coating of plastic, known as jacket (see Figure 4.6).

Figure 4.6 Optical Fibre

Optical fibres transmit a beam of light by means of total internal reflection. When a light beam from a source enters the core, the core refracts the light and guides the light along its path. The cladding reflects the light back into the core and prevents it from escaping through the medium (see Figure 4.7). These light pulses, which can be carried over long distances via optical fibre cable at a very high speed, carry all the information.

Figure 4.7 Signals Carried over an Optical Fibre

Optical fibre has the capability to carry information at greater speeds, higher bandwidth and data rate. A single optical fibre can pack hundreds of fibres, where each fibre has the capacity equivalent to that of thousands of twisted-pair wires. This capacity broadens communication possibilities to include services such as video conferencing and interactive services. In addition, fibre optic cables offer lower attenuation and superior performance and require fewer repeaters as compared to coaxial and twisted-pair cables. The major applications of the fibre-optic cables are cable TV, military applications, long-haul trunks, subscriber loops, local area networks, metropolitan trunks and rural trunks.

5. Why twisting of wires is necessary in twisted-pair cables?

Ans: Twisting is done in twisted-pair cables because it tends to minimize the interference (noise) between the adjacent pair of wires in cable thereby reducing the crosstalk. In case the two wires are parallel, they may not get affected equally by the electromagnetic interferences (noise and crosstalk) from nearby sources due to their different locations relative to the source. As a result, the receiver would receive some unwanted signals. On the other hand, if the wires are twisted, both wires are probable to be get affected equally by the external interferences thereby maintaining a balance at the receiver. To understand, suppose in one twist, one of the twisted wires is closer to noise source and the other is farther, then in the next twist, the opposite will be true. As a result, the unwanted signals of both the wires cancel out each other and the receiver does not receive any unwanted signal. Thus, crosstalk is reduced.

6. What are the different categories and connectors of UTP cables?

Ans: As per the standards developed by the Electronic Industries Association (EIA), UTP cables have been classified into seven categories, from 1 to 7. Each category is based on the quality of the cable and the higher number denotes higher quality. Table 4.2 lists the categories of UTP cables along with their specification and data rate.

To connect UTP cables to network devices, UTP connectors are required. The most common connector for the UTP cables is RJ45 (RJ stands for registered jack) as shown in Figure 4.8. Being a keyed connector, the RJ45 can be inserted in only one way.

Table 4.2 Categories of UTP Cables

Category (CAT) Specification Data rate (in Mbps)
1 UTP cables used in telephones <0.1
2 UTP cables used in T-lines 2
3 Improved CAT2 used in LANs 10
4 Improved CAT3 used in token ring network 20
5 Cable wire is normally 24AWG with a jacket and outside sheath; used in LANs 100
5E Extension to category 5 that reduces crosstalk and interference; used in LANs. 125
6 A new category with matched components coming from the same manufacturer. This cable must be tested at a data rate of 200 Mbps. This is used in LANs. 200
7 This is the shielded screen twisted-pair cable (SSTP). Shielding increases data rate and reduces crosstalk effect. This cable is used in LANs. 600

7. What are the different categories and connectors of coaxial cables?

Ans: According to the ratings provided by radio government (RG), the coaxial cables have been divided into three categories. Each category has a specific RG number that indicates a unique set of physical specifications. These specifications include the wire gauge of inner conductor, shield, type and size of outer casing and type and thickness of inner insulator. Different categories of the coaxial cables are listed in Table 4.3.

Figure 4.8 RJ45 UTP Connector

To connect coaxial cables to other devices, coaxial connectors are required. The three most popular coaxial connectors include the BNC connector, BNC terminator and BNC T connector as shown in Figure 4.9. The Bayone-Neill-Concelman (BNC) connector is the most commonly used connector that connects the coaxial cable to a device such as an amplifier or television set. The BNC terminator is used at the end of the cable to prevent the reflection of the signal and the BNC T connector is often used in Ethernet networks for branching out connections to other devices.

Table 4.3 Categories of Coaxial Cables

Category Use Impedance
RG-59 Cable TV 75 Ω
RG-58 Thin Ethernet 50 Ω
RG-11 Thick Ethernet 50 Ω

Figure 4.9 Coaxial Cable Connectors

8. Explain the different fibre-optic propagation modes.

Ans: Fibre-optic cables support two modes for propagating light, which are multimode and single mode. Each mode requires fibre with different physical characteristics.

Multimode Propagation

In this mode, many beams from a light source traverse the fibre along multiple paths and at multiple angles as shown in Figure 4.10(a). Depending upon the structure of core inside the cable, multimode can be implemented in two forms: step-index and graded-index. In multimode step-index fibre, the core's density is constant from the centre to the edges. A light beam moves through the core in a straight path until it meets the interface of the core and cladding. As the interface has a lower density than the core, there comes a sudden change in the angle of the beam's motion further adding to distortion of the signal as it moves on. The multimode graded-index fibre reduces such distortion of signal through the cable. As the density is high at the centre of the core, the refractive index at the centre is high which causes the light beams at the centre to move slower than the rays that are near the cladding. The light beams curve in a helical manner [see Figure 4.10(b)], thus, reducing the distance travelled as compared to zigzag movement. The reduction in path and higher speed allows light to arrive at the destination in almost the same time as straight lines.

Single-mode Propagation

This mode employs step-index fibre of relatively small diameter and less density than that of multimode fibre and a much focused light source. Because of the focused light source, the beams spread out to a small range of angles and propagate almost horizontally. Since all beams propagate through the fibre along a single path, distortion does not occur. Moreover, all beams reaching at the destination together can be recombined to form the signal. The single-mode propagation is well suited for long-distance applications such as cable television and telephones.

Figure 4.10 Propagation Modes for fibre-optic Cable

9. What are the different fibre sizes and connectors available?

Ans: The type of optical fibre is specified by the ratio of the diameter of its core to the diameter of its cladding. The commonly available fibre sizes are listed below in Table 4.4.

Fibre-optic cables use three types of connectors, which are SC connector, ST connector and MT-RJ connector as shown in Figure 4.11. SC (subscriber channel) connector uses a push–pull locking mechanism and is primarily used to connect fibre-optic cables for cable television. The ST (straight tip) connector is used to connect fibre-optic cables to network devices. This connector is more reliable than SC connector. MT-RJ (mechanical transfer-registered jack) connector is a small size connector which looks similar to RJ-45 connector. It is widely used for networking applications.

Table 4.4 Fibre Types

Type (core/cladding) Mode
50/125 Multimode, graded index
62.5/125 Multimode, graded index
100/125 Multimode, graded index
7/125 Single mode

Figure 4.11 Fibre-optic Cable Connectors

10. What are the advantages and disadvantages of fibre optic cables?

Ans: Fibre-optic cables are widely used in many domains such as telephone network and cable television network. Some advantages of fibre-optic cables are as follows:

Since transmission is light-based rather than electricity, it is immune to noise interference.
Transmission distance is greater than other guided media because of less signal attenuation (degradation in quality over distance).
It is extremely hard to tap into, making it desirable from the security viewpoint.
They are smaller and lighter than copper wire and are free from corrosion as well.
Fibre optic offers, by far, the greatest bandwidth of any transmission system.
Transmission through fibre optic cable requires lesser number of repeaters for covering larger transmission distances.

The disadvantages of fibre-optic cables are as follows:

The installation and maintenance of fibre-optic cables are quite expensive.
The propagation of light is unidirectional and often requires precise alignment.
Extending the fibre-optic cables by joining them together is a tough task.
It is more fragile when compared to copper wires.

11. What are the two kinds of light sources used in fibre-optic cables?

Ans: In fibre-optic cables, the light sources generate a pulse of light that is carried through the fibre medium. The two kinds of light sources used with the fibre-optic cables include light-emitting diodes (LEDs) and semiconductor laser diodes. These light sources are used to perform signalling and can be tuned in wavelengths by inserting Mach–Zehnder or Fabry–Pérot interferometers between the source and the fibre media. Table 4.5 lists the comparison between these two light sources.

Table 4.5 Comparison Between LEDs and Semiconductor Diodes

Characteristics LED Semiconductor laser diode
Data rate Low High
Cost Low cost Expensive
Fibre type Multimode Multimode/single mode
Lifetime Long life Short life
Temperature sensitivity Minor Substantial
Distance Short Long

 

12. Explain the use of electromagnetic spectrum for communication.

Ans: Electromagnetic signals include power, voice, radio waves, infrared light, visible light, ultraviolet light, X-rays and gamma rays. Each of these forms a portion of the electromagnetic spectrum (see Figure 4.12). The portions of the electromagnetic spectrum that can be used for transmitting information include radio wave, microwave, infrared light and visible light. Information can be transmitted using these portions by modulating the frequency, amplitude or phase of the electromagnetic waves. Though using UV, X-rays and gamma rays for transmitting information is a better choice due to their high frequency (HF), they are not generally used. This is because it is difficult to produce and modulate these rays as well as they cannot penetrate obstacles such as buildings. Moreover, they are dangerous for the living beings.

Figure 4.12 Electromagnetic Spectrum and Its Uses for Communication

The portion of the electromagnetic spectrum that can be used for transmitting information is parted into eight different ranges. These ranges are regulated by the government authorities and are known as bands. Some of the properties of bands are listed in Table 4.6.

Table 4.6 Properties of Bands

13. Explain the different propagation methods for unguided signals.

Ans: Unguided signals can propagate in three ways, which are ground wave, ionosphere and line-of-sight.

Ground Wave Propagation: In this propagation method, the radio waves pass through the lowest portion of the atmosphere, that is, the curvature of the earth (Figure 4.13). These low frequency radio waves when transmitted by an antenna disperse in all directions following the curvature of earth. The distance travelled is directly proportional to the power of the signal. That is, greater the amount of power in the signal is, the more will be the distance covered.

Figure 4.13 Ground Wave Propagation

Ionospheric Propagation: In this propagation method, the higher frequency radio waves transmitted by antenna travel upwards into ionosphere layer in the upper portion of atmosphere and bounced off by the layer towards earth; a low power signal can travel a greater distance (Figure 4.14). It operates in the frequency range of 2–30 MHz. As this type of propagation depends on the earth's ionosphere, it changes with the day timings and weather. This method of propagation is also known as sky wave propagation.

Figure 4.14 Ionosphere Propagation

Line of Sight Propagation: In this propagation method, very high frequency signals are transmitted which travel exactly in straight line (Figure 4.15). This method demands both transmitting and receiving antennas to be in line of sight of each other, that is, the receiving antenna must be in view of the transmitting antenna. It is sometimes called space waves or tropospheric propagation. It is limited by the curvature of the earth for ground-based stations (50 km).

Figure 4.15 Line-of-Sight Propagation

14. Explain in detail the different types of unguided media.

Ans: Unguided media are used in those cases when transmission of data through guided media is difficult. The three main types of unguided media are discussed in the following sections:

Radio Waves

The electromagnetic waves with frequency in the range of 3 kHz to 1 GHz are generally known as radio waves. These waves are omnidirectional, that is they are propagated in all directions when transmitted by an antenna. Thus, the antennas that send and receive the signals need not be aligned. However, the radio waves transmitted by two antennas using the same band or frequency may interfere with each other.

Radio waves present different characteristics at different frequencies. At low (VLF, LF) and medium (MF) frequencies, they follow the curvature of earth and can penetrate walls easily. Thus, a device such as a portable radio inside a building can receive the signal. At high frequencies (HF and VHF bands), as the earth absorbs the radio waves, they are propagated in sky mode. The higher frequency radio waves can be transmitted up to greater distances and thus are best suited for long-distance broadcasting. However, at all frequencies, radio waves are susceptible to interference from electrical equipments.

An omnidirectional antenna is used to transmit radio waves in all directions (see Figure 4.16). Due to their omnidirectional characteristics, radio waves are useful for multicast (one sender, many receivers) communication. Examples of multicasting are cordless phones, AM and FM radios, paging and maritime radio.

Figure 4.16 Omnidirectional Antenna

Microwaves

The electromagnetic waves with frequency in the range of 1–300 GHz are known as microwaves. Unlike radio waves, microwaves are unidirectional. The advantage of the unidirectional property is that multiple transmitters can transmit waves to multiple receivers without any interference. Since microwaves are transmitted using line-of-sight propagation method, the towers with mounted antennas used for sending and receiving the signal must be in direct sight of each other (Figure 4.17). In case, the antenna towers are located far away from each other, the towers should be quite tall so that the signals do not get block off due to curvature of earth as well as other obstacles. Moreover, repeaters should be often used to amplify the signal strength. Microwaves at lower frequencies cannot penetrate buildings and also, during propagation, refraction or delays can occur due to divergence of beams in space. These delayed waves can come out of phase with the direct wave leading to cancellation of the signal. This phenomenon is known as the multipath fading effect.

Figure 4.17 Microwave Transmission

Microwaves require unidirectional antennas that transmit signal only in one direction. Two such antennas are dish antenna and horn antenna (see Figure 4.18). A dish antenna works based on the geometry of a parabola. All the lines parallel to the line of sight when hit the parabola, they are reflected by the parabola curve at angles such that they converge at a common point called the focus. The dish parabola catches many waves and directs them on the focus. As a result, the receiver receives more of the signal. In a horn antenna, outgoing transmission is sent through a stem and as it hits the curved head, the transmission is deflected outward as a series of parallel beams. The received transmissions are collected by a horn similar to the parabolic dish, which deflects them back into the stem.

Since microwaves are unidirectional, they are best suited for unicast communication such as in cellular networks, wireless local area networks and satellite networks.

Figure 4.18 Unidirectional Antennas

Infrared Waves

The electromagnetic waves with frequency in the range of 300 GHz to 400 THz are known as infrared waves. These waves are widely used for indoor wireless LANs and for short-range communication; for example, for connecting a PC with a wireless peripheral device, in remote controls used with stereos, VCRs, TVs, etc. (Figure 4.19). Infrared waves at high frequencies are propagated using line-of-sight method and cannot penetrate solid objects. Therefore, a short range infrared system in a room will not be interfered by such a system present in an adjacent room. Furthermore, infrared waves cannot be used outside a building because the infrared rays coming from the sun may interfere with it and distort the signal.

The use of infrared waves has been sponsored by an association, known as Infrared Data Association (IrDA). This association has also established standards for the usage of infrared signals for communication between devices such as keyboards, printers, PCs and mouses. For example, some wireless keyboards are attached with an infrared port that enables the keyboard to communicate with the PC. Since infrared signals transmit through line-of-sight mode, the infrared port must be pointed towards the PC for communication.

Figure 4.19 Infrared Waves

Communication Satellites

A communication satellite can be referred to as a microwave relay station. A satellite links two or more ground-based (earth) stations that transmit/receive microwaves. Once a satellite receives any signal on a frequency (uplink), it repeats or amplifies that signal and sends it back to earth on a separate frequency (downlink). The area shadowed by the satellite (see Figure 4.20) in which the information or data can be transmitted and received is called the footprint.

Satellites are generally set in geostationary orbits directly over the equator, which rotates in synchronization with the earth and hence looks stationary from any point on the earth. These geostationary orbits are placed approximately 36,000 km above the earth's surface and satellites placed in this orbit are known as geostationary satellites.

Satellite transmission is also a kind of line-of-sight transmission and the best frequency for it is in the range of 1–10 GHz. The major applications for satellites are long-distance telephone transmission, weather forecasting, global positioning, television, and many more.

Figure 4.20 Satellite Transmission

Multiple Choice Questions

  1. A transmission medium is located under and controlled by the

    (a) transport layer

    (b) application layer

    (c) physical layer

    (d) session layer

     

  2. Guided transmission media include

    (a) coaxial cable

    (b) fibre-optic cable

    (c) twisted-pair cable

    (d) All of these

     

  3. Which of the following is not a type of twisted-pair cable?

    (a) UTP

    (b) FTP

    (c) STP

    (d) None of these

     

  4. BNC connectors are used with

    (a) satellites

    (b) fibre-optic cables

    (c) coaxial cables

    (d) twisted-pair cables

     

  5. The transmission medium with maximum error rate is

    (a) coaxial cable

    (b) twisted-pair cable

    (c) satellite link

    (d) optical fibre

     

  6. Optical fibres transmit a beam of light by means of

    (a) total internal reflection

    (b) total internal refraction

    (c) Both (a) and (b)

    (d) None of these

     

  7. In multimode step-index fibre, the density of the core_________from the centre to the edges.

    (a) increases

    (b) decreases

    (c) remains constant

    (d) None of these

     

  8. Which of the following is not a band?

    (a) VHF

    (b) UHF

    (c) VLF

    (d) SLF

     

  9. Unguided signals can propagate in__________ways.

    (a) two

    (b) eight

    (c) three

    (d) four

     

  10. The frequency at which a signal is received by a satellite is known as its________frequency.

    (a) downlink

    (b) microwave

    (c) terrestrial

    (d) uplink

Answers

1. (c)

2. (d)

3. (b)

4. (c)

5. (b)

6. (a)

7. (c)

8. (d)

9. (c)

10. (d)