Untitled Flashcards Set

Topic 4: Transmission Media and Data transmission

This topic covers the below sections;

s  Transmission Media and

s  Data transmission

1.      Transmission Media

In data communication terminology, a transmission medium is a physical path between the transmitter and the receiver i.e. it is the channel through which data is sent from one place to another.

Some factors to be considered when designing the transmission media:

• Bandwidth: All the factors are remaining constant, the greater the bandwidth of a medium, the higher the data transmission rate of a signal.

• Transmission impairment: When the received signal is not identical to the transmitted one due to the transmission impairment. The quality of the signals will get destroyed due to transmission impairment.

• Interference: An interference is defined as the process of disrupting a signal when it travels over a communication medium on the addition of some unwanted signal.

Causes of Transmission Impairment:

• Attenuation: Attenuation means the loss of energy, i.e., the strength of the signal decreases with increasing the distance which causes the loss of energy. Attenuation is measured in decibels (dB). It measures the relative strengths of two signals or one signal at two different point. Amplifiers are used to amplify the attenuated signal which gives the original signal back and compensate for this loss. Amplifiers are used to amplify the signals to compensate for this loss.

-          The below figure shows the effect of attenuation and amplification:

·       Distortion: Distortion occurs when there is a change in the shape of the signal. This type of distortion is examined from different signals having different frequencies. Each frequency component has its own propagation speed, so they reach at a different time which leads to the delay distortion.

·       Noise: When data is travelled over a transmission medium, some unwanted signal is added to it which creates the noise.

·       Any unwanted signal gets added to the transmitted signal by which the resulting transmitted signal gets modified and at the receiver side it is difficult to remove the unwanted noise signal. These noises are various kinds like impulse noise, thermal noise etc.

Ø   Impulse noise is a rapid rise in sound pressure that typically last less than one second. Impulse noise can also be defined as a noncontiguous series of irregular pulses or noise spikes of short duration, broad spectral density and of relatively high amplitude. Impulse noise can be caused by positioning a communications cable near a source of intermittent but strong electromagnetic pulses, such as an elevator motor

Ø  Thermal noise is the electronic noise generated by the thermal agitation of the electrons inside an electrical conductor at equilibrium.” In other words, it means that noise is always generated when a current is passed through the resistor.

Noise is diagrammatically represented as follows −

Transmission Media is broadly classified into two types:  

a). Guided and

b). Unguided Media

1. Guided Media: It is also referred to as Wired or Bounded transmission media. Signals being transmitted are directed and confined in a narrow pathway by using physical links. 

Features:  

·       High Speed

·       Secure

·       Used for comparatively shorter distances

There are three major types of Guided Media: 

(i) Twisted Pair Cable

Twisted pair cable transmits both analog and digital signals. It consists of two insulated copper wires arranged in a spiral pattern. Generally, several such pairs are bundled together in a protective sheath. Twisting helps to decrease the interference between the adjacent pairs of a cable. They are the most widely used Transmission Media.

                                 Twisted pair cable

There are two types of twisted pairs namely the Unshielded Twisted Pair (UTP) and shielded twisted pair (STP): 

a). Unshielded Twisted Pair (UTP): 
UTP consists of two insulated copper wires twisted around one another. This type of cable has the ability to block interference and does not depend on a physical shield for this purpose. It is used for telephonic applications.

Unshielded Twisted Pair

Advantages: 

·       Least expensive

·       Easy to install

·       High-speed capacity

Disadvantages: 

·       Susceptible to external interference

·       Lower capacity and performance in comparison to STP

·       Short distance transmission due to attenuation

b). Shielded Twisted Pair (STP): 
This type of cable consists of a special jacket (a copper braid covering or a foil shield) to block external interference. It is used in fast-data-rate Ethernet and in voice and data channels of telephone lines.

Shielded Twisted Pair

Advantages: 

§  Better performance at a higher data rate in comparison to UTP

§  Eliminates crosstalk (unwanted signals in a communication channel (as in a telephone, radio, or computer) caused by transference of energy from another circuit)

§  Comparatively faster

Disadvantages: 

§  Comparatively difficult to install and manufacture

§  More expensive

§  Bulky

Twisted pair categories:

Twisted pair connectors (RJ45)

Twisted Pair / RJ-45 Cabling Types

(ii) Coaxial Cable
it has an outer plastic covering containing an insulation layer made of PVC or Teflon and 2 parallel conductors each having a separate insulated protection cover. The coaxial cable transmits information in two modes: Baseband mode (dedicated cable bandwidth) and Broadband mode (cable bandwidth is split into separate ranges). Cable TVs and analog television networks widely use Coaxial cables. 

Coaxial cables are divided into two namely; ThickNet and ThinNet;

ThickNet - This type of coaxial cabling is used with Ethernet 10Base5 networks and is able to span distances of up to 500 meters. Originally used to directly connect computers, it eventually became popular in backbone implementations between LANs.

ThinNet- A much thinner and more flexible type of coaxial cable, ThinNet is used on Ethernet 10Base2 networks and can span distances of up to 185 meters. This was usually the media of choice for connecting computers on a LAN. In ThinNet networks, computers connect to the network via a BNC-T connector attached to the network card.

                                                BNC connectors

Advantages of Coaxial cables: 

·       High Bandwidth, the data can be transmitted at high speed.

·       Better noise Immunity, it has better shielding as compared to twisted pair cable.

·       Easy to install and expand

Disadvantages:  

·       It is more expensive as compared to twisted pair cable.

·       If any fault occurs in the cable causes the failure in the entire network.

-       (iii) Fiber Optic Cable / Optical fiber cable
This uses the concept of refraction of light through a core made up of glass or plastic. The core is surrounded by a less dense glass or plastic covering called the cladding. It is used for the transmission of large volumes of data. 

-       The cable can be unidirectional or bidirectional. The WDM (Wavelength Division Multiplexer) supports two modes, namely unidirectional and bidirectional mode.

Basic elements of Fibre optic cable:

• Core - The optical fibre consists of a narrow strand of glass or plastic known as a core. A core is a light transmission area of the fibre. The more the area of the core, the more light will be transmitted into the fibre.

• Cladding - The concentric layer of glass is known as cladding. The main functionality of the cladding is to provide the lower refractive index at the core interface as to cause the reflection within the core so that the light waves are transmitted through the fibre.

• Jacket - The protective coating consisting of plastic is known as a jacket. The main purpose of a jacket is to preserve the fibre strength, absorb shock and extra fibre protection.

Following are the advantages of fibre optic cable over copper:

• Greater Bandwidth - The fibre optic cable provides more bandwidth as compared copper. Therefore, the fibre optic carries more data as compared to copper cable.

• Faster speed- Fibre optic cable carries the data in the form of light. This allows the fibre optic cable to carry the signals at a higher speed.

• Longer distances -The fibre optic cable carries the data at a longer distance as compared to copper cable. It has less signal attenuation.

• Immunity to electromagnetic interference. The fibre optic cable is more reliable than the copper cable as it is immune to temperature changes which can cause obstruction in the connectivity of copper cable. It is Resistant to corrosive materials

• Thinner and Sturdier: Fibre optic cable is thinner and lighter in weight so it can withstand more pull pressure than copper cable.

Disadvantages:  

·       Difficult to install and maintain

·       High cost

·       Fragile

Others guided media include;

(iv) Stripline

Ø  Stripline is a transverse electromagnetic (TEM) transmission line medium invented by Robert M. Barrett of the Air Force Cambridge Research Centre in the 1950s.

Ø  Strip line consists of a central thin conducting strip of width ω which is greater than its thickness t. It is placed inside the low loss dielectric (εr) substrate of thickness b/2 between two wide ground plates. The width of the ground plates is five times greater than the spacing between the plates.

Ø  The thickness of metallic central conductor and the thickness of metallic ground planes are the same. The below figure shows the cross-sectional view of the strip line structure.

Ø  It uses a conducting material to transmit high-frequency waves it is also called a waveguide. This conducting material is sandwiched between two layers of the ground plane which are usually shorted to provide Electromagnetic Interference immunity.

Ø  Stripline is often compared to a flattened coaxial cable in that, like the cable, it consists of an inner conductor completely surrounded by dielectric material which is itself surrounded by a ground braid or foil. Of course, stripline circuits are planar, so that they appear as a sandwich of conductors in the middle, surrounded by dielectric layers, which in turn have parallel ground planes on the top and bottom.

(v) Microstripline

In this, the conducting material is separated from the ground plane by a layer of dielectric (insulator).

The strip line has a disadvantage that it is not accessible for adjustment and tuning. This is avoided in microstrip lines, which allows mounting of active or passive devices, and also allows making minor adjustments after the circuit has been fabricated.

A micro strip line is an unsymmetrical parallel plate transmission line, having di-electric substrate which has a metallized ground on the bottom and a thin conducting strip on top with thickness 't' and width 'ω'. This is illustrated in the figure below;

Stripline versus Microstripline- Why choose one transmission-line format over the other?

Both provide excellent electrical performance through millimeter-wave frequencies, depending upon the choice of Printed Circuit Boards (PCBs) materials. Printed circuit boards (PCBs) are usually a flat laminated composite made from non-conductive substrate materials with layers of copper circuitry buried internally or on the external surfaces.

Microstrip circuits are easier (and less expensive) to fabricate than stripline, with less processing steps and easier placement of circuit components. The stripline format affords more isolation between adjacent circuit traces, supporting more densely integrated circuits than with microstrip. Stripline circuits are also well suited for fabricating multilayer circuits, with good isolation between layers. The layers are interconnected by means of plated through holes (PTHs).

In both microstrip and stripline, the electrical behavior of the conductors are affected by the relative dielectric constants of the insulator materials as well as the proximity of the ground planes.

In microstrip there is one ground plane, while in stripline, there are two. In microstrip the effective dielectric constant impacting the impedance of a conductor is a combination of the relative dielectric constant of the insulator material as well as that of the air above the circuit (which is equal to 1). In stripline the effective dielectric constant is a combination of the relative dielectric constants of the substrate layers above and below the conductors.

b. Unguided Media: 
It is also referred to as Wireless or Unbounded transmission media. No physical medium is required for the transmission of electromagnetic signals. 

Features:  

·       The signal is broadcasted through air

·       Less Secure

·       Used for larger distances

There are three types of Signals transmitted through unguided media: 

(i) Radio waves

Ø  The basic building block of radio communications is a radio wave. Like waves on a pond, a radio wave is a series of repeating peaks and valleys. The entire pattern of a wave, before it repeats itself, is called a cycle.

Ø  The wavelength is the distance a wave takes to complete one cycle. Wavelength can be defined as the distance between two successive crests or troughs of a wave. It is measured in the direction of the wave.

Ø   The number of cycles, or times that a wave repeats in a second, is called frequency. Frequency is measured in the unit hertz (Hz), referring to a number of cycles per second. One thousand hertz is referred to as a kilohertz (kHz), 1 million hertz as a megahertz (MHz), and 1 billion hertz as a gigahertz (GHz). The range of the radio spectrum is considered to be 3 kilohertz up to 300 gigahertz.

Ø  A radio wave is generated by an electronic device called a transmitter which is connected into an antenna which radiates the waves. An antenna allows a radio transmitter to send energy into space and a receiver to pick up energy from space. The generated waves are received by another antenna connected to a radio receiver, which processes the received signal. Transmitters and receivers are typically designed to operate over a limited range of frequencies.

Ø  Radio waves are low-frequency signals and propagate in all directions. Therefore, it is not necessary to align the sending and receiving antennas. However, it is suitable for long distance broadcasting.

Ø  Radio waves are very widely used in modern technology for fixed and mobile radio communication, broadcasting, radar and radio navigation systems, communications satellites, wireless computer networks and many other applications.

Ø  These are easy to generate and can penetrate through buildings.

Ø  The sending and receiving antennas need not be aligned.

Ø  AM and FM radios and cordless phones use Radio waves for transmission. 

Ø  Radio can further be categorized as Terrestrial and Satellite. 

Ø  In Terrestrial radio, the radio waves are broadcast by a land-based radio station, while in satellite radio the radio waves are broadcast by a satellite in Earth orbit.

Ø  A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Most satellites have an electricity generation system for equipment on board, such as solar panels or radioisotope thermoelectric generators (RTGs). Most satellites also have a method of communication to ground stations, called transponders (e series of interconnected units that form a communications channel between the receiving and the transmitting antennas).

Ø  (ii) Microwaves
It is a line of sight transmission i.e. the sending and receiving antennas need to be properly aligned with each other. The distance covered by the signal is directly proportional to the height of the antenna. Frequency Range: 1GHz – 300GHz. These are majorly used for mobile phone communication and television distribution. 

Ø  Microwave technology is extensively used for point-to-point telecommunications (i.e. non-broadcast uses). Microwaves are especially suitable for this use since they are more easily focused into narrower beams than radio waves, allowing frequency reuse; their comparatively higher frequencies allow broad bandwidth and high data transmission rates, and antenna sizes are smaller than at lower frequencies because antenna size is inversely proportional to the transmitted frequency.

Ø  The point to point wireless is the ideal alternative for business communication between two buildings or sites where wired connection is either impossible, costly or impractical.

Ø  Microwaves are of two types namely Terrestrial and satellite;

Characteristics of Microwave:

• Frequency range: The frequency range of terrestrial microwave is from 4-6 GHz to 21-23 GHz.

• Bandwidth: It supports the bandwidth from 1 to 10 Mbps.

• Short distance: It is inexpensive for short distance.

• Long distance: It is expensive as it requires a higher tower for a longer distance.

• Attenuation: Attenuation means loss of signal. It is affected by environmental conditions and antenna size.

Advantages of Microwave:

• Microwave transmission is cheaper than using cables.

• It is free from land acquisition as it does not require any land for the installation of cables.

• Microwave transmission provides an easy communication in terrains as the installation of cable in terrain is quite a difficult task.

• Communication over oceans can be achieved by using microwave transmission.

Disadvantages of Microwave transmission:

• Eavesdropping - An eavesdropping creates insecure communication. Any malicious user can catch the signal in the air by using its own antenna.

• Out of phase signal - A signal can be moved out of phase by using microwave transmission.

• Susceptible to weather condition - A microwave transmission is susceptible to weather condition. This means that any environmental change such as rain, wind can distort the signal.

• Bandwidth limited - Allocation of bandwidth is limited in the case of microwave transmission.

Ø  Microwaves are of two types; Terrestrial and Satellite microwaves.

Terrestrial Microwave Transmission

• Terrestrial Microwave transmission is a technology that transmits the focused beam of a radio signal from one ground-based microwave transmission antenna to another.

• Microwaves are the electromagnetic waves having the frequency in the range from 1GHz to 1000 GHz.

• Microwaves are unidirectional as the sending and receiving antenna is to be aligned, i.e., the waves sent by the sending antenna are narrowly focused.

• In this case, antennas are mounted on the towers to send a beam to another antenna which is km away.

• It works on the line of sight transmission, i.e., the antennas mounted on the towers are the direct sight of each other.

Satellite Microwave Communication

• A satellite is a physical object that revolves around the earth at a known height.

• Satellite communication is more reliable nowadays as it offers more flexibility than cable and fibre optic systems.

• We can communicate with any point on the globe by using satellite communication.

So, how Does Satellite work?

The satellite accepts the signal that is transmitted from the earth station, and it amplifies the signal. The amplified signal is retransmitted to another earth station.

Advantages of Satellite Microwave Communication:

• The coverage area of a satellite microwave is more than the terrestrial microwave.

• The transmission cost of the satellite is independent of the distance from the centre of the coverage area.

• Satellite communication is used in mobile and wireless communication applications.

• It is easy to install.

• It is used in a wide variety of applications such as weather forecasting, radio/TV signal broadcasting, mobile communication, etc.

Disadvantages of Satellite Microwave Communication:

• Satellite designing and development requires more time and higher cost.

• The Satellite needs to be monitored and controlled on regular periods so that it remains in orbit.

• The life of the satellite is about 12-15 years. Due to this reason, another launch of the satellite has to be planned before it becomes non-functional.

(iii) Infrared
Infrared waves are used for very short distance communication. They cannot penetrate through obstacles. This prevents interference between systems.

The frequency of the infrared is in the range from 300 GHz to 400 THz.

It is used for short-range communication such as data transfer between two cell phones, TV remote operation, wireless mouse, keyboard, printer and data transfer between a computer and cell phone residing in the same closed area.

Characteristics of Infrared:

• It supports high bandwidth, and hence the data rate will be very high.

• Infrared waves cannot penetrate the walls. Therefore, the infrared communication in one room cannot be interrupted by the nearby rooms.

• An infrared communication provides better security with minimum interference.

• Infrared communication is unreliable outside the building because the sun rays will interfere with the infrared waves.

Key differences between: Radio waves, Microwaves and Infrared waves:

§  Radio waves are low-frequency signals and propagate in all directions. Therefore, it is not necessary to align the sending and receiving antennas. However, it is suitable for long distance broadcasting.

§  On the other hand, the microwave has a higher frequency than radio waves. But, the distance a signal can travel depends on the height of the antenna. Furthermore, the microwave requires line of sight transmission. Cellular phones, satellite networks, and wireless LANs use microwaves.

§  The infrared waves cannot pass much through obstacles. Therefore, they are used for short distance communication. Devices like TV remote controllers and VCR use infrared waves.

Difference between Guided and Unguided Media:

Important note:

When choosing the transmission media consider the following;

•       Transmission speed

•       Segment length

•       Cost

•       Resistance to environment conditions

2.      Data Transmission

What is data transmission?

ü  Data Transmission entails transfer of data between two devices. It is also known as a communication mode.

ü  A data transmission mode describes how two devices in a network communicate or exchange data. It specifics the direction in which signals travel over the media and the number of signals that can traverse the media at any given time.

Types of data transmission mode

There are three types of data transmission modes namely;

a)      Simplex

b)      Half-duplex and

c)      Full-duplex

a). Simplex

Simplex is also called one-way or unidirectional.

It allows communication in one direction only. Since signals travel in only one direction, the sender device uses the entire communication channel or all available bandwidth. The receiver device only receives signals. The receiver can't reply to the sender by using the same communication channel.

Examples of simple mode communication: TV remotes, keyboards, traditional monitors and smart speakers are some examples of Simplex. You can use the remote to control TV programs and functions, but you can't use the TV to control the remote in any way. The keyboard can only introduce input, the monitor can only give the output. 

Advantage of Simplex mode:

• In simplex mode, the station can utilize the entire bandwidth of the communication channel, so that more data can be transmitted at a time.

Disadvantage of Simplex mode:

• Communication is unidirectional, so it has no inter-communication between devices.

b). Half-duplex

Half-duplex allows communication in both directions but not at the same time. Signals travel in both directions over a medium but in one direction only at a time. Since signals travel in only one direction, a device can either send or receive data at a given time. A network card set to Half-duplex cannot receive data when it is sending data. To receive data, it needs to change the direction of data flow. To change direction, a special signal is used and acknowledged. The time required to turn over control to the other side is called the line turnaround time.

The half-duplex mode is used in cases where there is no need for communication in both directions at the same time. The entire capacity of the channel can be utilized for each direction. 

Railway tracks and walkie-talkies are examples of half-duplex. Only one train can run on a railway track at a time. If a train is on the track, the second train has to wait until the first train leaves the track.

c). Full-duplex

Full-duplex is also called two-way or bidirectional. It allows communication in both directions simultaneously. It divides the available channel into two parts and uses one part to send data and the other part to receive data. Since there is a separate path for sending and receiving data, a device can simultaneously perform both tasks at a given time.

In full-duplex mode, signals going in one direction share the capacity of the link with signals going in another direction, this sharing can occur in two ways: 

·       Either the link must contain two physically separate transmission paths, one for sending and the other for receiving.

·       Or the capacity is divided between signals traveling in both directions. 

Full-duplex mode is used when communication in both directions is required all the time. The capacity of the channel, however, must be divided between the two directions. 

Examples of full duplex:

-       Telephone Network in which there is communication between two persons by a telephone line, through which both can talk and listen at the same time.

-       A two-lane highway is an example of a full-duplex. A two-lane highway uses dedicated lanes for incoming and outgoing traffic.

Auto-sensing

A network interface card can operate in both half-duplex mode and full-duplex mode. All modern NICs run in full-duplex mode. Some older NICs only support half-duplex. Auto-sensing is a feature that allows a NIC to automatically detect whether the remote NIC supports full-duplex.

A summary of Simplex, Half duplex and full duplex communications: