Module 5. Principles of communication

Lesson 13

COMMUNICATION SYSTEMS

13.1  Introduction

This lesson will discuss various communication systems used for communication purpose and their applications. These topics will be useful to the students for selecting communication mediums based on their characteristics and systems used for various purposes.

13.2 Communication Systems

Basically communication systems are for smooth flow of information between two or more parties. These may be categorized based on the primary medium used for data transmission. Communication systems may also be classified as one-way, two-way, or multiple-way systems, depending on how many parties can exchange information through its various components. A few important communication systems are described as follows:

13.2.1 Radio communication system

In the radio communication system, information is transmitted with the help of a radio. Radio is a small part of the electromagnetic spectrum ranging from 3 KHz to 300GHz but in radio communication system waves ranging from 3 KHz to 300MHz  (i.e. VLF, LF, MF, HF and VHF bands) are used because wave above 100MHz behave in different manner therefore, require different technology for transmission. In radio communication, signals are transmitted by modulation of electromagnetic waves in this range. Radio communication system works with the help of a transmitter and a receiver both equipped with an antenna. On one end of these radio systems is a transmitter that will take the information and electronically convert it into radio waves. These radio waves travel to the other end of the radio communication system, which is designed to detect and decode the waves and convert them to recognizable information.

Radio waves can travel for long distances along earth curvature and can penetrate buildings easily, so they are widely used for communication, both indoors and outdoors. Radio waves are Omni directional, meaning that they travel in all directions from the source, so that the transmitter and receiver do not have to be carefully aligned physically. The properties of radio waves are frequency dependent. At low frequencies, radio waves pass through obstacles well, but the power falls off sharply with distance from the source. At high frequencies, radio waves tend to travel in straight lines and bounce off obstacles. They are also absorbed by rain. At all frequencies, radio waves are subject to interference from motors and other electrical equipment. Radio system is popular for wireless data services and is being widely used for Audible Radio through transistors and radio instruments at home; Wireless local area networks; Mobile phone communication etc.

Advantages:

Disadvantages:

·         Easy to generate

·         Can travel long distances,

·         Can penetrate buildings

·         Omini Directional

·         No proper alignment of transmitter and receiver is required

·         Range is limited

·         Not very reliable

·         Prone to electrical interference

 

13.2.2 Power line communication systems

These systems are used to transmit signals electronically from a source to destinations. A type of electronic system that often is referred to is cable television, widely known for its transmission of a video channels to houses in addition to their use for providing Internet access. Coaxial cable is generally used in cable TV network. Power line communication systems are often used because of their relatively low cost, even though there are other systems which are better in quality and efficiency. Low bandwidth is main limitation of this system.

13.2.3 Optical communication system

Optical communication system has revolutionized the wire-line communication system and offers many improvements over other type of systems. This system uses light as the medium of communication. These signals are faster, clearer, and more reliable than electrical or radio signals. These signals travel through optical fiber cables. Data/ information from source device is converted into an optical signal and reaches to the recipient. The receiver then decodes the signal and responds accordingly. Optical communication is being used extensively in data communication in Local Area Network, Public and Private communication networks (WAN, MAN) for providing backbone of the network etc. It is also being used in helicopters and aircraft for safe landing. The pilots receive light signals from the base and decide their next movements. Visible lights (Red, Yellow, Green) are used to control traffic on roads.

13.2.4 Terrestrial microwave communication system

Electromagnetic waves above 100MHz (i.e. microwaves) do not follow the curvature of earth and therefore travel in a straight line through air. Because of Earth’s curvature and the microwave transmission being in a straight line, this system requires relay stations (repeaters) with a dish antenna at suitable distance for the reception, amplification and transmission of data to increase the distance served by terrestrial microwave communication system. Transmitting and receiving antennas must be accurately aligned with each other. The higher the towers are, the further apart relay stations can be. The distance between relay stations goes up very roughly with the square root of the tower height. For 100-m high towers, repeaters can be spaced 80km apart. Terrestrial microwave with repeaters provides the basis for telephone systems worldwide.

Unlike radio waves at lower frequencies, microwaves do not pass through buildings well. In addition, even though the beam may be well focused at the transmitter, there is still some divergence in space. Some waves may be refracted off low-lying atmospheric layers and may take slightly longer to arrive than direct waves.

Microwave signals propagate in one direction at a time, which means that two frequencies are necessary for two-way communication such as a telephone conversation. One frequency is reserved for transmission in one direction and the other for transmission in the other. Each frequency requires its own transmitter and receiver. Today, both pieces of equipment usually are combined in a single piece of equipment called a transceiver, which allows a single antenna to serve both frequencies and functions.

Advantages:

Disadvantages:

·         Less expensive to install than cables for moderate distances.

·         Offers high data rates in comparison to Radio system

·         Require little or no maintenance

·         Easy to install

·         Limited by the curvature of the earth

·         LOS is subject to environmental and atmospheric conditions.

·         Noise problem

·         Prone to electrical interference

 

13.2.5 Satellite communication system

Satellite communication is another mode of data communication system for line-of-sight microwave transmission. It uses an artificial satellite positioned in the space to facilitate the communication among the various points on the earth. SHF and EHF microwaves are generally used for satellite communication since these waves can travel at faster speed in straight line and can penetrate into the space crossing ionosphere.  The principle is same as in terrestrial microwave communication with a satellite acting as a super tall antenna and repeater. Satellite communication allows a microwave signal to covers a large span of the earth with a single bounce. Satellite contains several transponders, each of which listens to some portion of the spectrum, amplifies the incoming signal, and then rebroadcasts it at another frequency, to avoid interference with the incoming signal. Satellite majorly works on the solar power which is continuously received by the satellite’s solar panels.

There are two components in satellite communication namely Ground station or the Earth base station and space component (satellite). A signal is transmitted from earth base station towards satellite stationed in space. Satellite receives signal with the help of antenna. Received signals are amplified to an optimum level and then with the help of transponders they are retransmitted back to the earth. The Earth station then receives the signal from the satellite, and re-amplifies it and helps in the communication. Transmission of signals from Earth station to satellite is called uplink and transmission from satellite to the Earth station is called down link. Common frequency bands used for satellite communication are as follows (table-13.1):

Table 13.1 Satellite frequency bands

Band

Downlink (GHz)

Uplink (GHz)

C

3.7 - 4.2

5.925 – 6.425

Ku

11.7 – 12.2

14.0 – 14.5

Ka

17.7 – 21.7

27.5 – 31.0

Q

36

46

V

46

56

 

Different type of satellite systems are being used for different purpose and requirement of connectivity for communication. Some important types of satellite system are described as given below:

13.2.5.1 Geosynchronous satellites (or Fixed satellite system)

Satellites that are placed in the Earth’s orbit and move with the same speed of Earth to provide continuous communication at all time during 24hours in a day are called geosynchronous satellites. These satellites seem to remain fixed above a certain spot. One satellite in orbit has line-of-sight contact with a vast number of Earth stations, but the curvature of the Earth still keeps much of the planet out of sight. Therefore, one geosynchronous satellite cannot cover the whole earth. It takes a minimum of three satellites equidistant from each other in geosynchronous orbit to provide full global transmission. The fixed satellite system helps in the transfer of numerous data and information across the countries through fixed point on the earth’s surface.

13.2.5.2 Mobile satellite system

It is helpful in connecting ships, aircrafts at distant and remote places.

13.2.5.3 Research satellite system

Research Satellite System is primarily helpful in various research processes for the scientists. The scientists can gather all the necessary and useful data through the research satellite system.

13.2.5.4 VSAT

It is a low-cost micro station, sometime called VSATs (Very Small Aperture Terminals). These tiny terminals have 1-meter antenna and can put out about 1 watt of power. In many VSAT systems, the micro stations do not have enough power to communicate directly with one another (via the satellite). Instead, a special ground station, the hub with a large, high-gain antenna is needed to relay traffic between VSATs.

13.2.5.5 Applications

·         Television Distribution

·         Long distance Telephone Transmission

·         Private Business Networks

Advantages:

Disadvantages:

·         Can cover large area of the earth

·         Cost is independent of distance covered

·         Mobility can be easily achieved

·         Ideal for TV broadcast

·         Very expensive technology

·         Less secure

·         Vulnerable to interference

·         Effected by atmospheric conditions.

 

13.2.6 Infrared communication system

Infrared communication systems use the infrared (IR) electromagnetic waves (300GHz-200THz) for communication. IR behaves in different ways therefore require different technology to use IR in communication. Unguided infrared and millimeter waves are widely used for short-range communication among computer peripherals. IR enabled devices (like remote controls, printers, PDAs, etc.) use infrared light-emitting diodes (LEDs) to emit infrared radiation which is focused by a plastic lens into a narrow beam. The beam is modulated, i.e. switched on and off, to encode the data. The receiver uses a silicon photodiode to convert the infrared radiation to an electric current. It responds only to the rapidly pulsing signal created by the transmitter, and filters out slowly changing infrared radiation from ambient light. Infrared communications are useful for indoor use in areas of high population density. Infrared is the most common way for remote controls to command appliances. Infrared remote control protocols like RC-5, SIRC, are used to communicate with infrared.

They are relatively directional, cheap and easy to build, but have a major drawback: they do not pass through solid objects. An advantage of this is that an infrared system in one room of a building will not interface with a similar system in adjacent rooms. Security of infrared systems against eaves dropping is better than radio systems. No government license is needed to operate an infrared system, in contrast to radio systems.

13.2.6.1 Applications

The remote controls used on televisions, VCRs and stereos all use infrared communication. Infrared imaging is used extensively for military and civilian purposes. Military applications include target acquisition, surveillance, night vision, homing and tracking. Non-military uses include thermal efficiency analysis, remote temperature sensing, short-ranged wireless communication, spectroscopy, and weather forecasting

IR can be used in indoor wireless LANs. The computers and offices in a building can be equipped with relatively unfocused (somewhat omni directional) infrared transmitters and receivers. In this way, portable computers with infrared capability can be on the local LAN without having to physically connect to it. When several people show up for a meeting with their portables, they can just sit down in the conference room and be fully connected without having to plug in. Infrared communication cannot be used outdoors because the sun shines as brightly in the infrared as in the visible spectrum.

13.3 Computer Networks

A Computer Network may be defined as an interconnected collection of autonomous computers. Two computers are said to be interconnected if they are able to exchange information. The connections may be through copper wires, optical fibers, and wireless electromagnetic or optical media. Autonomous means that there is no master/slave relationship between the connected devices.  Typically, each device in a network serves a specific purpose for one or more individuals. For example, a PC can provide access to information or software. On the other hand another PC may be a file server devoted to managing a disk drive containing shared files.

13.3.1 Network relationship types

This term refers to different concepts about how one computer makes use of another computer’s resources over network. There are two types of network relationship as described below:

·    Peer-to-pear network relationship: In this relationship, each device or computer on the network communicates with each other as equals and channel is also shared equally. Each computer is responsible for making its resources available to other computers on the network.

·    Client/Server network relationship: In this relationship a distinction is made between the computers on the network. A computer is made responsible for providing services and is called as a server while other computers use these services and called as clients.

13.3.2 Categories of networks

The category of a network is determined by its size, its ownership, the distance it covers, and its physical architecture. A computer network is generally divided into three categories:

·         Local area network (LAN): It may cover a small geographic area connecting devices in a single building or group of buildings.

·         Metropolitan area network (MAN): It covers medium size area such as a city.

·         Wide area network (WAN): It covers a large area such as a state, country or the world

13.3.3 Internetworks and intranet

A collection of interconnected networks is called an internetwork or just internet. The term internet (lower i) should not be confused with Internet (uppercase I). The first is generic term used to mean an interconnection of networks. The second is the name of a specific worldwide network. Intranet is a closed network set up for exclusive use and managed by an organization.

13.3.4 Goals of computer networks

Network goals can be summarized in terms of the uses of networks for companies, organizations, people etc. These uses can be viewed as the facilities provided by computer networks. Some of the goals or objectives can be summarized as:

I.                   Resource sharing:  Goal is to make all programs, data, and equipment available to anyone on the network without regard to the physical location of the resource and the user. This provides a high availability of resources to users.

II.                Load sharing: This is another aspect of resource sharing. Sharing load between multiple computers connected together can reduce the delays for carrying out time intensive applications.

III.             High reliability: High reliability can be achieved due to alternative sources of supply. For example, all files could be replicated on two or three machines. So, if one of them is unavailable (due to a hardware failure), the other copies could be used. In addition, the presence of multiple CPUs means that if one goes down the others may be able to take over its work.

IV.             Cost effectiveness: Small computers have a much better price/performance ratio than large ones. Therefore linking of small size computer gives high performance just equivalent to large systems.

V.                Scalability: A closely related point is the ability to increase system performance gradually as the workload increases just by adding more PCs.

VI.             Powerful communication medium: A real time communication can be possible between two persons sitting on-line and far apart (distant geographical locations).

13.3.5 Local area networks

Generally called LANs, are privately - owned networks within a single office single building or campus of up to a few kilometers in size. They are widely used to connect personal computers and workstations in company offices and factories to share resources (e.g., printers) and exchange information. LANs with their emphasis on low cost and simplicity have been based on the broadcast approach. LANs are distinguished from other kinds of networks by three characteristics: (1) their size, (2) their transmission technology. (3) their topology.

13.3.5.1 Size 

LANs are restricted in size, which means that the worst-case transmission time is bounded and known in advance.

13.3.5.2 Transmission Technology

LANs often use a transmission technology consisting of a single cable to which all the machines are attached. Traditional LANs run at speeds of 10 to 100 Mbps, have low delay (tens of microseconds), and make very few errors. Newer LANs may operate at higher speeds, up to hundreds of megabits sec.

13.3.5.3 Topology

Topology of a network is the geometric representation of the relationship (physical and logical) of all links and linking devices to each other. Topologies used in LAN are:

·    Star: Each device has a dedicated point-to-point link only to a central controller called a hub or switch. The devices are not directly linked to each other. It is less expensive (less cables required), easy to install and reconfigure and robust. Link failure of one device will isolate that device from the network. The whole network will be down only when central device (switch/ hub) is faulty. This topology is scalable but not infinitely. 

·    Tree: It is a variation of star topology. The devices are connected to a hub and that hub is further connected to another hub or a central hub. Thus it makes a tree like shape. The advantages and disadvantages are same as star topology. As the tree size, it will adversely affect throughput of the network.

·    Bus: All the devices in the network are connected to a single long cable that acts as a backbone of the network. A bus topology is a kind of broadcast network i.e. multipoint line configuration. It is easy to install but difficult to reconfigure and fault isolation. Scalability is also an issue as only limited number of devices can be attached to one patch of cable. If there is fault in the backbone then whole network will not be functional. 

·    Ring: Each device is connected with the two devices on either side of it through a dedicated point-to-point link. Data is transferred from one computer to another in sequential order. It is easy to install and reconfigure. Each device is linked to its immediate neighbors therefore addition and removal of computers is easy from the network. The physical placement of devices is immaterial. Device can be placed in rectangular, triangular or in any shape. If one link is breakdown than network will not be in working order.

These topologies are graphically depicted in the figure 13.1 given below:

Fig. 13.1 LAN topologies