COMMUNICATION SYSTEM: PRINCIPLE MODEL & TRANSMISSION MEDIA

 Introduction

Data communications are the exchange of data between two devices via some form of transmission medium such as a wire or air. For data communications to occur, the communicating devices must be part of a communication system made up of a combination of hardware and software. The effectiveness of a data communications system depends on four fundamental characteristics. These are:

1. Delivery: The system must deliver data to the correct receiver. Data must be received by the intended device or user only.

2. Accuracy: The system must deliver the data accurately. Data that have been altered in transmission must be corrected before delivery.

3. Timeliness: The system must deliver data in a timely manner. Data delivered late are useless. In the case of video and audio, timely delivery means delivering data as they are produced, in the same order that they are produced, and without significant delay.

4. Jitter: Jitter refers to the variation in the packet arrival time. It is the uneven delay in the delivery of audio or video packets. For example, let us assume that video packets are sent every 20 ns. If some of the packets arrive with 20ns delay and others with 300ns delay, then the quality of video will be uneven.

Data communication: The process of electronic transfer of information between two locations is known as data communication. A data communications system has five components. Figure shows the five components of data communication

1. Message. The message is the information (data) to be communicated. Popular forms of information include text, numbers, pictures, audio, and video.

2. Sender. The sender is the device that sends the data message. It can be a computer, workstation, telephone handset, video camera, and so on.

3. Receiver. The receiver is the device that receives the message. It can be a computer, workstation, telephone handset, television, and so on.

4. Transmission medium. The transmission medium is the physical path by which a message travels from sender to receiver. Some examples of transmission media include twisted-pair wire, coaxial cable, fiber-optic cable, and radio waves.

5. Protocol. A protocol is a set of rules that govern data communications. It represents an agreement between the communicating devices. Without a protocol, two devices may connected but not communicating, just as a person speaking Tamil cannot be understood by a person who speaks only Telugu.

Data Communication may occur in a simple point-to-point mode or in a multipoint mode where more than two computers are connected together in the form of a network. A network is a system of interconnected computers that can communicate with one another to share applications and data. 

 Modes of Transmission

There are three ways, or modes, of transmitting data from one point to another. These are simplex, half-duplex, and full-duplex.

1. Simplex: In simplex mode, the communication is unidirectional. Only one of the two devices on a link can transmit and the other can only receive. Keyboards and monitors are examples of simplex devices. The keyboard can only send input; the monitor can only receive output. The advantage of simplex mode is that the entire capacity of the channel can be used to send data in one direction.

2. Half-Duplex: In half-duplex mode, each station can both transmit and receive, but only one at a time. When one device is transmitting, the other can only accept, and vice versa. The half- duplex mode is like a one-lane road with traffic allowed in both directions. When cars are traveling in one direction, cars going the other way must wait. In a half-duplex transmission, the entire capacity of a channel is taken over by whichever of the two devices is transmitting at the time. Walkie-talkies and CB (citizens band) radios are both half-duplex systems. 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. 

3. Full-Duplex (Duplex): In full-duplex mode, both stations can send and receive simultaneously. The full-duplex mode is like a two-way street with traffic flowing in both directions at the same time. In full-duplex mode, signals going in one direction share the capacity of the link with the signals going in the other 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 of the channel is divided between signals traveling in both directions. One common example of full-duplex communication is the telephone network. When two people are communicating by a telephone line, both can talk and listen at the same time.

 Transmission Medium

A transmission medium can be broadly defined as anything that can carry information from a source to a destination

Generally, transmission media are of two types:

(i) Guided Transmission Medium: Guided transmission media are also called bounded media or wired media. They comprise of cables or wires through which data is transmitted between communicating devices. They are called guided since they provide a physical conduit from the sender device to the receiver device. The signals traveling through these media are bounded by the physical limits of the medium..

The most popular guided media are:

• Twisted pair cable

• Coaxial cable

• Fiber optics

(ii) Unguided Transmission Medium: Unguided transmission media are also called wireless media. They transport data in the form of electromagnetic waves that do not require any cables for transmission. These media are bounded by geographical boundaries.

Unguided signals can travel in three ways-

•  Ground propagation: In this, radio waves travel through the lowest portion of the atmosphere, hugging the Earth. These low-frequency signals emanate in all directions from the transmitting antenna and follow the curvature of the planet.

• Sky propagation: In this, higher-frequency radio waves radiate upward into the ionosphere where they are reflected back to Earth. This type of transmission allows for greater distances with lower output power.

• Line-of-sight propagation: In this type, very high-frequency signals are transmitted in straight lines directly from antenna to antenna.

The commonly used unguided transmissions are: 

• Radio transmission

• Microwave transmission

• Infrared transmission

1. Twisted Pair Cable: It consists of two separately insulated conductor wires twisted around each other to reduce interference by adjacent wires. They are the most widely used transmission media.

(i) Unshielded Twisted Pair (UTP): This type of cable has the ability to block interference and other than plastic insulation nothing else shields it from outside interference. It is used for telephonic applications.

Advantages:

•  Least expensive

• Easy to install

• Digital and Technological Solutions

• High speed capacity

• Susceptible to external interference

• Lower capacity and performance in comparison to STP

• Short distance transmission due to attenuation

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

Advantages:

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

• Eliminates crosstalk

• Comparatively faster

• Comparatively difficult to install and manufacture

• More expensive

• Bulky

• Plastic cover

• Metal shield

• Plastic cover

2. Coaxial Cable: Coaxial Cables are a group of wrapped and insulated wires capable of transmitting
data at higher rates. They consist of a central copper wire surrounded by a PVC insulation over which there is a sleeve of copper mesh. The copper mesh sleeve is shielded again by PVC material. Signal is transmitted by inner copper wire and is shielded by outer mesh. 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.



Advantages: 

• High Bandwidth

• Better noise Immunity

• Easy to install and expand 

• Inexpensive

• Long distance transmission

Disadvantages:

• Expensive than twisted pair cables.

• Not compatible with twisted wire cable.

• Coaxial cables experience signal leakage at the point of connection.

• Coaxial Cables are not suitable for long-distance transmissions due to significant data loss ove the extended distance.

3. Optical Fibre Cable: Optical fibers are long, thin strands of glass or plastic having diameter of a human hair. The optical fiber consists of three concentric elements, the core, the cladding and the outer coating, often called the buffer. The core is usually made up of glass or Coating. plastic. The core is the light-carrying portion of the fiber. The cladding surrounds the core. The cladding is made of a material with a slightly lower index of refraction than the core. This difference in the indices causes total internal reflection to occur at the core- cladding boundary along the length of the fiber. Light is transmitted down the fiber and does not escape through the sides of the fiber. The outer layer serves as a "shock absorber" to protect the core and cladding from damage. The coating usually comprises one or more coats of a plastic material to protect the fiber from the physical environment. Sometimes metallic sheaths are added to the coating for further physical protection. The cable can be unidirectional or bidirectional.

Advantages:

• Increased capacity and bandwidth

• Light weight

• Less signal attenuation

• Immunity to electromagnetic interference

• Resistance to corrosive materials

• Analog and digital transmissions

• Security from tampering
• 
  

Disadvantages:

• Difficult to install and maintain

• High cost

• Fragile

4. Radio Waves: The below figure shows the part of the electromagnetic spectrum, ranging from 3 kHz to 900 THz, used for wireless communication.

Electromagnetic waves ranging in frequencies between 3 KHz and 1 GHz are normally called radio waves.Radio waves are omnidirectional. When an antenna transmits radio waves, they are propagated in all directions. This means that the sending and receiving antennas do not have to be aligned. A sending antenna sends waves that can be received by any receiving antenna. The omnidirectional property has disadvantages, too. The radio waves transmitted by one antenna are susceptible to interference by another antenna that may send signals using the same frequency or band.

Applications of Radio Waves

(i) The omnidirectional characteristics of radio waves make them useful for multicasting in which there is one sender but many receivers.

(ii) AM and FM radio, maritime radio, cordless phones, and paging are the examples

5. Microwaves: Electromagnetic waves having frequencies between 1 and 300 GHz are called microwaves. Microwaves are unidirectional. When an antenna transmits microwaves, they can be narrowly focused. This means that the sending and receiving antennas need to be aligned. The unidirectional property has an obvious advantage. A pair of antennas can be aligned without interfering with another pair of aligned antennas. For increasing the distance served by terrestrial microwave, repeaters can be installed with each antenna.The signal received by an antenna can be converted into transmittable form and relayed to next antenna

The following describes some characteristics of microwaves propagation:

(i) Microwave propagation is line-of-sight. Since the towers with the mounted antennas need to be in direct sight of each other, towers that are far apart need to be very tall.

(ii) Very high-frequency microwaves cannot penetrate walls. This characteristic can be a disadvantage if receivers are inside the buildings.

(iii) The microwave band is relatively wide, almost 299 GHz. Therefore, wider sub-bands can be assigned and a high data rate is possible.

(iv) Use of certain portions of the band requires permission from authorities.

(v) They are used in cellular phones, satellite networks and wireless LANs. 

Disadvantages of Microwave Transmission

• It is very costly

6. Satellite Microwave : This is a microwave relay station which is placed in outer space. The satellites are launched either by rockets or space shuttles. These are positioned 36000 Km above the equator with an orbit speed that exactly matches the rotation speed of the earth. As the satellite is positioned in a geo-synchronous orbit, it is stationary relative to earth and always stays over the same point on the ground. This is usually done to allow ground stations to aim antennas at a fixed point in the sky.

 Features of Satellite Microwave

• Bandwidth capacity depends on the frequency used.

•  Satellite microwave deployment for orbiting satellites is difficult.

Advantages of Satellite Microwave

• Transmitting station can receive back its own transmission and check whether the satellite has transmitted information correctly.

• A single microwave relay station which is visible from any point.

Disadvantages of Satellite Microwave

• Satellite manufacturing cost is very high

• Cost of launching satellite is very expensive

• Transmission highly depends on whether conditions, it can go down in bad weather 

7. Infrared Waves: Infrared waves, with frequencies from 300 GHz to 400 THz, can be used for short-range communication. Infrared waves, having high frequencies, cannot penetrate walls. This advantageous characteristic prevents interference between one system and another, a short-range communication system in one room cannot be affected by another system in the next room.

When we use infrared remote control, we do not interfere with the use of the remote by our neighbours. However, this same characteristic makes infrared signals useless for long-range communication. In addition, we cannot use infrared waves outside a building because the sun's rays contain infrared waves that can interfere with the communication.

Applications of Infrared Waves

The infrared band, almost 400 THz, has an excellent potential for data transmission. Such a wide bandwidth can be used to transmit digital data with a very high data rate.

Advantages

• Higher bandwidth means superior throughput to radio

• Inexpensive to produce

• No longer limited to tight interroom line-of-sight restrictions

Disadvantage

•  Limited in distance

•  Cannot penetrate physical barriers like walls, ceilings, floors, etc.

COMPUTER NETWORKS

A computer network is a set of devices (often referred to as nodes) connected by transmission media. A node can be a computer, printer, scanner, fax machine, switch, router or any other device capable of sending and/or receiving data generated by other nodes on the network.

Uses of networking

Computer network is a system of interconnected computers that enable the computers to communicate with each other and share their resources, data and applications. Some of the main uses of networking are as follows:

1. Information and Resource Sharing: Computer networks allow organisations having units which are placed apart from each other, to share information in a very effective manner. Programs and software in any computer can be accessed by other computers linked to the network. It also allows sharing of hardware equipment, like printers and scanners among varied users.

2. Retrieving Remote Information: The information is stored in remote databases to which the user gains access through information systems like the World Wide Web

3. Interpersonal Communication: A computer network facilitates interpersonal communications allowing users to communicate efficiently and easily via various means: email, instant messaging, chat rooms, telephone, video telephone calls, and video conferencing that has completely revolutionized the teaching learning process.

4. E-Commerce: Computer networks have paved the way for a variety of business and commercial transactions online, popularly called e-commerce. Users and organisations can pool funds, buy or sell items, pay bills, manage bank accounts, pay taxes, transfer funds and handle investments electronically.

5. Highly Reliable Systems: Computer networks allow systems to be distributed in nature, by the virtue of which data is stored in multiple sources. This makes the system highly reliable. If a failure occurs in one source, then the system will still continue to function and data will still be available from the other sources.

6. Cost-Effective Systems: Computer networks have reduced the cost of establishment of computer systems in organisations. Previously, it was imperative for organisations to set up expensive mainframes for computation and storage. With the advent of networks, it is sufficient to set up interconnected personal computers (PCs) for the same purpose. Users may access and use resources provided by devices on the network, such as printing a document on a shared network printer also reduces the overall setup cost.

7. VoIP : VoIP or Voice over Internet protocol has revolutionized telecommunication systems. Through this, telephone calls are made digitally using Internet Protocols instead of the regular analog phone lines.

Types of Network based on size

The types of network are classified based upon the size, the area it covers and its physical architecture. The three primary network categories are LAN, WAN and MAN. Each network differs in their characteristics such as distance, transmission speed, cables and cost.

1. LAN (Local Area Network) is a group of interconnected computers within a small area (room, building, campus). Two or more pc's can from a LAN to share files, folders, printers, applications and other devices. Coaxial cables are normally used for connections. Due to short distances, errors and noise are minimum. Data transfer rate is 10 to 100 mbps. Example: A computer lab in a school.

2. MAN (Metropolitan Area Network) is a design to extend over a large area. It connects number of LANs to form larger network, so that resources can be shared. Networks can be up to 5 to 50 km. Owned by organization or individual. Data transfer rate is low compare to LAN. Example: Organization with different branches located in the city.

3. WAN (Wide Area Network) is a country and worldwide network. It contains multiple LANs and MANs. It is distinguished in terms of geographical range. The network uses satellites and microwave relays as transmission medium. Data transfer rate depends upon the ISP provider and varies over the location. Best example is the internet.

Other types

WLAN (Wireless LAN) A LAN that uses high frequency radio waves for communication. It provides short range connectivity with high speed data transmission.

PAN (Personal Area Network) is a network organized by the individual user for its personal use.

 SAN (Storage Area Network) connects servers to data storage devices via fiber-optic cables.