COMMUNICATION NETWORK

COMMUNICATION NETWORK

A communication network, consists of a set of equipment and facilities that provide a service, the transfer of information between users located at various geographical points. For example telephone networks provides telephone service, computer networks, television broadcast networks, cellular networks and Internet.

Communication network and transportation networks have become essential infrastructure in every society.

The capacity to transfer high volume of data for a long distance almost immediately are the features of network-based services. For example Internet provides e-mail, information search and retrieval, various forms of electronic service.

EVOLUTION OF NETWORK ARCHITECTURE AND SERVICES

A communication service which is used of transfer of information. Different services differ in the details of how and in what form information is transferred.

Let us consider three example networks
i) Telegraph networks
ii) Telephone networks
iii) Computer networks

TELEGRAPH NETWORKS AND MESSAGE SWITCHING

In the year 1987 Samuel B. Morse demonstrated a practical telegraph that provided the basis for telegram service, the transmission of text message over long distance. Here the text was encoded into sequence of dots and dashes. Each dot and dash which is used to be represented by short and long pulses of electrical current over a copper wire. By relying on two signals, telegraphy made use of digital transmission system. In Morse code the pause between letter was 3 dots and for word is 5 dots.

In store-and-forward process, the message is completely received and stored at an intermediate point and then forward to the next node depending on the availability of link. The process of selecting an appropriate link is known as routing. If the information is message, then it is known as message switching. In simpler terms, message switching is a switching method that sends data from point-to-point with each intermediate node storing the data , waiting for a free transmission channel and forwarding the data to the next point until the destination is reached.

In the beginning communication channel used to carry information sent by one person. Naturally transmission rate (in bits/second) was less. To increase the transmission rate multiplexing was developed. Multiplexing is a technique used to place multiple signals on a single communication channel. Multiplexing partitions a channel into many separate channels, each capable of transmitting its own independent signal, thereby enabling many different transmission over a single medium.

One approach of multiplexing involves modulation. Binary symbols can be transmitted by sending a sinusoidal signal of a given frequency for a given period of time. Multiple sequence of binary symbols could be transmitted simultaneously by using multiple pairs of frequencies for the different telegraphy signals. This concept is used in Modems.

We know characters can be represented by ASCII ( American Standard Code for Information Interchange) , is a 7 bit code. The equipments can communicate with ASCII code.

The telegraph service involves the transmission of text messages between geographically far places. To provide the service , the architecture of the telegraph network contains the following key elements or functions.

i) Digital transmission takes in the network that is information is represented either 0 or 1. It can make use of ASCII code also. The transmission medium can be copper wire and radio.
ii) To identify the messages exactly, messages are covered by beginning and ending by sequence of characters. This is known as framing.
iii) There should be destination address that is to whom it want to deliver. Routing procedure determines the path that a message follows across a network of telegraph stations interconnected by digital lines

TELEPHONE NETWORKS AND CIRCUIT SWITCHING

In 1876 Alexander Graham Bell developed a device that can transmit voice signals. The device that is known as Telephone. The telephone network provides a two-way, real-time transmission of voice signals across a network.

Telephone service became popular due to its voice service and expert operator with knowledge of Morse code is not required like in the case of telegraph. Here voice signals are converted into equivalent electrical signal and passed through cable.

COMMUNICATION FUNDAMENTALS

COMMUNICATION FUNDAMENTALS

Communication concepts

Communication is the process of transmitting information. It’s a big, vague, fuzzy word and broadly applied. The “Communications Department “ at a university may include speech teachers, public relations specialists, writers and broadcasters, who have nothing to do with hardware. By that definition , a writer is in the communication business.

Evolution of communications

We the human beings are communicating from early days that is by drawing the pictures on cave walls. It can be how to attack the animal or how to the save the life from the threats or it may be a route to go to another cave.

The earlier long-distance communications was by signal fires that relayed simple information. The signal used to represent “yes” or “no” – the question being asked had to be known beforehand. It’s only a single bit information, not a detailed message.

A written letter or a human messenger could carry more information. The first systems we might call telecommunications were series of hilltop towers, built by French engineer Claude Chappe in the 1790s. The towers had to be in sight of each other, with an operator in each one. The operator relayed a message by moving arms on top of the tower. The operator of the next tower looked through a telescope to watch the arms move, reading the message, recording it, then moving the arms of his tower to relay it to the next tower. Chappe invented a code that used arm position to indicate letters or certain common words. His system was called an optical telegraph.

Samuel Morse’s electrical telegraph eventually replaced Chappe’s optical one. It also required operators, but they used keys that completed an electrical circuit, sending bursts of currents through a wire. The Morse code was a series of dots an dashes ( short and long pulses) that represented letters.

The electrical telegraph spread across the continents and in 1866 across the Atlantic. Its wires formed a network running between major cities. Telegraphers received signals and either sent them for local delivery or relayed them to more distant stations. We can think of each dot and dash of the telegraph as a bit of information.

Later Alexander Graham Bell developed telephone. He borrowed some principles from the telegraph , but instead of transmitting dots and dashes, it sent a continuously changing electrical current that represented a speaker’s voice. The telephone network was bigger and more complex than the telegraph network.

After that radio waves are used to carry signals through air. Radio first carried telegraph signals. Wires worked fine on land, but only radio could send telegrams to ships at sea and relay urgent messages such as request for help. It was radio rescue calls that sent ships to help the survivors of the sinking of the Titanic. Radio communications started for ships and airplanes in the world war II.

Radio communication started at low frequencies, but gradually moved to higher ad higher frequencies as electronics improved. The higher the frequency the more information the signal can carry. Pictures need more transmission capacity than sound alone, so television are broadcast at higher frequencies than audio radio.

By the 1970s , satellites were beaming radio signals around the globe. The telephone system had become global and known popularly “telecommunication network” . We could make phone calls to anywhere in the world.

After that fiber optics arrived. Fiber provides high bandwidth. It carries high capacity information at faster rate. Now it is a integral part of telecommunication network.

Protocols and standards

Protocols and standards

The Communication occurs between entities in different systems. An entity is anything capable of sending or receiving information. But two entities cannot just send bit streams to each other and expect to be understood. For communication to occur the entities must agree on a protocol. The key elements of a protocol are syntax, semantics and timing.

Syntax : Syntax refers to the structure or format of the data, meaning the order in which they are presented. For example , a simple protocol might expect the first eight bits of data to be the address of the sender , the second eight bits to be the address of receiver and rest of the stream to be the message itself.

Semantics : Semantics refers to the meaning of each section of bits. How is a particular pattern to be interpreted and what action is to be taken based on that information.

Timing : Timing refers to two characteristics. When data should be sent and how fast it can be sent. For example , if a sender produces data at 100 Mbps but the receiver can process data at only 1 Mbps, the transmission will overload and receiver and data will be largely lost.

Standards

Standards are essential in creating and maintaining an open and competitive market fro equipment manufacturers and in guaranteeing national and international operational ability of data and telecommunication technology and processes. They provide guidelines to manufacturers, vendors, government agencies and other service providers to ensure the king of interconnectivity necessary in today’s marketplace and in international communications.

Data communications standards fall into two categories

1. de facto ( by fact)
2. de jure ( by law)

De jure standards are those that have been legislated by an officially recognized body.

De facto standards are often established originally by manufacturers seeking the functionality of a new product or technology.

Standards organization

Standards are developed by cooperation among standard creation committees forums and government regulatory agencies.

i) The International Standards Organization (ISO)
ii) The International Telecommunication Union (ITU-T)
iii) ANSI (American National Standards Institute)
iv) The Institute of Electrical and Electronics Engineer (IEEE)
v) The Electronic Industries Association (EIA)

The ISO is an organization dedicated to worldwide agreement on international standards in variety of fields. ITU-T is an international standards organization related to the United Nations that develops standards for telecommunications. Two popular standards developed by ITU-T are the V series and X series. ANSI , a nonprofit organization is the United States, voting representative to both the ISO and ITU-T. The IEEE is the largest national professional group involved in developing standards for computing , communication, electrical engineering and electronics. It sponsored an important standard for Local Area Network (LAN) called project 802. EIA is an association of electronics manufacturers in the United States. It is responsible for developing the EIA-232-D and EIA-530 standards.

System Functions

System Functions

Communications systems serve many functions. It is possible to divide into four categories on functions.
a) Point-to-point
b) Broadcasting
c) Switched
d) Networked

Point-to-point

A point-to-point configuration provides a dedicated link between two devices. The entire capacity of the channel is reserved for transmission between those two devices. Most point-to-point line configuration use an actual length of wire or cable to connect the two ends.

Broadcasting

A broadcast system send the same signal to everyone who receives it. In its usual form , transmission is one way, from the signal source to the individual. Local radio and television transmission are good examples of pure one-way broadcast systems.

Switched systems

A switched system makes temporary connections between terminals so they can exchange information. The telephone is a good example.

Networking

A network connects many terminals that can communicate with each other. For example Local Area Network is belongs to this category.

Services

Services

We know the operation of modern communication networks is a very complex process that involves the interaction of many systems. A communication network , is a set of equipment and facilities that provides a service, the transfer of information between users located at various geographical points. The ability of communication networks to transfer communication at extremely high speeds, allows users to gather information in large volumes almost instantly with the help of computers for long distances.

Radio and television broadcasting are probably the most common communication services. Various stations transmit an ensemble of signals simultaneously over radio or cable distribution networks.

Telephone service is the most common real-time service provided by a network. Two people are able to communicate by transmitting their voices across the network. The service is considered as connection-oriented in the sense that users must first interact with the network to set up a connection

Telephone networks provide a broad class of call management services that use the originating number or the destination number to determine the handling of a call. For example , in call return the last originating number is retained to allow it to be automatically called by the destination user at a later point in time. Caller ID allows the originating number and sometimes name, of the originating call to be displayed to the destination user when the receiving device is display capable. Voice mail allows a destination user to have calls forwarded to a message receiving device when the destination user is not available.

Cellular telephone service extends the normal telephone service to mobile users who are free to move within a regional area covered by an interconnected array of smaller geographical areas called cells. Each cell has a radio transmission system that allows it to communicate with users in its area. Cellular system should handle “handoff” when user moves from one area to another area.

Electronic mail (e-mail) is another popular network service. The user provides a text message and a name and/or address to a mail application. The application interacts with a local mail server, which in turn transmits the message to a destination server across a computer network. The destination user retrieves the message by using a mail application. E-mail is not a real-time service , where large delays can be tolerated. Here security and privacy is matter of concern.

Many applications that involve an interaction between processes running in two computers may be characterized by client/server interaction. Internet is an example of distributed system. The World Wide Web (WWW) is accessed through a browser program that displays the document and allows the user to access other documents by clicking the links. Each link provides the browser with a uniform resource locator (URL) that specifies the name of the machine where the document is located as well as the name of the file that contains the requested document.

Video on demand characterization another type of interactive service. The objective of the service is to provide access to a video library. The user initiates the service by accessing a menu from which a selection is made. Video which requires lot of bandwidth to pass from source computer to the destination.

Streamed audiovisual services over the internet provide an example of a service with some of the feature of video on demand. An application like “real player “ can be used to access a channel that provides an audio visual stream to the client machine.

Another class of service is audio-visual conferencing. Here both audio and video are transmitted over a distance. This service requires real-time response requirement. This service is being used by many companies for training, client interaction from a site, group discussion etc.

modern-communication-networks

Computing environments

Computing environments

Introduction

We know, in information technology era, sharing of resources and easy communication are acting as a backbone of any network. Popular example is our Internet. With the help of internet we are able to exchange information and share resources. Hence in this chapter we study different advantages of the network and different forms of the network and their features.

The learning objectives of this chapter are to know
What is a computer network ?
How network is advantageous ?
What are different forms of networks ?
What is a e-mail ?

Networking of computers and its advantages.

Computer network is defined as an interconnection of autonomous computer. Here autonomous means, there is no master and slave relationship. All computers are equal. Computer network enables to share the resources. Computer networking also refers to connecting computers to share data, application software and hardware devices. Networks allow sharing of information among various computers and permit users to share files. For example a students accesses compilers sitting at one place, where compiler may be stored on the other machine. The students takes printout with the help of one printer connected to the network. The printer can be shared among many students.

Network offers the opportunity to communicate more efficiently with others through electronic mail. Networks allow companies to share software and peripherals such as printers, plotters, scanners and so on. With networking all the computers in an office can be connected to a single printer and scanner. It also helps in using storage devices efficiently.

Computer network acts as a very powerful communication medium. It means people exchange their information. When compared to mainframe computers, network of computers saves money.

Types of networks

Depending the nature of the distances, protocols ( the set of rules used for communication) the network can be classified into LAN (Local Area Network ), MAN (Metropolitan Area Network ) and WAN ( Wide Area Network).

Local Area Network (LAN)
A LAN (local area network) is a group of computers and network devices connected together, usually within the same building. By definition, the connections must be high-speed and relatively inexpensive (e.g., token ring or Ethernet). They function to link computers together and provide shared access to printers, file servers, and other services.

Any individual computer connected to a network is called workstation. A workstation may not need a floppy disk or hard disk. A LAN or local area network connects computers and peripherals in a limited area. LAN requires cables to connect workstations. For example LAN is used in a hall or within a building. Figure 5.1 shows Local Area Network , where various departments are connected.

Metropolitan Area Network (MAN)

A MAN ( Metropolitan Area Network) is used to connect computers to cover the city or town. The range may be approximately 50 Kilometers. Normally cables and fiber optic cables are used to connect computers. The routing of the messages are fast. Normally central library in a city may be connected by a MAN, so users can access the information. Figure 5.2 shows a typical view of Metropolitan Area Network.

Wide Area Network (WAN)
A WAN (Wide Area Network) covers large distance like state, country or continents. The WAN uses the fiber optics, cables and even satellites also. Here communication circuits are connected with the help of hardware device called routers. Routers forward small pieces of information called packets from one to another. Internet is the popular one comes under WAN. Some of the examples makes use of internet are reservation of airplane tickets, railway tickets and even cinema tickets. Another facility called e-commerce, where business is carried out through internet. Here people can buy books, articles and so on through registering their wants through the internet. Figure 5.3 shows a typical view of Wide Area Network.

Internet
The internet is a massive wide area network ,connecting thousands of computer networks around the world. The internet is a world wide “network of networks”. It is a collection of thousands of smaller networks in different countries around the world. It links thousands of academic government, military and public computers, enabling millions of people to share information and other resources.

Internet pathways are used to exchange digitized computer data. The basic services that form the foundation of the internet are e-mail , telnet and FTP. With internet we can easily exchange electronic mail with friends and family anywhere in the world.

Telnet allows you to connect to a remote computer. We can access any of the public services or tools and library databases at the remote site. FTP(File Transfer Protocol) provides for transferring files from one computer to another across the internet.

Internet has many uses. For individuals, the most important uses of internet are e-mail and surfing the Web. One can read the topics of interest like sports, a hobby, a country or any place of interest.

E-mail:
The e-mail stands for electronic mail. One of the major features of computer networking is that messages can be sent electronically to various terminals on the network. The messages are sent very quickly and accurately. E-mail uses the concept of Storing and forwarding messages. It saves a lot of money for the users. Here user registers his/her account with one of the providers. The e-mail account normally contains username and the provider name. For example abc @ yahoo.com , represents abc is the name of the user , who is registered in yahoo provider.

Summary:
• Computer network is defined as a interconnection of autonomous computers.
• Computer networking enables us to connect computers to share data, application software and hardware devices.
• Networks allow sharing of information among various computers and permit users to share files.
• A LAN or local area network connects computers and peripherals in a limited area.
• A MAN covers the city or town.
• A Wide Area Network covers a large area.
• The internet is a massive wide area network, connecting thousands of computer networks around the world.
• E-mail is a cost effective communication

Quiz questions

1. Computer network defined as a interconnection of ———- computers
2. Computer network allows user to —– the resources
3. ——– network confined to hall or building.
4. ——- is one example of WAN.
5. —— business is carried our through internet.
6. Small piece of information called —- are forwarded by router in Internet.

Answers
1. Autonomous
2. share
3. Local area
4. Internet
5. E-commerce
6. packets

Applications and Layered architecture

Applications and Layered architecture

Communication network must support wide range of services. Normally people use networks to communicate, send e-mails, transfer of files and so on. Industry people use communication network for transfer of funds, update information about the product and so on. Hence, to provide support for current service and future services, a complete plan is required. This necessitates developing a complete flexibility in network architecture.

communications functions are grouped into the following tasks
• The transport across a network of data from a process in one machine to the process at another machine.
• The routing and forwarding of packets across multiple hops in a network
• The transfer of a frame of data from one physical interface to another.

To reduce their design complexity, most networks are organized as a series of layers or levels, each one built upon its predecessor. The number of layers, the name of the each layer, the contents of each layer and the function of each layer differ from network to network.

Interaction between the layers must be defined precisely. Interaction is done with definition of the service provided by each layer and to the layer above. Interface between layers through which a service is requested and through which results are conveyed. New services that build on existing services can be introduced even at the later stage. The layered approach accommodates incremental changes readily.


We know , in all networks, the purpose of each layer is to offer certain services to the higher layers. The entities comprising the corresponding layers on different machines are called peer processes. Between each pair of adjacent layers there is an interface. The interface defines which primitive operations and services the lower layer offers to the upper one. The set of layers and protocols is called the network architecture.

A protocol is a set of rules that governs how two or more communicating devices are to interact. HTTP protocol enables retrieval of web pages and TCP protocol enables the reliable transfer of streams of information between computers.

HTTP

Let us consider a client/server architecture, a server process in a computer waits for incoming requests by listening to a port. Port is an address that identifies which process is to receive a message that is delivered to a given machine. The server provide response to the requests. The server process always runs a process in the background called daemon. httpd refers to server daemon for HTTP. The documents are prepared using Hyper Text Markup Language (HTML) which consists of text, graphics and other media are interconnected by links that appear within the documents. The www is accessed through a browser program that displays the documents and allows the user to access other documents by clicking one of these links. Each link provides the browser with a uniform resource Locator (URL) that specifies the name of the machine where the document is located and the name of the file that contains the requested document. The HTTP ( Hyper Text Transfer Protocol ) specifies rules by which the client and server interacts so as to retrieve a document.


In HTTP, we use two-way connection that transfer a stream of bytes in correct sequential order and without errors. The TCP protocol provides this type of communication service between two processes in two machines connected to a network. Each HTTP inserts its messages into a buffer and TCP transmits the contents of the buffer to the other TCP in blocks of information called segments. Each segment contains port number information in addition to the HTTP message information. The following figure shows how communication is carried between HTTP client and HTTP server.

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