Network Notes 1

8/9/2019 Network Notes 1

1/23

1

Introduction to Computer Networks

In simple terms it means an interconnected set of some objects. For decades we are

familiar with the Radio, Television, Railway, Highway, Bank and other types of networks.

In recent years, the network that is making significant impact in our day-to-day life is the

Computer Network.

It simply means a number of computers and some other devices connected to each other

by some medium.

By computer network we mean an interconnected set of autonomous computers. The term

autonomous implies that the computers can function independent of others. However,

these computers can exchange information with each other through the communication

network system.

Computer networks have emerged as a result of the convergence of two technologies of

this century Computer & Communication as shown in below given figure.

Evolution of Computer Networks


2/23

1

Data Communications

When we communicate, we are sharing information. This sharing can be local or remote. The term telecommunication, which includes telephony, telegraphy, and television, means communication

at a distance (tele is a Greek word meaning far).

The word data refers to information presented in whatever form is agreed upon by the parties creatingand using the data.

Data communications is the exchange of data between two devices via some form of transmissionmedium such as a wire cable.

For data communications to occur, the communicating devices must be part of a communication systemmade up of a combination of hardware (physical equipment) and software (programs).

The effectiveness of a data communications system depends on four fundamental characteristics: delivery,accuracy, timeliness, and jitter.

1. Delivery The system must deliver data to the correct destination. Data must be received by theintended device or user and only by that device or user.

2. Accuracy The system must deliver the data accurately. Data that have been altered in transmissionand left uncorrected are unusable.

3. Timeliness The system must deliver data in a timely manner. Data delivered late are useless. In thecase 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. This kind of delivery is called real-timetransmission.

4. Jitter Jitter refers to the variation in the packet arrival time. It is the uneven delay in the delivery ofaudio or video packets. For example, let us assume that video packets are sent every 30 ms. If some of

the packets arrive with 30-ms delay and others with 40-ms delay, an uneven quality in the video is the

result.

Components of Data Communications

A data communications system has five components (see Figure 1.1).


3/23

2

Message The message is the information (data) to be communicated. Popular forms of information include

text, numbers, pictures, audio, and video.

Sender The sender is the device that sends the data message. It can be a computer, workstation, telephone

handset, video camera, and so on.

Receiver The receiver is the device that receives the message. It can be a computer, workstation, telephone

handset, television, and so on.

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.

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 be connected but not communicating, justas a person speaking French cannot be understood by a person who speaks only Japanese.


4/23

1

Protocols & Standards

Protocols

In computer networks, communication occurs between entities in different systems. An

entity is anything capable of sending or receiving information. However, two entitiescannot simply send bit streams to each other and expect to be understood.

For communication to occur, the entities must agree on a protocol. A protocol is a set

of rules that govern data communications. A protocol defines what is communicated,

how it is communicated, and when it is communicated.

The key elements of a protocol are syntax, semantics, and timing.

Syntax The term 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 8 bits of

data to be the address of the sender, the second 8 bits to be the address of the receiver, and

the rest of the stream to be the message itself.

Semantics The word 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

interpretation? For example, does an address identify the route to be taken or the final

destination of the message?Timing The term timing refers to two characteristics when data should be sent and

how fast they 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 the receiver and

some data will be lost.

The protocols include guidelines that regulate the following characteristics of a network

Access Method, Allowed Physical Topologies, Types of Cabling, and Speed of Data

Transfer. The most common protocols are

Ethernet, Local Talk, Token Ring, FDDI and ATM.


5/23

2

Standards

Standards are essential in creating and maintaining an open and competitive market forequipment manufacturers and in guaranteeing national and international interoperability

of data and telecommunications technology and processes.

Standards provide guidelines to manufacturers, vendors, government agencies, andother service providers to ensure the kind of interconnectivity necessary in today's

marketplace and in international communications.

Data communication standards fall into two categories: de facto (meaning "by fact" or"by convention") and de jure (meaning "by law" or "by regulation").

De facto Standards that have not been approved by an organized body but have beenadopted as standards through widespread use are de facto standards. De facto standards

are often established originally by manufacturers who seek to define the functionality of

a new product or technology.

De jure Those standards that have been legislated by an officially recognized body arede jure standards.

Standards Organizations

Standards are developed through the cooperation of standards creation committees,forums, and government regulatory agencies.

(a) Standards Creation Committees

While many organizations are dedicated to the establishment of standards, data

telecommunications in North America rely primarily on those published by the following:

1. International Organization for Standardization (ISO) It is a multinational bodywhose membership is drawn mainly from the standards creation committees of various

governments throughout the world. The ISO is active in developing cooperation in the

realms of scientific, technological, and economic activity.

2. International Telecommunication Union Telecommunication Standards Sector(ITU-T) By the early 1970s, a number of countries were defining national standards

for telecommunications, but there was still little international compatibility. The United


6/23

3

Nations responded by forming, as part of its International Telecommunication Union

(ITU), a committee, the Consultative Committee for International Telegraphy and

Telephony (CCITT). This committee was devoted to the research and establishment of

standards for telecommunications in general and for phone and data systems in

particular. On March 1, 1993, the name of this committee was changed to the

International Telecommunication Union Telecommunication Standards Sector (ITU-

T).

3.American National Standards Institute (ANSI) Despite its name, the AmericanNational Standards Institute is a completely private, nonprofit corporation not affiliated

with the U.S. federal government. However, all ANSI activities are undertaken with the

welfare of the United States and its citizens occupying primary importance.4. Institute of Electrical and Electronics Engineers (IEEE) It is the largest

professional engineering society in the world. International in scope, it aims to advance

theory, creativity, and product quality in the fields of electrical engineering, electronics,

and radio as well as in all related branches of engineering. As one of its goals, the IEEE

oversees the development and adoption of international standards for computing and

communications.

5. Electronic Industries Association (EIA) Aligned with ANSI, the EIA is anonprofit organization devoted to the promotion of electronics manufacturing

concerns. Its activities include public awareness education and lobbying efforts in

addition to standards development. In the field of information technology, the EIA has

made significant contributions by defining physical connection interfaces and electronic

signaling specifications for data communication.

(b) Forums

Telecommunications technology development is moving faster than the ability ofstandards committees to ratify standards.

Standards committees are procedural bodies and by nature slow-moving.


7/23

4

To accommodate the need for working models and agreements and to facilitate thestandardization process, many special-interest groups have developed forums made up

of representatives from interested corporations.

The forums work with universities and users to test, evaluate, and standardize newtechnologies.

By concentrating their efforts on a particular technology, the forums are able to speedup the acceptance and use of those technologies in the telecommunications community.

The forums present their conclusions to the standards bodies.(c) Regulatory Agencies

All communications technology is subject to regulation by government agencies such asthe Federal Communications Commission (FCC) in the United States.

The purpose of these agencies is to protect the public interest by regulating radio,television, and wire/cable communications.

The FCC has authority over interstate and international commerce as it relates tocommunications.


8/23

1

Types of Data Flow

Communication between two devices can be simplex, half-duplex, or full-duplex asshown in figure given below.

Simplex

In simplex mode, the communication is unidirectional, as on a one-way street. Only oneof the two devices on a link can transmit; the other can only receive. Keyboards andtraditional monitors are examples of simplex devices. The key-board can only introduce

input; the monitor can only accept output. The simplex mode can use the entire

capacity of the channel to send data in one direction.

Half-Duplex

In half-duplex mode, each station can both transmit and receive, but not at the sametime. When one device is sending, the other can only receive, 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.

Data flow (Simplex, Half-Duplex, and Full-Duplex)


9/23

2

Full-Duplex

In full-duplex made (also, called duplex), both stations can transmit and receivesimultaneously. The full-duplex mode is like a two-way street with traffic flowing in

both directions at the same time.

One common example of full-duplex communication is the telephone network. Whentwo people are communicating by a telephone line, both can talk and listen at the same

time.


10/23

1

Categories of Networks (Wired)

There is no generally accepted taxonomy into which all computer networks fit, but two

dimensions stand out as important: Transmission Technology and Scale. The

classifications based on these two basic approaches are considered in this section.

Classification of Networks based on Transmission Technology

Computer networks can be broadly categorized into two types based on transmission

technologies:

Broadcast networks

Point-to-point networks

Broadcast Networks

Broadcast network have a single communication channel that is shared by all themachines on the network as shown below.

All the machines on the network receive short messages, called packets in certaincontexts, sent by any machine.

An address field within the packet specifies the intended recipient. Upon receiving apacket, machine checks the address field. If packet is intended for itself, it processes the

packet; if packet is not intended for itself it is simply ignored.

This system generally also allows possibility of addressing the packet to all destinations(all nodes on the network).

Broadcast Network based on Shared Bus

Broadcast Network based on Satellite Communication


11/23

2

As an example, consider someone standing at the end of a corridor with many roomsoff it and shouting ''Hey! Sunil, come here. I need you.'' Although the packet may

actually be received (heard) by many people, but only Sunil responds. The others just

ignore it.

When such a packet is transmitted and received by all the machines on the network.This mode of operation is known as Broadcast Mode.

Some Broadcast systems also supports transmission to a sub-set of machines,something known asMulticasting.

Point-to-Point Networks

It consists of many connections between individual pairs of machines. To go from thesource to the destination, a packet on this type of network may have to first visit one or

more intermediate machines.

As a general rule smaller, geographically localized networks tend to use broadcasting, whereas larger networks usually are point-to-point. Point-to-Point transmission with

one sender and one receiver is sometimes called Unicasting.

The end devices that wish to communicate are called stations. The switching devices arecalled nodes. Some Nodes connect to other nodes and some to attached stations.

It uses FDM or TDM for node-to-node communication. There may exist multiple paths between a source-destination pair for better network

reliability.

Communication Network based on Point-to-Point Communication


12/23

3

As a general rule smaller, geographically localized networks tend to use broadcasting,whereas larger networks normally use are point-to-point communication.

Classification of Networks based on Scale

Alternative criteria for classifying networks are their scale. They are divided into Local Area

(LAN), Metropolitan Area Network (MAN) and Wide Area Networks (WAN).

Local Area Network (LAN)

LAN


13/23

4

Local Area Network (LAN)

LAN is usually privately owned and links the devices in a single office, building orcampus of up to few kilometers in size.

These are used to share resources (may be hardware or software resources) and toexchange information.

LANs are distinguished from other kinds of networks by three categories: their size,transmission technology and topology.

LAN typically used transmission technology consisting of single cable to which allmachines are connected. Traditional LANs run at speeds of 10 to 100 Mbps (Quite

higher speeds these days).

The most common LAN topologies are bus, ring and star.

Metropolitan Area Networks (MAN)

Conventional MAN (Cable TV Network)


14/23

5

Metropolitan Area Networks (MAN)

MAN is designed to extend over the entire city. It may be a single network as a cable TV network or it may be means of connecting a

number of LANs into a larger network so that resources may be shared.

For example, a company can use a MAN to connect the LANs in all its offices in a city.MAN is wholly owned and operated by a private company or may be a service provided

by a public company. (Example Vishal Mega Mart)

The main reason for defining MANs as a special category is that a standard has beenadopted for them. It is DQDB (Distributed Queue Dual Bus) or IEEE 802.6.

Wide Area Network (WAN)

WAN provides long-distance transmission of data, voice, image and information overlarge geographical areas that may comprise a country, continent or even the whole

world.

In contrast to LANs, WANs may utilize public, leased or private communicationdevices, usually in combinations, and can therefore span an unlimited number of miles.

A WAN that is wholly owned and used by a single company is often referred to asenterprise network.

Wide Area Network


15/23

6

Wide Area Network

Stream of Packets from Sender to Receiver.


16/23

1

Categories of Networks (Wireless)

Wireless Networks

Wireless LANs use high frequency radio signals, infrared light beams, or lasers tocommunicate between the workstations and the file server or hubs.

Each workstation and file server on a wireless network has some sort of transceiver/antennato send and receive the data.

Information is relayed between transceivers as if they were physically connected. For longerdistance, wireless communications can also take place through cellular telephone technology,

microwave transmission, or by satellite.

Wireless networks are great for allowing laptop computers or remote computers to connectto the LAN.

Wireless networks are also beneficial in older buildings where it may be difficult orimpossible to install cables.

(a) Bluetooth configuration (b) Wireless LAN

(a) Individual mobile computers (b) A flying LAN


17/23

2

The two most common types ofInfrared Communications used in schools and collegesare Line-of-Sight (it is generally Weak Signal, Example wifi system in colleges) and

Scattered Broadcast (Very Strong Signal, Example CBI Office/ Parliament House).

In Line-of-sight communication means that there must be an unblocked direct line betweenthe workstation and the transceiver. If a person walks within the line-of-sight while there is a

transmission, the information would need to be sent again. This kind of obstruction can slow

down the wireless network.

Scattered infrared communication is a broadcast of infrared transmissions sent out inmultiple directions that bounces off walls and ceilings until it eventually hits the receiver.

Wireless LANs have several disadvantages as poor security, and are susceptible to

interference from lights and electronic devices. They are also slower than LANs usingcabling.


18/23

1

Network Topology

The Network (Physical) topology of a network refers to the configuration of cables,computers, and other peripherals. Physical topology should not be confused with logical

topology which is the method used to pass information between workstations.

Linear Bus Topology

A linear bus topology consists of a main run of cable with a terminator at each end. Allnodes (file server, workstations, and peripherals) are connected to the linear cable.

Ethernet and LocalTalk networks use a linear bus topology.

Advantages of a Linear Bus Topology

Easy to connect a computer or peripheral to a linear bus.Requires less cable length than a star topology.

Disadvantages of a Linear Bus Topology

Entire network shuts down if there is a break in the main cable.

Terminators are required at both ends of the backbone cable.

Difficult to identify the problem if the entire network shuts down.

Not meant to be used as a stand-alone solution in a large building.Star Topology

A star topology is designed with each node (file server, workstations, and peripherals)

connected directly to a central network hub or concentrator.

Data on a star network passes through the hub or concentrator before continuing to its

destination. The hub or concentrator manages and controls all functions of the

Linear Bus to olo


19/23

2

network. It also acts as a repeater for the data flow. This configuration is common with

twisted pair cable; however, it can also be used with coaxial cable or fiber optic cable.

Advantages of a Star Topology

Easy to install and wire.

No disruptions to the network then connecting or removing devices.

Easy to detect faults and to remove parts.

Disadvantages of a Star Topology

Requires more cable length than a linear topology.

If the hub or concentrator fails, nodes attached are disabled.

More expensive than linear bus topologies because of the cost of the concentrators.

The protocols used with star configurations are usually Ethernet or Local-Talk.

Token Ring uses a similar topology, called the star-wired ring.

Star-Wired Ring Topology

A star-wired ring topology may appear (externally) to be the same as a star topology.

Internally, the MAU (Multi-Station Access Unit) of a star-wired ring contains wiring

that allows information to pass from one device to another in a circle or ring. The

Token Ring protocol uses a star-wired ring topology.

Tree Topology

A tree topology combines characteristics of linear bus and star topologies. It consists ofgroups of star-configured workstations connected to a linear bus backbone cable. Tree

topologies allow for the expansion of an existing network, and enable schools to

configure a network to meet their needs.

Advantages of a Tree Topology

Point-to-point wiring for individual segments.

Supported by several hardware and software venders.


20/23

3

Disadvantages of a Tree Topology

The type of cabling used limits overall length of each segment.

If the backbone line breaks, the entire segment goes down.

More difficult to configure and wire than other topologies.

Tree TopologyStar Topology

Topologies for Point To Point Subnet (a) Star (b) Ring (c) Tree (d) Complete

(e) Intersecting Rings (f) Irregular)


21/23

1

Distributed Computing/ Processing

It involves the distribution of applications & business logic across multiple processingplatforms.

Distributed processing implies that processing will occur on more than one processor inorder for a transaction to be completed.

In other words, processing is distributed across two or more machines and the processesare most likely not running at the same time, i.e. each process performs part of an

application in a sequence. Often the data used in a distributed processing environment is

also distributed across platforms.

Distributed Computing also refers to the use of distributed systems to solve computationalproblems. In distributed computing, a problem is divided into many tasks, each of which issolved by one computer.

Alternatively distributed system consists of multiple autonomous computers thatcommunicate through a computer network. These computers interact with each other in

order to achieve a common goal.

A Computer Program that runs in a distributed system is called a Distributed Program,and Distributed Programming is the process of writing such programs.


22/23

1

Line Configuration

It is the way two or more communication devices attach to a link. A Link is the physical communication pathway that transfers data from one device to

another.

Line configuration can be of two types Point to Point & MultipointPoint to Point

Point-to-point telecommunications generally refers to a connection restricted to twoendpoints, usually host computers.

Multipoint

It is also known as Multi-drop Line Configuration. It refers to a communicationconfiguration in which several terminals or stations are connected together.

Multipoint is a single communications channel to which two or more stations or logicalunits are attached although only one can transmit at a time.

Using it, the telecommunication system permits three or more locations tointercommunicate in a conference call.


23/23

Applications of Computer Networks

Business Applications Commercial networks generally have with multiple clients and

one server. With the advancements in technology Electronic Commerce is getting popular.

Home Applications

Access to remote information

Person-to-person communication

Interactive entertainment

Mobile Applications

Laptop computer used in offices or schools or colleges.PDAs used in Shopping Malls for billings by sales personnel.

Social Applications These are some portal (http://pgportal.gov.in) to sort out public

grievances and spreading general information.

Person to person Communication

Documents

Network Notes 1