UNIT 3 Computer Network and Communication

 

UNIT 3

 

Computer Network and Communication

Communication Network

Ever since the beginning of time, people have had an uncontrollable need to communicate. Our nature drives us to exchange ideas, information and opinions. Without communication, we can’t learn, without learning, we can’t grow. and without growth, we shrivel up and disappear.

Creativity and communication drive the human race more than any other force. They are the essential elements of life.

Networking is the ultimate level of communication. It transcends words and pictures to provide the pathway for thoughts, ideas and dreams. Networking that exists today is the result of millions of years of evolution and growth.

If we go through the history of communication, Charles Babbage and Countess of Lovelace are listed as the pioneer in the field of the present communicating world. Suddenly, the alphabet shrunk from twenty-six characters to two i.e. 0 and 1.

The world was thrust into a technological era that would spawn numerous networking inventions including the telephone, phonograph, motion pictures and the television. Finally, data communications were born a century later when computers were linked across distances.

NASA uses data communications to control the space shuttle and realign geosynchronous satellites, surgeons perform computerized operations from miles away, and because of data communications, today’s workforce is better to balance home and work by working from home or telecommuting. Data communications have had a profound effect on humanity.

The world will go on changing. It will become a small global village. In the new information age, humans will become very big fish in a very small pond.

Computer Network

A more comprehensive definition of the network: “A collection of distributed, intelligent machines that share data and information through interconnected lines of communication is called network”.

When two or more computers are brought together in connection with cable or without cable, that may extend within a limited room or to the entire world, then the computers are said to be in network connection. They merely perform the task of data sharing, transmission of data and communication.

Network Elements

Network elements fall into a place to create a compact LAN. Network communications provide the electronic pathway that computers use to communicate with each other. These elements include physical cabling, data signals and LAN standards.

Networking represents the rules of the road, defining protocols that instruct different machines how to speak (Internetwork Packet Exchange/Sequence Packet Exchange) for NetWare, TCP/IP (Transmission Control Protocol/Internet Protocol) for the Internet and AppleTalk for Macintosh computers.

Finally, network services simply define the things networks can do. Service requesters (clients) ask for resources and service providers (servers) grant them in the case of Client Server architecture. Common network services include file services, print services and messaging. A service provider is any combination of hardware and software that fulfills a particular job or function.

Network elements define the pathway and protocols for electronic communications. We are more concerned with what the message look like and how it travels from point A to point B in the imaginary computers in the network.

Here are three basic elements of networks:

1.     Network service

2.     Network Standards & Protocols

3.     Transmission media

Network Services

Network services are the things that a network can do. These are the services provided by the computer. The major services that a network can provide are:

1.     File Services

2.     Print Services

3.     Message Service

4.     Application Services

5.     Database Services

1. File Service: File Services includes file transfer and storage, data migration, file update synchronization and archiving. These features are the most popular reasons for networking.

2.Print-Service:
Print Services produce shared access to valuable printing devices. A printer that is used as sharing in the network acts as the print server.

3.Message-Service:
Message Services facilitate e-mail, manage integrated email and voicemail and co-ordinate object-oriented applications. Messaging is an exciting new player at the network party.

4.Application-Service:
Application Services allows you to centralize high-profile applications. The server holds the applications (generally network versions) and they are accessed by the users in the network.

5.Database-Service:
Database Services involve the coordination of distributed data and replication. The main server holds the data and determines the number of users that can access the database and modify the data. It also provides the print facility of the data in the network.

Network Standards & Protocols

The term protocol refers to the set of rules and procedures that govern the transmission of messages over a physical networking medium. Topology describes the geographic orientation and arrangement of networking components.

Together, protocol and topology combine to create a networking standard. These standards are developed and controlled by the Institute of Electrical and Electronics Engineers (IEEE). The four major industry standards are:

1.     Ethernet

2.     Token Ring

3.     ARCnet and

4.     FDDI (Fiber Distributed Data Interface)

Topologies define the geographic arrangement of server and workstations. File servers and workstations are arranged according to a variety of factors: speed, cost, reliability, distance and load requirements. Each topology is ideally used for different combinations of these factors:

·         The bus is fast, cheap and unreliable.

·         The star is slow, large and reliable.

·         The ring is fast, expensive and reliable.

·         The starring is fast, cheaper and reliable.

Ethernet Standard for Bus Topology

The Ethernet topology was developed at the University of Hawaii, to connect computers on the various islands. It was a radio-based design. The term Ethernet is a compound word of either meaning air and net meaning network. Later, Robert Metcalfe went to Xerox’s Palo Alto Research Center (PARC) laboratories.

The radio portion was eliminated and changed to co-axial cabling. This was the string from station to station. This station to station topology was named linear bus.

Token-Passing Standard or Protocol

The token-passing protocol relies on a control signal called the token. A token is a 24-bit packet that circulates throughout the network, from NIC to NIC, in an orderly fashion. If a workstation wants to transmit a message, first, it must seize the token. At that point, the workstation has complete control over the communications channel.

The existence of only one token eliminates the possibility of signal collisions. This means that the only station can speak at a time on a multi-station ring.

ARC Net Standard or Protocol

The Attached Resource Computer Network (ARC Net Standard was created in 1977, at the Data point Corporation by a scientist named John Murphy. This standard predates the token-passing design adopted by IBM but uses very much the same technology ARC Net does not offer the same overall connectivity as Ethernet, but its hardware components are standardized sufficiently so that any ARC Net device from any manufacturer can be used on any other ARC Net LAN.

 

Direction of communication flow

Local Area Networks (LANs)have some specific architecture for communicating among the computers in the connection. The networks made by connecting IBM PCs and Macintosh computers have different methods of communication between themselves. The architecture defines how the computers communicate in these networks. The different architectures are:

1.     Peer-to-Peer Architecture

2.     Client Server Architecture

Peer-to-Peer Architecture

Some of today’s most popular server-centric operating systems include Novell’s NetWare, Microsoft’s Windows NT Advanced Server, Windows 2000 Advanced Server, IBM’s LAN Server and Banyan’s VINES. Not every network is server-centric.

Some system called peer-to-peer network software usually has no central point of control, relying on the peers to perform the various functions for each other. Some of today’s most popular peer-to-peer operating systems includes Microsoft Windows 95, Microsoft Windows for Workgroups, Microsoft Windows Nt, LAN tactics and Novell’s Personal NetWare.

 

In a peer-to-peer architecture, there is no dedicated server. The data signal sent by a computer is recognized by another computer and it is accessed by using intelligent methods. In this case, every computer can act as a server as well as the client. The computer that provides servers is a server and the computer that receives the information is a client at the time of data transmission and communication.

Client Server Architecture

In the LAN, the computers are confined with the limited boundary of 1 to5 km by using repeaters. The data transmission is the rate of 20 MBPS (million bits per second). In this case, a dedicated server is installed in a computer and all the others computers are clients.

 

The operating system, applications and other resources are generally kept in the file server only. A server can handle 20-50 computers ( depends on the server capacity). All the computers are connected by using co-axial cable. The operating systems used in client / server is the computer that provides the services and the computers that requests data, program and the query is known as a client computer.

Types of Networks: Computer Network Models

Network models classify networks by size, distance and structure. These models help us to understand the relationship among centralized, distributed and collaborative computing systems. There are three fundamental classifications of network models:

1.     Local Area Networks (LANs)

2.     Metropolitan Area Networks (MANs)

3.     Wide Area Network (WANs)

Local Area Network (LAN)

A local area network combines computers hardware and transmission media in the relatively small area. These systems are usually contained within one department, building or campus. In addition, LANs typically comprise only one transmission media type such as coaxial cable or a twisted pair but never both.

LANs are characterized by comparatively high-speed communications. These high speeds are possible because LANs use one kind of cable, which generally is limited to 5 km or less. MANs and WANs, on the other hand, typically rely on inbound communication media which equates to much slower speeds.

LANs occupy a relatively small area, including a relatively small number of machines and all components connect directly to the communications media. LAN hardware gives the system its processing communications, system fault tolerance and most important, connectivity. LAN software endows the system with productivity, user interface, network management and most important, user transparency. The two most important components are LAN are the file server (hardware) and network operating system ( software).

Characteristics of LAN

The following are the major characteristics of LAN which are physically connected.

1.     The distance between two nodes is not more than 5 km.

2.     LAN is confined within the same management system.

3.     The data transfer rate ranges from 1 byte to 10 GB per second.

4.     This is used in office automation system

5.     There exists node-to-node connection.

Capacities of LAN

The following are the major capacities of LAN in addition to other capacities which are not discussed here.

1.     LAN helps to access distance data, processes them and prints.

2.     Shares hardware resources and software.

3.     It can control and transmit data on the basis of priority.

4.     Acts and bridge between two networks.

5.     Acts as a gateway for internetwork communication.

Metropolitan Area Network (MAN)

A Metropolitan Area Network (MAN) is larger than a LAN, as its name implies, it covers the area of a single city. MANs rarely extend beyond 100 KM and frequently comprise a combination of different hardware and transmission media. In addition, they have somewhat slower data communication rates than that of LAN’s, partly because of their reliance on unbound media over great distances.

The most important characteristics of MAN’s are their diversity. MAN’s typically are used when you need to connect dissimilar systems within a single metropolis. As shown in figure, we have connected bank, home and work LAN’s into one MAN. This level of continuity is synonymous with twenty-first-century computing.

The two most important components of MAN’s are security and standardization. Security is important because information is being shared between similar systems. Many users are still wary of MAN computing because of the sensitivity of their data. Standardization is necessary to ensure reliable data communications.

Wide Area Network (WAN)

A Wide Area Network (WAN) is simply a LAN of LANs. WAN connect LANs that may be on opposite sides of a building, across the country or around the world. On one of the three network models, WAN’s are characterized by the slowest data communication rates and the largest distances. Furthermore, WANs can connect different types of networks together.

For example, you can connect a centralized computing system (mainframe) with a distributed LAN. These connections frequently are made through a special type of device called a gateway. WAN connects LANs from around the world. WANs can be characterized as either enterprise or global WANs.

WAN connects LANs from around the world. WANs can be characterized as either enterprise or global WANs. An enterprise WAN connects an entire organization, including all LANs at various sites. This term is used for large, widespread organization such as corporations, universities and governments. Even though the LANs may be in different part of the country; they must all belong to a single company or institution.

Global WANs also span the world, but they do not have to connect LANs within a single organization. The internet is an example of a global WAN. It connects diverse locations, organizations and institutions throughout the world. Finally, global WANs can be public or private. The Internet, for example, is a public global network, which means that anyone can attach to it from anywhere in the world. Private WANs (intranet) are also available.

As more networks are connected together, the current classifications may begin to disappear. Technological progress is expected to result in the development of a single computer networking infrastructure. Faster and cheaper data communications will transcend the need for LANs, MANs and WANs.

 

LAN Topologies

LAN physical topology defines the geographical arrangement of networking devices. Topologies are driven fundamentally by two network connection types:

A point-to-point connection is a direct link between two devices. For example, when you attach your computer to a printer, you have created a point-to-point link. In networking terms, most of the today’s point-to-point connections are associated with modems and PSTN (Public Switched Telephone Network) communications because only two devices share point-to-point connections, it defeats the purpose of a shared network.

A multipoint connection, on the other hand, is a link between three or more devices. Historically, multipoint connections were used to attach central CPUs to distributed dumb terminals.

In today’s LAN environments, multipoint connections link many network devices in various configurations.

The major topologies of LAN are:

1.     Bus Topology

2.     Ring Topology

3.     Star Topology

4.     Mesh Topology

5.     Cellular Topology

6.     Hybrid Topology

Bus Topology

The physical bus topology is the simplest and most widely used of the network designs. It consists of one continuous length of cabling (trunk) and a terminating resistor (terminator) at each end. The data communications message travels along the bus in both directions until it is picked up by a workstation or server NIC.

If the message is missed or not recognized, it reaches the end of the cabling and dissipates at the terminator. All nodes in the bus topology have equal access to the trunk – no discriminating here. This is accomplished using short drop cables or direct T-connectors.

This design is easy to install because the backbone trunk traverses the LAN as one cable segment. This minimizes the amount of transmission media required. Also, the number of devices and length of the trunk can be easily expanded.

Advantages of Bus Topology:

1.     It uses established standards and it is relatively easy to install.

2.     Requires fewer media than other topologies.

Disadvantages of Bus Topology:

1.     The bus networks are difficult to reconfigure, especially when the acceptable number of connections or maximum distances have been reached.

2.     They are also difficult to troubleshoot because everything happens on a single media segment. This can have dangerous consequences because any break in the cabling brings the network to its knees.

 

 

Ring Topology

As its name implies, the physical ring topology is a circular loop of point-to-point links. Each device connects directly or indirectly to the ring through an interface device or drop cable. Messages travel around the ring from node to node in very organized manner. Each workstation checks the messages for a matching destination address.

If the address doesn’t match, the node simply regenerates the message and sends it on its way. If the address matches, the node accepts the message and sends a reply to the originating sender. Initially, ring topologies are moderately simple to install; however, they require more media than bus systems because the loop must be closed.

Once your ring has been installed, it’s a bit more difficult to reconfigure. Ring segments must be divided or replaced every time they’re changed. Moreover, any break in the loop can affect all devices on the network.

Advantages of Ring Topology:

1.     They are very easy to troubleshoot because each device incorporates a repeater.

2.     A special internal feature called becoming, allows the troubled workstation to identify themselves quickly.

Disadvantages of Ring Topology:

1.     It is considerably difficult to install and reconfigure ring topology.

2.     Media failure on unidirectional or single loop causes complete network failure.

Star Topology

The Physical star topology uses a central controlling hub with dedicated legs pointing in all directions – like points of a star. Each network devices has a dedicated point-to-point link to the central hub. This strategy prevents troublesome collisions and keeps the line of communication open and free of traffic.

Star topologies are somewhat difficult to install because each device gets its own dedicated segment. Obviously, they require a great deal of cabling. This design provides an excellent platform for reconfiguration and troubleshooting.

Changes to the network are as simple as plugging another segment into the hub. In addition, a break in the LAN is easy to isolate and doesn’t affect the rest of the network.

Advantages of Star Topology:

1.     Relatively easy to configure.

2.     Easy to troubleshoot.

3.     Media faults are automatically isolated to the failed segment.

Disadvantages of Star Topology:

1.     Requires more cable than most topologies.

2.     Moderately difficult to install.’

Mesh Topology

The mesh topology is the only true point-to-point design. It uses a dedicated link between every device on the network. This design is not very practical because of its excessive waste of transmission media. This topology is difficult to install and reconfigure.

Moreover, as the number of devices increases geometrically, the speed of communication also become slow. ATM (Asynchronous Transfer Mode) and switched Hubs are the example of high-speed Mesh implementation.

Advantages of Mesh Topology:

1.     Easy to troubleshoot because each link is independent of all others.

2.     You can easily identify faults and isolate the affected links. Because of the high number of redundant paths, multiple links can fail before the failure affects any network device.

Disadvantages of Mesh Topology:

1.     It is difficult to install and reconfigure especially as the number of devices increases.

Cellular Topology

A cellular topology combines wireless point-to-point and multipoint designs to divide a geographic area into cells. Each cell represents the portion of the total network area in which a specific connection operates. Devices within the cell communicate with a central station or hub. Hubs are then interconnected to route data between cells.The cellular topology relies on the location of wireless media hubs. Cellular networks exhibit interesting characteristics since this topology do not depend on cables. Troubleshooting is easy because each hub interacts independently with each device. A cellular installation depends on the accessibility hub locations.

Advantages of Cellular Topology:

1.     It is relatively easy to install.

2.     It does not require media reconfiguration when adding or removing users.

3.     Fault isolation and troubleshooting is fairly simple.

Disadvantages of Cellular Topology:

1.     All devices using a particular hub are affected by a hub failure.

Hybrid Topology

By modifying or combining some of the characteristics of the ‘pure’ network topologies, a more useful result may be obtained. These combinations are called hybrid topologies. Some of the hybrid topologies are:

1.     Tree network

2.                 Star-Ring or interconnected

 

Communication Transmission Media

INTRODUCTION T0 TRANSMISSION MEDIA

Transmission media provide the physical path through which electrons flow. At the physical layer of OSI model of networking, electrons represent network data as binary 0s and 1s. Transmission media provides these electrons with a bound or unbound communications path. Telephone lines are well proven and commonly used communication media.

Basically, there are two methods by which the communication takes place. They are:

1.     Wired Transmission Media or Bound Transmission Media and

2.     Wireless Transmission Media or Unbound Transmission Media

Bound Transmission Media

As said earlier, bound transmission media consists of a central conductor surrounded by a physical jacket. This property offers advantages in security, reliability and speed. Bound media are ideal for LANs but distance limitations can be the problem for WANs. The four types of bound media are:

i. Unshielded Twisted Pair (UTP)
ii. Shielded Twisted Pair (STP)
iii. Coaxial Cable
iv. Optical Fiber

i. Unshielded Twisted Pair (UTP)

UTP is the most common type of telecommunication media these days. It is most suited for data and voice communication. It consists of two metal conductors (usually copper) hat which are insulated separately with their own colored plastic insulation. It can transmit up to 96000 bps. UTP usually is intended for analog communications not digital.

ii. Shielded Twisted Pair (STP)

STP cable has a metal foil or braided mesh covering that covers each pair of insulated conductors. The metal foil is used to prevent infiltration of electromagnetic noise. This shield also helps to eliminate crosstalk during the telephone conversation. STP provides shielded protection against EMI (Electromagnetic interference) whereas UTP doesn’t have.

iii. Coaxial Cable

Coaxial cabling is commonly used for television transmissions. It provides higher bandwidth and better reliability. Unlike twisted pairs that have two wires, coaxial cables have the single central conductor, which is made up of solid wire. This conductor is surrounded by an outer jacket made up of PVC. A coaxial cable is capable of transmitting data at the rate of 10 Mbps.

iv. Optical Fiber

Fiber optics plays by a completely different set of rules. You won’t find any electricity here. Instead, fiber-optic cabling uses pulses of light (photos) for network communications i.e. it relies on photonics instead of electronics. As a result, fiber optics is completely immune to EMI (Electromagnetic Interference) and is extremely fast.

Analog and Digital Signal

The major role of the physical medium is to move information from one communicating device to another. However, information to be transmitted should be first transformed into electromagnetic signals. Information over any medium is transmitted by two methods.

 

They are:

i. Analog Signal
ii. Digital Signal

i. Analog Signal

An analog signal is a continuous waveform that changes smoothly over time. The analog signals represent the continuous variation in the data. When data are plotted on the Y-axis with time, we get curves called analog signal. During data transmission, the signals are modulated or changed in analog form.

ii. Digital Signal

Digital Signal data is the data stored in the form of 0s and 1s. When the signal is at a high point, its value is 1 and when it is low, its value is 0. A signal in digital format has precise voltages that are not affected by noise or attenuation compared to analog signals, which are very prone to noise. Digital data represent two states either 0 or 1.

Unbound Transmission Media or Wireless Communication

Unbound transmission media extend beyond the limiting confines of cabling. They provide an excellent communication alternative for WANs. The lack of physical restrictions provides larger bandwidth as well as wide-area capabilities. Unbound media typically operate at very high frequencies. The four types of unbound media are:

1.     Radio Wave

2.     Microwave

3.     Infrared

4.     Satellite Communication

1. Radio Wave

Although radio waves are prevalent and well understood, we are just beginning to realize their enormous potential as a networking medium. Radio waves can operate on single or multiple frequency bands. In this case, the signals are carried over carrier waves which have frequencies in the range of radio frequency spectrum.

There are three types of RF (radio frequency) propagation, namely, ground wave, ionosphere and line of sight. During the time of radio wave propagation, it should be modulated. The following are the different types of modulation.

1.     Amplitude Modulation (AM)

2.     Frequency Modulation (FM)

3.     Phase Modulation (PM)

2. Microwave

Microwave, in contrast, have been used in data communications for a long time. They have a higher frequency than radio wave and therefore, they can handle larger amounts of data. There are, of course, problems with microwaves attenuation and environmental interference.

Microwave transmission is the line of sight transmission. The transmit station must be in visible contact with the receive station. This sets the limit on the distance between stations depending on the local geography.

3. Infrared

Infrared offers a great unbound photonic solution. Like fiber-optic cabling, infrared communications use light. So, they are not bound by the limitations of electricity.

4. Satellite Communication

Satellite communication is also kind of line of sight transmission. Satellites are set in geostationary orbits directly over the equator, which rotates in synchronization to the earth and hence looks stationary from any point on the earth. These geostationary orbits are placed 36,000km above the earth’s surface.The communication is carried through uplinks and downlinks. Uplinks and downlinks are also called earth stations because they are located on the earth. The area shadowed by the satellite in which the information or data can be transmitted and received is called footprints. There are many satellites in the space for communication.

Computing Models

It is better to appreciate the importance of network models from the evolution of computing. In the beginning, electronic computers were huge machines that used thousands of vacuum tubes. This was the dark ages of computing before transistors and silicon chips. Computing, in its earliest form, was centralized in the mainframes. Mainframe computers stored, organized and processed information.

Data were fed into these centralized machines through remote terminals and keyboards. All data were stored on the mainframe and processing was performed there as well.

In a distributed computing model, 95% of the processing is handled by distributing computers. The central file server handles housekeeping tasks only, such as security and printer sharing. A relatively new trend called collaborative computing has emerged largely because of distributed computing and the capabilities of personal computer networks.

Unlike the distributed computing model (in which individual PCs carry out independent tasks), collaborative computing involves two or more computers working together synergistically.

Today’s networks integrate personal computers, mainframes and a host of other computing and other communication services. To classify this complexity, you need network models that transcend these definitions.

 

Network Transmission Devices

INTRODUCTION

Network transmission devices connect individual workstations and servers into a synergistic LAN. These devices start simple connectors and evolve in complexity and sophistication. The following are the transmission devices:

1.     Media Connectors

2.     Network Interface Card (NIC)

3.     Repeaters

4.     Hubs

Media Connectors

Transmission media connectors attach directly to the medium itself and serve as the physical interface between cabling and network nodes. BNC T-connector, RJ- 45 Connector for UTP, IBM data connector for STP and DIX connector for thick coaxial cabling are media connectors. Some of the media connector are described below:

·         BNC T-connector for Thin Coaxial Cabling

The BNC connector was one of the first for network communications. It was introduced so long ago that none is really sure what the letters stand for. Some authorities claim that it means “British Naval Connector”, whereas others insist that it stands for “Bayonet Nut Coupler”. BNC has become part of standard networking language.

·         RJ-45 Connector for UTP (Unshielded Twisted Pair)

In the business world, coaxial cabling is one way out, but manufacturers are still using it. Nearly, all new cable installations use twisted-pair cabling with eight wires per cable. The eight wires are separated into four twisted pairs.

The pairs are then terminated in and RJ-45 connector. This port looks like the one attached to your home telephone except that it is a bit larger with room for eight for wires instead of four.

·         DIX Connector for Thick Coaxial Cabling

DIX is an acronym for Digital, Intel and Xerox. The companies that invented Ethernet these early connectors are D-shell shaped with 15 pins each (organized in two rows and eight pins, respectively). The drop cable that plugs into this port looks a lot like your video connector except that it is little longer and narrower.

Network Interface Boards or Network Interface Card (NIC)

The NIC contains the electronic circuitry needed to ensure the reliable communications between workstations and servers. The NIC is the electronic interface between the computer and the LAN cabling.

The card itself uses a bus-specific edge connector to plug into the computer motherboard. On the exposed side of the NIC, there are cabling ports that plug directly into the LAN cabling.

Repeaters

As the name implies, repeaters repeat network data. In general, repeaters operate at the electronic level and contain no real intelligence. A repeater accepts weak signals, electrically regenerates them and then sends the messages on their way. There are two types of repeaters: amplifiers and signal-regenerating repeaters.

Hubs

Technically speaking, a hub is simply a multi-port repeater. In addition to regenerating network data, hubs add form and function to the layout of the LAN. In many topologies, the hub is the central component of the network transmission media. There are three types of hubs. They are the passive hub, active hub and intelligent hub.

Internetwork Transmission Devices

A LAN links a small group of functionally similar workstations within a local geographic area. Once your network expands to include other floors or even other LANs, it ceases to be local and becomes a WAN. To connect multiple LANs into a WAN, you need flexible advanced internetwork transmission devices such as:

1.     Bridges

2.     Routers

3.     Gateways

1. Bridges

Like repeaters, bridges extend the maximum distance of your network by connecting separate segments together. However, unlike repeaters, bridges do it intelligently. Bridges analyze network packets from multiple segments and determine who gets through and who doesn’t. This is based on physical address.

2. Routers

Routers are more intelligent than bridges. Instead of being limited to the physical address, routers can use both physical and logical addressing, which allows you to break your internet work into logical subnets.

3. Gateways

Gateways represent the pinnacle of internetwork transmission devices. They perform more software translation than anything. Routing capabilities which are built into the gateway device do the packet routing. Gateways are required when network messages travel between two entirely different systems.

A classic example is the exchange of data between centralized and distributed CPUs. The gateway needs to read the network address, reconfigure the packet protocols from IPX to SNA, translate the operating software and in most cases, completely rewrite the data alphabet.

Communication Transmission Devices & Techniques

Communication devices provide basic data communications functions. They transmit, translate and transform data into signals that can be transmitted into the physical world. The most common communication devices are:

1.     Modems

2.     Multiplexers

3.     CSU/DSU (Channel Service Unit/Digital Service Unit)

4.     Circuit Switching

5.     Message Switching

6.     Packet Switching

1. Modems

Modems are necessary because computers and transmission media speak completely different languages. Computers are digital (0s and 1s) and most transmission media are analog (UTP or radio waves). For digital computers to communicate over analog media, they need translation device.

That’s where a modem comes in. The modem actually performs two distinct tasks: modulation and demodulation. During modulation, the modem translated digital information into an analog waveform. Next, the data travels along the medium to the destination device. There, the analog wave is demodulated into digital 0s and 1s.

Modem speeds are typically measured in baud (or loosely but per second). Today’s faster modems can support speed up to 96,000 bps (96kbps).

2. Multiplexers

Multiplexers allow you to send multiple signals across the single transmission medium. Multiplexing (or mixing) refers to the process of funneling multiple data connections into one circuit for transport across a single medium. An excellent example can be found in TV cabling.

If you think about it for a moment, you have 100 or more channels arriving through one piece of coaxial cabling. Your cable box or VCR demultiplexes the signal and separates the channels.

3. CSU/DSU

Modems allows you to connect digital computers to analog transmission media. Most WAN providers require that you lease the CSU/DSU device from them. This ensures system-wide continuity and safety.

4. Circuit Switching

Circuit switching connects the sender and receiver by a single path during the conversion. Telephone switching equipment, for example, uses address number (in the form of a country area, office, trunk codes, etc) to establish a path that connects the sender’s telephone to the receiver’s telephone.

In circuit switching, a complete path must exist before communications can take place. The computer which is initiating the data transfer establishes the dedicated path and ensures reliable packet delivery.

5. Message Switching

Message switching does not establish a dedicated path between two stations. Instead, conversions are divided into messages. Each message is packaged with its own destination address and then transmitted from the router through the internetwork.

Each router receives the messages, stories briefly and then transmits it to the next device. This type of network is sometimes called a store-and-forward WANs.

6. Packet Switching

Packet switching combines the advantage of both circuit and message switching. Packet switching breaks the datagrams into small parts called packets. Each packet is constructed with source and destination address that allow it to work its way through internetwork and find its destination.

Because packets have strictly defined maximum lengths, they can be stored in RAM instead of disk, which makes store-and-forwarding faster and easier. In general packet switching routers use one of the two different Strategies:

1.     Datagram packet switching

2.     Virtual packet switching

Advantages & Disadvantages of Network

The following are the distinct notes in favor of networking:

1.     The computers, staff and information can be well managed.

2.     A network provides the means to exchange data among the computers and to make programs and data available to the people.

3.     It permits the sharing of the resources of the machine.

4.     Networking also provides the function of back-up. The backup capability is especially useful in systems like air traffic control where the counterpart can assume its functions and workload.

5.     Networking provides a flexible networking environment. Employees can work at home by using through networks tied through networks into the computer at the office.

Open System Interconnection (OSI) Model

INTRODUCTION

OSI model has become the primary architecture for the computer network. It was originated in 1982 and has become the most efficient ISO standard network model. The OSI model describes how information moves through a network medium. There are seven layers of OSI model. Each model is specific in its task.

As the message travels up the OSI Model on the other side, the headers and footers are stripped away, finally revealing the message at the top. To differentiate the evolution of the message as it moves along the model, different names are used to identify the message. The following names are commonly used.

LAYER	DATA NAME
Application Layer	Message
Presentation Layer	Message
Session Layer	Message
Transport Layer	Datagrams or segments
Network Layer	Packets or datagrams
Data Link Layer	Frames
Physical Layer	Bits

Physical Layer

The physical layer handles low-level rules for transmitting bits. This layer encodes or decodes bits and sends or receives the stream of data. This is the pavement of the information superhighway. The physical layer defines:

·         Electrical properties

·         Transmission media

·         Transmission devices

·         Physical topology

·         Data signaling

·         Data synchronization

·         Data bandwidth

Electrical properties define the rules how the message travels over transmission media, which then create bound or unbound pathways for transmission of bits. Again, these media compose the concrete of the information superhighway. Transmission devices provide midpoints and functionality to the transmission media. They are the raw implementation tools. Without translation devices, bits would scatter aimlessly through space. Physical topologies provide the form and function. They are defined by connection types and geographical arrangements of networking nodes. They often determine which medium is best certain circumstances.

An example includes bus, star, ring and cellular topologies. Data signaling encompasses coding and timing rules for digital and analog communications. These rules govern how the bits are received at the other side. Data synchronization defines one of two states for the electronic transmission message: asynchronous or synchronous.

Finally, data bandwidth is the capacity of transmission media, which determines the communications methodology and transmission speed. Most LANs are baseband communications, whereas WANs operate in the broadband arena.

Data Link Layer

The Data Link Layer organizes physical bits into logical groups called frames. These are contiguous series of bits grouped together as a unit of data. The Data Link Layer also detects and sometimes corrects errors. It also controls data flow and identifies computers on the network through physical addressing.

To accomplish all this, the Data Link Layer is organized into two sub-layers:

·         Media Access Control (MAC)

·         Logical Link Layer (LLC)

Media Access Control (MAC):

These protocols are the rules of the road. Your data signals must use one of the three different protocols to gain access to the media. They are:

·         Contention

·         Token passing and

·         Polling

Physical addressing distinguishes one location from another. The data frames need address so that they know where to go and what to do. Addressing works on the hierarchy structure. The data frames need three addresses. They are:

·         Physical address

·         Logical address

·         Service address

Logical Link Control (LLC):

This layer consists of the Functions:

Frame synchronization: It forces senders and receivers to agree on the boundaries of data frames. This takes place as follows:

1.     Asynchronous – sends each frame separately

2.     Synchronous – sends on the basis of synchronized clocks

3.     Isochronous – incorporates advantages of above both

Flow Control: They are typically made up of devices with different transmission speeds like storage and processing capabilities. In diverse world, we need to develop rules that protect slower devices but don’t hinder faster ones. These rules are called flow control. Data Link flow control regulates how much data can be sent over the transmission media in a specified period of time. That is achieved by:

·         Window flow control

·         Guaranteed rate flow control

Error Checking: It uses a complex calculation called the Cyclical Redundancy Check (CRC). CRC ensures the integrity of the packet by calculating a 16-32 bit number based on the contents of the packet.

Network Layer

Network Layer is concerned primarily with moving data from point A to point B. In general, network layer has four functions:

·         Logical Addressing

·         Switching

·         Routing

·         Network Control

Logical Addressing

Logical addressing helps to determine a logical path during internetwork communications. The network relies on three types of addresses:

1.     Physical address

2.     Logical address

3.     Service address

Switching

Most large internetworks consist of multiple physical paths between the sender and receiver. Intermediate routers are responsible for moving the packets across these diverse links. Switching makes this possible.

·         Circuit Switching: It connects the sender and receiver by a single path during the conversation. Telephone switching equipment, for example, uses address number (in the form of the country area, office, truck codes, etc) to establish a path that connects the sender’s telephone to the receiver’s telephone.

·         Message Switching: It does not establish a dedicated path between two stations. Instead , a conversation is divided into messages. Each message is packaged with its own destination address and then transmitted from router to router through the internetwork.

·         Packet Switching: It combines the advantage of both circuit and message switching. Packet switching breaks the datagrams into small parts called packets. Each packet is constructed with source and destination address that allow it to work its way through internetwork and find its destination.

Routing

Routing intermediate network devices are used to make intelligent decisions about how the message should travel from point A to point B. Routing involves two simple steps:

1.     Route discovery and

2.     Route selection

Network Control

Network control administers internetworking and routing. These tasks will ensure that we maintain a steady speed and rejoin with all the rest of the datagrams at the other end. Network layer administrative task include:

·         Network flow control

·         Network sequencing

·         Network error checking

Transport Layer

It organizes packets into segments and delivers them reliably to upper- layer services. If packets are not delivered to the destination correctly by the network layer, transport functions come to rescue. This layer can initiate retransmissions and inform the upper layers that we are trying again.

The OSI Transport Layer consists of the following components:

1.     Service addressing

2.     Segmentation

3.     Transport control

Service addressing: It provides a doorway to upper-layer services. Remember, this is the last point of connectivity before we get to the actual network services. Each message is destined for a particular service. The transport layer uses connection Ids, ports and sockets to make sure that we find the right one.

Segmentation: It is the housekeeping task that package messages into acceptable sizes. Many upper-layer services need specified-sized messages and segmentation does the trick.

Transport control: It includes error checking and flows control. Error checking is the Transport Layer’s main function. At this point, we are concerned mostly with the sage arrival of the network messages. We don’t care what they look like. That’s the Data Link Layer’s job.

Session Layer

The Session Layer opens a dialogue between the sender and receiver. It also monitors conversations and makes sure that everyone is getting along.

The session Layer does all this by using three simple steps:

1.     Connection establishment

2.     Data transfer and

3.     Connection release

Connection establishment: It initiates the dialogue between two systems. It uses networking protocols to find a dialogue path and communications media to send messages back and forth (data transfer).

Data Transfer: During data transfer,the session layer manages the dialogue and ensures reliable conversations through simplex, half duplex or full duplex transmissions. Finally, connection release ends the dialogue and closes the connection. Incidentally, connection release can be either planned or accidental.

Connection release: Finally, connection release ends the dialogue and closes the connection. Incidentally, connection release can be either planned or accidental.

Presentation Layer

The Presentation Layer is the OSI translator. It transforms the upper-layer message into a mutually agreed-upon format. At this point, the dialogue is open and data transfer has begun, but before the Application Layer can read the message, the Presentation Layer must take care of two things.

1.     Translation

2.     Encryption

Translation: It is the Presentation Layer’s main task. It is necessary when two system speaking different language try to communicate most of the time. The translation can happen in the variety of ways like Bit order, byte order, the character code and file syntax.

Encryption:It is a secondary task for the Presentation Layer. It is necessary for sensitive data and operating system security, such as default authentication. This level of encryption and decryption supports both public and private keys.

Application Layer

The Application Layer sits at the very top of the OSI Model. It is the ultimate goal of networking i.e. service providing. The Application Layer does the services task as:

1.     Service Advertisement and

2.     Service Availability

The Application Layer uses special networking protocols to provide file, print, message, application and database services. Service advertisement lets other systems and users know what services are available. Providers use active or passive techniques to define the scope of their network services.

Service Availability is the next step. Once a service has been advertised, it must be made available. Service availability can be accomplished in several ways including OS Call Interception, remote operation, and collaborative computing.