A computer network consists of two or more computers that are linked in order to share resources such as printers and CD-ROMs, exchange files, or allow electronic communications. The computers on a computer network may be linked through cables, telephone lines, radio waves, satellites, or infrared light beams.

 

Computer network can be classified on the basis of following features :

 

By Scale: Computer networks may be classified according to the scale :

 

• Local Area Network (LAN)
• Metropolitan Area Network (MAN)
• Wide Area Network (WAN)

 

By Connection Method: Computer networks can also be classified according to the hardware technology that is used to connect the individual devices in the network such as Optical fibre, Ethernet, Wireless LAN.

 

By Functional Relationship (Network Architectures) : Computer networks may be classified according to the functional relationships which exist between the elements of the network. This classification also called computer architecture. There are two type of network architecture :

 

• Client-Server
• Peer-to-Peer Architecture
•  

By Network Topology: Network Topology signifies the way in which intelligent devices in the network see their logical or physical relations to one another. Computer networks may be classified according to the network topology upon which the network is based, such as :

 

• Bus Network
• Star Network
• Ring Network
• Mesh Network
• Star-Bus Network
• Tree or Hierarchical Topology Network
•  

Advantages of Network

 

The following are the distinct notes in favor of computer network.

 

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

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

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

d. Networking also provides the function of back-up.

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

 

Explain Network Services

 

1. Network services are the thing that a network can do. The major networking services are

2. File Services: This includes file transfer, storage, data migration, file update, synchronization and achieving.

3. Printing Services: This service produces shared access to valuable printing devices.

4. Message Services: This service facilitates email, voice mails and coordinate object oriented applications.

5. Application Services: This services allows to centralize high profile applications to increase performance and scalability

6. Database Services: This involves coordination of distributed data and replication

Switching: Every time in computer network you access the internet or another  computer network outside your immediate location, your messages are sent through a maze of transmission media and connection devices. The mechanism for moving information between different computer network and network segment is called switching in computer network.

 

For example: - whenever a telephone called is placed, there are numerous junctions in the communication path that perform this movement of data from one network onto another network.

File servers. -With a file server, the Computer Network client passes requests for computer network files or file records over a computer network to the file server. This form of computer network data service requires large bandwidth and can slow a computer network with many users down considerably. Traditional LAN (Local area Network) computing allows users to share resources, such as data files and peripheral devices, by moving them from standalone PCUs onto a Networked File Server (NFS).

 

 

Database servers-In database servers, clients passes SQL (Structured Query Language) requests as messages to the server and the results of the query are returned over the network. The code that processes the SQL request and the data resides on the server allowing it to use its own processing power to find the requested data, rather than pass all the records back to a client and let it find its owndata as was the case for the file server.

 

Transaction servers- Clients invoke remote procedures that reside on servers which also contains an SQL database engine. There are procedural statements on the server to execute a group of SQL statements (transactions) which either all succeed or fail as a unit. The applications based on transaction servers are called On-line Transaction Processing (OLTP) and tend to be mission-critical applications which require 1-3 second response time, 100% of the time and require tight controls over the security and integrity of the database.

 

The communication overhead in this approach is kept to a minimum as the exchange typically consists of a single request/reply (as opposed to multiple SQL statements in database servers). Application servers are not necessarily database centered but are used to server user needs, such as. download capabilities from Dow Jones or regulating a electronic mail process. Basing resources on a server allows users to share data, while security and management services, which are also based in the server, ensure data integrity and security.

IEEE 802.4 Token Bus : In token bus Computer network station must have possession of a token before it can transmit on the computer network. The IEEE 802.4 Committee has defined token bus standards as broadband computer networks, as opposed to Ethernet's baseband transmission technique. Physically, the token bus is a linear or tree-shape cable to which the stations are attached

The topology of the computer network can include groups of workstations connected by long trunk cables. Logically, the stations are organized into a ring. These workstations branch from hubs in a star configuration, so the network has both a bus and star topology. Token bus topology is well suited to groups of users that are separated by some distance. IEEE 802.4 token bus networks are constructed with 75-ohm coaxial cable using a bus topology. The broadband characteristics of the 802.4 standard support transmission over several different channels simultaneously.

 When the logical ring is initialized, the highest numbered station may send the first frame. The token and frames of data are passed from one station to another following the numeric sequence of the station addresses. Thus, the token follows a logical ring rather than a physical ring. The last station in numeric order passes the token back to the first station. The token does not follow the physical ordering of workstation attachment to the cable. Station 1 might be at one end of the cable and station 2 might be at the other, with station 3 in the middle.

 In such a case, there is no collision as only one station possesses a token at any given time. In token bus, each station receives each frame; the station whose address is specified in the frame processes it and the other stations discard the frame.

                         

 

 

MAC Sublayer Function

 

 

 

• When the ring is initialized, stations are inserted into it in order of station address, from highest to lowest.

 

• Token passing is done from high to low address.

 

• Whenever a station acquires the token, it can transmit frames for a specific amount of time.

 

• If a station has no data, it passes the token immediately upon receiving it.

 

• The token bus defines four priority classes, 0, 2, 4, and 6 for traffic, with 0 the lowest and 6 the highest.

 

• Each station is internally divided into four substations, one at each priority level i.e. 0,2,4 and 6.

 

• As input comes in to the MAC sublayer from above, the data are checked for priority and routed to one of the four substations.

 

• Thus each station maintains its own queue of frames to be transmitted.

 

• When a token comes into the station over the cable, it is passed internally to the priority 6 substation, which can begin transmitting its frames, if it has any.

 

• When it is done or when its time expires, the token is passed to the priority 4 substation, which can then transmit frames until its timer expires. After this the token is then passed internally to priority 2 substation.

 

• This process continues until either the priority 0 substation has sent all its frames or its time expires.

 

• After this the token is passed to the next station in the ring.

 

 

 

Frame format of Token Bus

 

 

 

The various fields present in the frame format are

 

 

 

1. Preamble: This. Field is at least 1 byte long. It is used for bit synchronization.

                      

 

2. Start Delimiter: This one byte field marks the beginning of frame.

 

3. Frame Control: This one byte field specifies the type of frame. It distinguishes data frame from control frames. For data frames it carries frame's priority. For control frames, it specifies the frame type. The control frame types include. token passing and various ring maintenance frames, including the mechanism for letting new station enter the ring, the mechanism for allowing stations to leave the ring.

 

4. Destination address: It specifies 2 to 6 bytes destination address.

 

5. Source address: It specifies 2 to 6 bytes source address.

 

6. Data: This field may be upto 8182 bytes long when 2 bytes addresses are used & upto 8174 bytes long when 6 bytes address is used.

 

7. Checksum: This 4 byte field detects transmission errors.

 

8. End Delimiter: This one byte field marks the end of frame.

 

The various control frames used in token bus are:

                         

The two different computer network connection types are

(A) Point-to-Point Connection
(B) Multipoint Connection.

A point-to-point connection is a direct link between two devices such as a computer and a printer. It uses dedicated link between the devices. The entire capacity of the link is used for the transmission between those two devices. Most of today's point-to-point connections are associated with modems and PSTN (Public Switched Telephone Network) communications. In point to point networks, there exist many connections between individual pairs of machines.

To move from sources to destination, a packet (short message) may follow different routes. In networking, the Point-to-Point Protocol (PPP) is a data link protocol commonly used in establishing a direct connection between two networking nodes. It can provide connection authentication, transmission encryption, and compression PPP is used over many types of physical networks including serial cable, phone line, trunk line, cellular telephone, specialized radio links, and fiber optic links such as SONET. PPP is also used over Internet access connections (now marketed as "broadband").

 

Internet service providers (ISPs) have used PPP for customer dial-up access to the Internet, since IP packets cannot be transmitted over a modem line on their own, without some data link protocol. Two encapsulated forms of PPP, Point-to-Point Protocol over Ethernet (PPPoE) and Point-to-Point Protocol over ATM (PPPoA), are used most commonly by Internet Service Providers (ISPs) to establish a Digital Subscriber Line (DSL) Internet service connection with customers.

PPP is commonly used as a data link layer protocol for connection over synchronous and asynchronous circuits, where it has largely superseded the older Serial Line Internet Protocol (SLIP) and telephone company mandated standards (such as Link Access Protocol, Balanced (LAPB) in the X.25 protocol suite). PPP was designed to work with numerous network layer protocols, including Internet Protocol (IP), TRILL, Novell's Internetwork Packet Exchange (IPX), NBF and AppleTalk.

                           

B) Multipoint Connection.

A multipoint connection is a link between three or more devices. It is also known as Multi-drop configuration. The networks havjng multipoint configuration are called Broadcast Networks. In broadcast network, a message or a packet sent by any machine is received by all other machines in a network. The packet contains address field that specifies the receiver. Upon receiving a packet, every machine checks the address field of the packet. If the transmitted packet is for that particular machine, it processes it; otherwise it just ignores the packet. 

                             

Broadcast network provides the provision for broadcasting & multicasting. Broadcasting is the process in which a single packet is received and processed by all the machines in the network. It is made possible by using a special code in the address field of the packet. When a packet is sent to a subset of the machines i.e. only to few machines in the network it is known as multicasting. Historically, multipoint connections were used to attach central CPs to distributed dumb terminals. In today's LAN environments, multipoint connections link many network devices in various configurations

Types of Computer Networks can be classified on various properties. The Computer networks can also be classified on the basis of Computer network technology used by them. There are two types of Computer networks in this category.

1. Broadcast Networks. In broadcast networks, a single communication channel is shared among all the computers of the network. This means, all the data transportation occurs through this shared channel. The data is transmitted in the form of packets. The packets transmitted by one computer are received by all others in the network. The destination of packet is specified by coding the address of destination computer in the address field of packet header.

 

On receiving a packet, every computer checks whether it is intended for it or not. If the packet is intended for it, it is processed otherwise, it is discarded. There is another form of broadcast networks in which the packets transmitted by a computer are received by a particular group of computers. This is called as "Multicasting".

 

2. Point to Point or Store and Forward Networks. This is the other type of networks on the basis of transmission technology. The store and forward networks consist of several interconnected computers and networking devices. The data is transmitted in the form of packets. Each packet has its own source and destination address.

 

To go from a source to a destination, a packet on this type of network may first have to visit one or more intermediate devices or computers that are generally called as "routers". The packets are stored on an intermediate router unless the output line is free. When the output line is free, it is forwarded to the next router. The routing algorithms are used to find a path from the source to destination. The routing algorithms play a very important role in this type of network.

 

Modem is abbreviation for Modulator – Demodulator. Modems are used for data transfer from one computer network to another computer network through telephone lines. The computer network works in digital mode, while analog technology is used for carrying massages across phone lines.

Modulator converts information from digital mode to analog mode at the transmitting end and demodulator converts the same from analog to digital at receiving end. The process of converting analog signals of one computer network into digital signals of another computer network so they can be processed by a receiving computer is referred to as digitizing.

When an analog facility is used for data communication between two digital devices called Data Terminal Equipment (DTE), modems are used at each end. DTE can be a terminal or a computer.

                          

The modem at the transmitting end converts the digital signal generated by DTE into an analog signal by modulating a carrier. This modem at the receiving end demodulates the carrier and hand over the demodulated digital signal to the DTE.

                           

The transmission medium between the two modems can be dedicated circuit or a switched telephone circuit. If a switched telephone circuit is used, then the modems are connected to the local telephone exchanges. Whenever data transmission is required connection between the modems is established through telephone exchanges.

Types of Modems

• Modems can be of several types and they can be categorized in a number of ways.

• Categorization is usually based on the following basic modem features:

1. Directional capacity: half duplex modem and full duplex modem.

2. Connection to the line: 2-wire modem and 4-wire modem.

3. Transmission mode: asynchronous modem and synchronous modem.

Half duplex and full duplex Modems

Half duplex

 

1. A half duplex modem permits transmission in one direction at a time.

2. If a carrier is detected on the line by the modem, I gives an indication of the incoming carrier to the DTE through a control signal of its digital interface.

3. As long as they camel' IS being received; the modem does not give permission to the DTE to transmit data.

                           

Full duplex

 

• A full duplex modem allows simultaneous transmission in both directions.

• Therefore, there are two carriers on the line, one outgoing and the other incoming. Wire and 4-wire Modems

• The line interface of the modem can have a 2-wire or a 4-wire connection to transmission medium. 4-wire Modem

• In a 4-wire connection, one pair of wires is used for the outgoing carrier and the other pair is used for incoming carrier.

• Full duplex and half duplex modes of data transmission are possible on a 4- wire connection.

• As the physical transmission path for each direction is separate, the same carrier frequency can be used for both the directions.

                             

2-wire Modem

• 2-wire modems use the same pair of wires for outgoing and incoming carriers.

• A leased 2-wireconrlection is usually cheaper than a 4-wire connection as only one pair of wires is extended to the subscriber's premises.

• The data connection established through telephone exchange is also a 2-wire connection.

• In 2-wire modems, half duplex mode of transmission that uses the same frequency for the incoming and outgoing carriers can be easily implemented.

• For full duplex mode of operation, it is necessary to have two transmission channels, one for transmit direction and the other for receive direction.

• This is achieved by frequency division multiplexing of two different carrier frequencies. These carriers are placed within the bandwidth of the speech channel.

                                  

                                  

Asynchronous & Synchronous Modems

Asynchronous Modem

• Asynchronous modems can handle data bytes with start and stop bits.

• There is no separate timing signal or clock between the modem and the DTE.

• The internal timing pulses are synchronized repeatedly to the leading edge of the start pulse .

                                

Synchronous Modem

• Synchronous modems can handle a continuous stream of data bits but requires a clock signal.

• The data bits are always synchronized to the clock signal.

• There are separate clocks for the data bits being transmitted and received.

• For synchronous transmission of data bits, the DTE can use its internal clock and supply the same to the modem.

                              

 

Modulation techniques used for Modem:

 

 

The basic modulation techniques used by a modem to convert digital data to analog signals are :

 

 

 

• Amplitude shift keying (ASK).

 

• Frequency shift keying (FSK).

 

• Phase shift keying (PSK).

 

• Differential PSK (DPSK).

 

 

 

These techniques are known as the binary continuous wave (CW) modulation.

 

• Modems are always used in pairs. Any system whether simplex, half duplex or full duplex requires a modem at the transmitting as well as the receiving end.

 

• Thus a modem acts as the electronic bridge between two worlds - the world of purely digital signals and the established analog world.

Ethernet : IEEE 802.3 Local Area Network (LAN) Protocols : Ethernet protocols refer to the family of local-area network (LAN) technology covered by the IEEE 802.3. It is working examplc of the more general carrier sense multiple access with collision detect (CSMA/CD). In the Ethernet Computer Network standard, there are two modes of operation: half-duplex and full-duplex modes. In the half duplex mode, data are transmitted using the popular Carrier-SenseMultiple Access/Collision Detection (CSMA/CD) protocol on as hared medium.

The main disadvantages of the half-duplex are the efficiency and distance limitation, in which the link distance islimited by the minimum MAC frame size. This restriction reducesthe efficiency drastically for high-rate transmission. Therefore, thecarrier extension technique is used to ensure the minimum framesize of 512 bytes in Gigabit Ethernet to achieve a reasonable linkdistance.Four data rates are currently defined for operation over opticalfiber and twisted-pair cables :

 

• 10 Mbps      -     10Base-T Ethernet (IEEE 802.3)
• 100 Mbps    -    Fast Ethernet (IEEE 802.3u)
• 1000 Mbps -    Gigabit Ethernet (IEEE 802.3z)
• 10-Gigabit  -     10 Gbps Ethernet (IEEE 802.3ae).

The Ethernet is a multi-access network, meaning that a set of nodes send and receive frames over a shared link you can, therefore, think of an Ethernet or being like a bus that has multiple stations plugged into it. The "carrier sense" in CSMA/CD means that all the nodes can distinguish between an idle and a busy link and "collision detect" means that a node listens as it transmits and can therefore detect when a frame it is transmitting has collided with a frame transmitted by another node.

The Ethernet has its root in an early packet radio network, called ALOHA, like, the ALOHA, the problem faced by the Ethernet is how to mediate access to a shared medium fairly and efficiently. In ALOHA, the medium was the atmosphere, while in Ethernet the medium is coax cable.

In the earliest days, 10-Mbps Ethernet war used, but now it has been extended to include a 100-Mbps version called Fast Ethernet and a 1000-Mbps version called Gigabit Ethernet.

 

The Ethernet System consists of three basic elements :

 

(1) The physical medium used to carry Ethernet signals between computers,

(2) a set of medium access control rules embedded in each Ethernet interface that allow multiple computers to fairly arbitrate access to the shared Ethernet channel, and

(3) an Ethernet frame that consists of a standardized set of bits used to carry data over the system.

As with all IEEE 802 protocols, the ISO data link layer is divided into two IEEE 802 sub-layers, the Media Access Control (MAC) sub-layer and the MAC-client sub-layer. The IEEE 802.3 physical layer corresponds to the ISO physical layer.

Each Ethernet-equipped computer operates independently of all other stations on the network: there is no central controller. All stations attached to an Ethernet are connected to a shared signaling system, also called the medium. To send data a station first listens to the channel, and when the channel is idle the station transmits its data in the form of an Ethernet frame, or packet.

After each frame transmission, all stations on the network must contend equally for the next frame transmission opportunity. Access to the shared channel is determined by the medium access control (MAC) mechanism embedded in the Ethernet interface located in each station. The medium access control mechanism is based on a system called Carrier Sense Multiple Access with Collision Detection (CSMA/CD).

As each Ethernet frame is sent onto the shared signal channel, all Ethernet interfaces look at the destination address. If the destination address of the frame matches with the interface address, the frame will be read entirely and be delivered to the networking software running on that computer. All other network interfaces will stop reading the frame when they discover that the destination address does not match their own address.

During 20th century the most important technology has been the information gathering, its processing and distribution. The computers and communications have been merged together and their merger has had a profound effect on the manner in which computer systems are organized.

The old model in which a single computer used to serve all the computational needs of an organization has been replaced by a new one in which a large number of separate but interconnected computers do the job. Such systems are called as computer networks.

 

• Two computers are said to be interconnected if they interchange information. The connection between the separate computers can be done via a copper wire, fiber optics, microwaves or communication satellite.

• A printer, computer, or any machine that is capable of communicating on the network is referred to as a device or node.

• We can also say that computer network is an interconnection of various computers to share software, hardware and data through a communication medium between them. The computers connected in a network share files, folders, applications and resources like scanner, web-cams, printers etc.

 

• The best example of computer network is the Internet.

 

A computer network is an interconnection of various computers to share software, hardware, resources and data through a communication medium between them.

 

A Computer Networking is a set of autonomous computers that permits distributed processing of the information and data and increased Communication of resources.

 

Any Computer Networking communication need a sender, a receiver and a communication medium to transfer signal or Data from sender to the receiver. We need sender, receiver, communication channel, protocols and operating system to establish a computer networking.

 

A networks model describes the organization of various computers in a network for using resources.

Computer Network Model

A computer networks communication can be based on centralized, distributed or collaborative computing. Centralized computing involves many workstations or terminals, connected to one central mainframe or other powerful computer. Distributed computing interconnects one or more personal computers and allows various services like Data sharing, hardware sharing resources sharing or network sharing. The collaborative computing is the combination of centralized and distributed computing.

 

1. Centralized computing.

• It is also known as client-server computing.

• In this type of system, multiple computers are joined to one powerful mainframe computer.

• The server or mainframe computer has huge storage and processing capabilities.

• The computers that are connected to the mainframe or server are called Clients or Nodes.

• These nodes are not connected to each other; they are only connected to server.

 

2. Distributed computing

 • If one computer can forcibly start, stop or control another the computers are not autonomous. A system with one control unit and many slaves, or a large computer with remote printers and terminals is not called a computer network, it is called a Distributed System.

• Distributed computing means that the task is divided among multiple computers.

• Distributed computing interconnects one ore more personal computers or Workstations.

• In distributed computing, the nodes are capable of processing their own data and rely on network for services other than data processing.

• It allows various services like network sharing, hardware sharing and file sharing.

 

3. Collaborative computing / Hybrid computing

 

• It is the combination of centralized and distributed computing

             

• In collaborative computing, the nodes are able to serve the basic needs of their users but they are dependent on some other computers for processing some specific request.

Computer Network Classification

The local area network communication can be constructed by using server based model or peer to peer model. In peer to peer networks, the individual clients share data and resources but no one computer is treated as server.

Networks can be classified into local area Networks, metropolitan area Networks and wide area networks. Local area network is the small network that cover a small area of Network. Metropolitan area networks are created by combining various local area networks. Wide area networks are the biggest networks that provide connectivity across the globe.

Networks provide the benefits of exchanging information or Data, sharing resources, reducing system costs, increased reliability and flexible working environment.

Computer Network topology

The physical arrangement of computers in a communication network is called as topology. In star topology, every system on the network is connected to a central controller called Hub and all the data is transmitted through this. Star topology is very easy to install and configure. In bus topology, a single cable acts as a backbone of the communication network and all the nodes or computers are attached to it by using T connectors.

 

Uses of Computer Networks

 

 

The computer networks are playing an important role in providing services to large organizations as well as to the individual common man.

 

 

 

Service Provided by the Network for Companies:

 

 

 

• Many organizations have a large number of computers in operation. These computers may be within the same building, campus, city or different cities.

 

• Even though the computers are located in different locations, the organizations want to keep track of inventories, monitor productivity, do the ordering and billing etc.

 

• The computer networks are useful to the organizations in the following ways:

 

 

 

1. Resource sharing.

 

2. For providing high reliability.

 

3. To save money.

 

4. It can provide a powerful communication medium.

 

 

 

1. Resource sharing:

 

 

 

• It allows all programs, equipments and data available to anyone on the network irrespective of the physical location of the resource and the user.

 

• Show in Fig (a) and (b) which shows a printer being shared and different information being shared.

                          

 

                                

 

2. High reliability due to alternative sources of data:

 

 

 

• It provides high reliability by having alternative sources of data. For e.g. all files could be replicated on more than one machines, so if one of them is unavailable due to hardware failure or any other reason, the other copies can be used.

 

• The aspect of high reliability is very important for military, banking, air traffic control, nuclear reactor safety and many other applications where continuous operations is a must even if there are hardware or software failures.

 

 

 

3. Money saving:

 

 

 

• Computer networking is an important financial aspect for organizations because it saves money.

 

• Organizations can use separate personal computer one per user instead of using mainframe computer which are expensive.

 

• The organizations can use the workgroup model (peer to peer) in which all the PCs are networked together and each one can have the access to the other for communicating or sharing purpose.

 

• The organization, if it wants security for its operation it can go in for the domain model in which there is a server and clients. All the clients can communicate and access data through the server.

 

• The whole arrangement is called as client -server model.

 

                                     

 

Client: The individual workstations in the network are called as clients.

 

Server:

 

The central computer which is more powerful than the clients and which allows the clients to access its software and database is called as the server .

 

• Server computers typically are more powerful than client computers or are optimized to function as servers.

 

 

 

Communication in client-server configuration:

 

 

                                     

 

• The client places a request on the server machine when he wants an access to the centralized resources.

 

• The server responds to this request and sends the signal accordingly to the client.

 

• The software run at the client computer is called as client program. This software configures the computer to act as a client.

 

• Similarly the software run on the server computer IS called as server program. It configures a computer to act as a server.

 

 

 

4. Communication medium:

 

 

 

• A computer network provides a powerful communication medium among widely separated employees.

 

• Using network it is easy for two or more employees, who are separated by geographical locations to work on a report, document or R and D simultaneously i.e. on -line.

 

 

 

Networks for People:

 

 

 

• Starting in 1990s, the computer networks began to start delivering services to the private individuals at home.

 

• The computer networks offer the following services to an individual person:

 

 

 

1. Access to remote information

 

2. Person to person communication

 

3. Interactive entertainment.

 

 

 

1. Access to remote information:

 

 

 

Access to remote information involves interaction· between a person and a remote database. Access to remote information comes in many forms like:

 

(i) Home shopping, paying telephone, electricity bills, e-banking, on line share market etc.

 

(ii) Newspaper is. On-line and is personalized, digital library consisting of books, magazines, scientific journals etc.

 

(iii) World wide web which contains information. about the arts, business, cooking, government, health, history, hobbies, recreation, science, sports etc.

 

 

 

2. Person to person communication:

 

 

 

Person to person communication includes:

 

(i) Electronic-mail (e-mail)

 

(ii) Real time e-mail i.e. video conferencing allows remote users to communicate with no delay by seeing and hearing each other. Video-conferencing is being used for remote school, getting medical opinion from distant specialists etc.

 

 

 

(iii) Worldwide newsgroups in which one person posts a message and all other subscribers to the newsgroup can read it or give their feedbacks.

 

 

 

3. Interactive entertainment:

 

 

 

Interactive entertainment includes:

 

 

 

(i) Multiperson real-time simulation games.

 

(ii) Video on demand.

 

(iii) Participation in live TV programmes likes quiz, contest, discussions etc.

 

In short, the ability to merge information, communication and entertainment will surely give rise to a massive new industry based on computer networking.

Router: Routers are devices (computers) containing software that help in determining the best path out of the available paths, for a particular transmission. They consist of a combination of hardware and software. The hardware includes the physical interfaces to the various networks in the internet work. The two main pieces of software in a router are the operating system and the routing protocol.

 

 

Routers use logical and physical addressing to connect two or more logically separate networks. They accomplish this connection by organizing the large network into logical network segments or sub-networks. Each of these sub networks is given a logical address. This allows the networks to be separate but still access each other and exchange data when necessary. Data is grouped into packets, or blocks of data. Each packet, in addition to having a physical device address, has a logical network address.

 

Routers are frequently used to interconnect identical networks as well as to interconnect networks with different types of hardware. Compared to one giant LAN, a series of smaller LANs connected through routers has some highly desirable benefits.

 

1. The foremost of these is security. LANs operate in a broadcast mode. Information retrieved or transmitted goes on to the network and traverses the entire cable system. Only the station specifically addressed actually reads the data, but the data is physically presented to each station.
2. Another benefit of routers is' reliability. If one network goes down because the server has stopped functioning or because of a fault in the cable, other networks and departments served by routers are not affected. The internet ting routers isolate the affected network, hence the unaffected networks although connected, experience no work stoppage or data loss.
3. Another benefit of using routers is performance enhancement within the individual network. Suppose that a network has 12 workstations, each of which generates approximately the same amount of traffic. In a single-network environment, all the traffic for those 12 workstations goes on the same cable. But if the network is split into 2 networks of 6 workstations each, the traffic load is cut to half. Each network has its own server and hard disk and is largely self-contained; so fewer PCs make demands on the network cabling system.
4. A final benefit of routers is greater networking range. In some networks, for instance, cable length cannot exceed 1,000 meters. A router effectively nullifies this limitation by performing the function of a repeater and reconstituting the signal. Physical range can be whatever is required by the particular installation, provided that a router is installed before the maximum cable range is exceeded.

 

Types of Routers

 

Many kinds of Routers are present in the current market that are depending on the need of the any enterprises. Cisco is the major manufacturer of router mostly Cisco router are used to set up network of large corporation and even of the ISPs. Broadly Routers are divided into three categories that depending upon the corporate needs.

 

1. Internet connectivity routers

 

These routers are used in Border Gateway Protocol for exchange information. These type of routers are further divided into four more categories.

 

1.1. Edge router

 

A Edge router or edge device is routes data packets between LANs and an Asynchronous Transfer Mode (ATM) network.

 

1.2. Subscriber edge router (SER)

 

Subscriber edge router (SER) Also known as Customer Edge router.Customer Edge router use EBGP protocol, it used in (enterprise) organization.

 

1.3. Inter provider border router

 

Inter provider border router are used for Interconnecting ISPs. Inter provider border router use BGP protocol to speake.

 

1.4. Core router

 

When a router become the integral part of the LAN(Local Area Network) is called core router. Core router uses internal BGP protocol.Core routers have specialized functions in VPN,that is based on a combination of BGP and Multi-Protocol Label Switching protocols.

 

One Another Type of Router is SOHO routers. It is used for connection other networks in small geographical area it is known as SOHO connectivity.

 

How Routers Works

 

Routers are interconnectivity devices that is used to transfer the datum packets along networks by visualizing the networks path. Routers visualizing the networks path to many networks such as Electronic networks, Transport networks and phone networks. Two ways are exist for routers operation using either control plane or forwarding plane. In control plane the router sends the precise data packets to their specific location. On the other hand in forwarding plane router does not remember the sending or receiving information about the packets.

 

Advantages of Routers

 

Routers route the data in an organized way. Routers generate a reliable connection between hosts. Routers is used for alternatively incase the main is fail to transfer data.

Computer Networks fall into three classes regarding the size, distance and the structure namely: LAN (Local Area Network), MAN (Metropolitan Area Network), WAN (Wide Area Network).

                        

Types of Networks

LAN (Local Area Network)

A Local Area Network is a privately owned computer network covering a small Networks geographical area, like a home, office, or groups of buildings e.g. a school Network. A LAN is used to connect the computers and other network devices so that the devices can communicate with each other to share the resources. The resources to be shared can be a hardware device like printer, software like an application program or data. The size of LAN is usually small. The various devices in LAN are connected to central devices called Hub or Switch using a cable.

 

Now-a-days LANs are being installed using wireless technologies. Such a system makes use of access point or APs to transmit and receive data. One of the computers in a network can become a server serving all the remaining computers called Clients.

 

For example, a library will have a wired or wireless LAN Network for users to interconnect local networking devices e.g., printers and servers to connect to the internet.

LAN offers high speed communication of data rates of 4 to 16 megabits per second (Mbps). IEEE has projects investigating the standardization of 100 Gbit/s, and possibly 40 Gbit/s. LANs Network may have connections with other LANs Network via leased lines, leased services.

 

Types of LAN

 

There are basically two types of Local Area Networks namely: ARCnet and Ethernet.

 

ARCNET (Attached Resource Computer NETwork)

 

ARCNET is one of the oldest, simplest, and least expensive types of Local-Area Network protocol, similar in purpose to Ethernet or Token Ring. ARCNET was the first widely available networking system for microcomputers and became popular in the 1980s for office automation tasks. ARCnet was introduced by Datapoint Corporation in 1977.  

A special advantage of ARCNET is that it permits various types of transmission media - twisted-pair wire, coaxial cable, and fiber optic cable - to be mixed on the same network. The specification is ANSI 878.1. It can have up to 255 nodes per network.

A new specification, called ARCnet Plus, will support data rates of 20 Mbps

Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired local area network technologies. Ethernet uses a bus or star topology Network and supports data transfer rates of 10 Mbps.

Ethernet Network uses the CSMA/CD access method to handle simultaneous demands. It is one of the most widely implemented LAN standards. A newer version of Ethernet Network, called 100Base-T (or Fast Ethernet), supports data transfer rates of 100 Mbps.

And the newest version, Gigabit Ethernet supports data rates of 1 gigabit (1,000 megabits) per second. Ethernet is a physical and data link layer technology for local area networks (LANs). Ethernet Network was invented by engineer Robert Metcalfe.

MAN (Metropolitan Area Networks)

MAN stands for Metropolitan Area Networks is one of a number of types of networks. A MAN is a relatively new class of network. MAN is larger than a local area network and as its name implies, covers the area of a single city. MANs rarely extend beyond 100 KM and frequently comprise a combination of different hardware and transmission media. It can be single network such as a cable TV network, or it is a means of connecting a number of LANs into a larger network so that resources can be shared LAN to LAN as well as device to device.

 

                              

A MAN can be created as a single network such as Cable TV Network, covering the entire city or a group of several Local Area Networks (LANs). It this way resource can be shared from LAN to LAN and from computer to computer also. MANs are usually owned by large organizations to interconnect its various branches across a city.

 

MAN is based on IEEE 802.6 standard known as DQDB (Distributed Queue Dual Bus). DQDB uses two unidirectional cables (buses) and all the computers are connected to these two buses. Each bus has a specialized device that initiates the transmission activity. This device is called head end. Data that is to be sent to the computer on the right hand side of the sender is transmitted on upper bus. Data that is to be sent to the left hand side of the sender is transmitted on lower bus.

                            

The two most important components of MANs are security and standardization. Security is important because information is being shared between dissimilar systems. Standardization is necessary to ensure reliable data communication.

A MAN usually interconnects a number of local area networks using a high-capacity backbone technology, such as fiber-optical links, and provides up-link services to wide area networks and the Internet.

The Metropolitan Area Networks (MAN) protocols are mostly at the data link level (layer 2 in the OSI model), which are defined by IEEE, ITU-T, etc.

WAN (Wide Area Networks)

A wide area network (WAN) is a telecommunication network. A wide area network is simply a LAN of LANs or Network of Networks. WANs connect LANs that may be on opposite sides of a building, across the country or around the world. WANS are characterized by the slowest data communication rates and the largest distances. WANs can be of two types: an enterprise WAN and Global WAN.

                           

Computers connected to a Wide Area Networks are often connected through public networks, such as the telephone system. They can also be connected through leased lines or satellites. The largest WAN in existence is the Internet. Some segments of the Internet, like VPN based extranets, are also WANs in themselves. Finally, many WANs are corporate or research networks that utilize leased lines.

Numerous WANs have been constructed, including public packet networks, large corporate networks, military networks, banking networks, stock brokerage networks, and airline reservation networks.

Organizations supporting WANs using the Internet Protocol are known as Network Service Providers (NSPs). These form the core of the Internet.

By connecting the NSP WANs together using links at Internet Packet Interchanges (sometimes called "peering points") a global communication infrastructure is formed.

WANs (wide area networks) generally utilize different and much more expensive networking equipment than do LANs (Local Area Networks). Key technologies often found in WANs (wide area networks) include SONET, Frame Relay, and ATM.

Clarify Enterprise WANs.

An enterprise WAN (wide area networks) connects an entire organization including all LANs (Local Area Networks) at various sites. This term is used for large, widespread organizations such as corporations, universities and governments.

Clarify Global WANs.

Global WANs (wide area networks) also span the world but they do not have to connect LANS (Local Area Networks) within a single organization. The Internet is an example of a global WAN. It connects diverse locations, organizations and institutions throughout the world. Global WANS (wide area networks) can be public or private. Private WANs (wide area networks) are called Intranet which belongs to an organization. Public WANs (wide area networks) are open to everybody so that anybody can connect and use the resources and services available.

WLANs - Wireless Local Area Networks

WLANs (Wireless Local Area Networks or sometimes referred to as LAWN, for local area wireless network) provide wireless network communication over short distances using radio or infrared signals instead of traditional network cabling.WLANs (Wireless Local Area Networks) is one in which a mobile user can connect to a local area network (LAN) through a wireless (radio) connection

Norman Abramson, a professor at the University of Hawaii, developed the world’s first wireless computer communication network,

A WLAN typically extends an existing wired local area network. WLANs (Wireless Local Area Networks) are built by attaching a device called the access point (AP) to the edge of the wired network. Clients communicate with the AP using a wireless network adapter similar in function to a traditional Ethernet adapter. 

Network security remains an important issue for WLANs (Wireless Local Area Networks). Random wireless clients must usually be prohibited from joining the WLAN. Technologies like WEP raise the level of security on wireless networks to rival that of traditional wired networks. 

The IEEE 802.11 group of standards specify the technologies for wireless LANs. 802.11 standards use the Ethernet

WLAN (Wireless Local Area Networks) hardware was initially so expensive that it was only used as an alternative to cabled LAN in places where cabling was difficult or impossible.

 

All components that can connect into a wireless medium in a network are referred to as stations. All stations are equipped with wireless network interface controllers (WNICs). Wireless stations fall into one of two categories: access points, and clients. Access points (APs), normally routers, are base stations for the wireless network.

They transmit and receive radio frequencies for wireless enabled devices to communicate with. Wireless clients can be mobile devices such as laptops, personal digital assistants, IP phones and other smartphones, or fixed devices such as desktops and workstations that are equipped with a wireless network interface. 

WLAN (Wireless Local Area Networks) types

Private home or small business WLAN

Commonly, a home or business WLAN employs one or two access points to broadcast a signal around a 100- to 200-foot radius. You can find equipment for installing a home WLAN in many retail stores.

With few exceptions, hardware in this category subscribes to the 802.11a, b, or g standards (also known as Wi-Fi); some home and office WLANs now adhere to the new 802.11n standard. Also, because of security concerns, many home and office WLANs adhere to the Wi-Fi Protected Access 2 (WPA2) standard.

Enterprise class WLAN

 

An enterprise class WLAN employs a large number of individual access points to broadcast the signal to a wide area. The access points have more features than home or small office WLAN equipment, such as better security, authentication, remote management, and tools to help integrate with existing networks. These access points have a larger coverage area than home or small office equipment, and are designed to work together to cover a much larger area. This equipment can adhere to the 802.11a, b, g, or n standard, or to security-refining standards, such as 802.1x and WPA2.

Examples:

For WLANs that connect to the Internet, Wireless Application Protocol (WAP) technology allows Web content to be more easily downloaded to a WLAN and rendered on wireless clients like cell phones and PDAs.

 

Storage Area Network (SAN):

A storage area network (SAN) is a type of local area network (LAN) is a high-speed special-purpose network. A SAN typically supports data storage, retrieval and replication on business networks using high-end servers, multiple disk arrays and Fibre Channel interconnection technology.

Storage Area Networks (SANs) technology is similar but distinct from network attached storage (NAS) technology. While SANs traditionally employ low-level network protocols for transferring disk blocks, a NAS device typically works over TCP/IP and can be integrated fairly easily into home computer networks.

The term SAN can sometimes refer to system area networks instead of a storage area network. System area networks are clusters of high performance computers used for distributed processing applications requiring fast local network performance. Storage area networks, on the other, are designed specifically for data management.

SANs support disk mirroring, backup and restore, archival and retrieval of archived data, data migration from one storage device to another and the sharing of data among different servers in a network. SANs can incorporate sub networks with network attached storage (NAS) systems.

Storage Area Networks Make Your Life Easier

Simplification of Storage Administration is now possible because of Storage Area Networks cause cables and storage devices doesn’t need to be moved physically. Moving data from one server into another is now a breeze. Thanks to Storage Area Networks. Life is much easier.

Before, storage area networks process can take as little as half an hour. But this was before and now we can accelerate it.

The boo-table features of Storage Area Networks can also be effective and enable during recovery of data because of certain disaster such as server failure or human error. Storage area networks are great tools in recovering important data and back ups. Distant location doesn’t effect the storage area networks as long as the secondary storage array is working.

This enables storage replication either implemented by disk array controllers, by server software, or by specialized SAN devices. Since IP WAN’s are often the least costly method of long-distance transport, the Fibre Channel over IP (FCIP) and iSCSI protocols have been developed to allow SAN extension over IP networks.

 

In the old model like in physical SCSI layer, it supported a few meters of distance and no guarantee of business continuity when disaster strike.In storage area networks, the disk arrays has accelerated and consolidated in the features like I/O caching, volume cloning and snap shotting making business continuance possible or BCV’s (Business Continuance Volumes).

Campus Area Network (CAN) 

A campus area networks (CANs) is a computer network interconnecting a few local area networks (LANs) within a university campus or corporate campus Network.Campus area network may link a variety of campus buildings.A campus area network is larger than a local area network but smaller than a metropolitan area network (MAN) or wide area network (WAN). CAN can also stand for corporate area network. 

Personal Area Network (PAN)

A personal area network is a computer network organized around an individual person. Personal area networks typically involve a mobile computer,Personal area networks can be constructed with cables or wirelessly.Personal area networks generally cover a Network range of less than 10 meters (about 30 feet).

PAN (Personal Area Network) first was developed by Thomas Zimmerman and other researchers at M.I.T.'s Media Lab and later supported by IBM's Almaden research lab.

Wireless Personal Area Network (WPAN) which is virtually a synonym since almost any personal area network would need to function wirelessly. Conceptually, the difference between a PAN ( personal area network ) and a wireless LAN ( Local Area Network) is that the former tends to be centered around one person Network while the latter is a local area network (LAN) that is connected without wires Network and serving multiple users.

 

Wireless Networks

 

 

• The fastest growing segment of the computer industry is the mobile computers such as notebook computers and personal digital assistant (PDAs).

 

• The wireless networks are becoming increasingly important because the wired connection is not possible in cars or aero planes.

 

• Wireless networks can have many applications. A very common example is the portable office.

 

• People traveling on road often want to make use of their portable electronic equipment for telephone calls, e-mails, faxes, read remote files etc.

 

• Wireless networks can exist on trucks, buses, taxies, aero planes etc. They are used where the telephone systems are destroyed in the event of disasters such as. fires, floods and earthquakes etc.

 

• The wireless networks are important for military.

 

• Wireless networks and mobile computing are related but they are not identical because portable computers are sometimes wired and some wireless computers are not portable.

 

• But some applications are truly mobile wireless applications such as a portable office, inventories being handled by PDAs, etc.

 

• Wireless LAN is another example of wireless network. Direct digital cellular service CDPD (Cellular Digital Packet Data) is now becoming available:

 

• It is possible to have combinations of wired and wireless networking.

The term Network Topology defines the geographic arrangement of computer networking devices. The term Topology refers to the way in which the various nodes or computers of a network are linked together. It describes the actual layout of the computer network hardware. Two or more devices connect to a link; two or more links form a topology. Topology determines the data paths that may be used between any pair of devices of the network.

The selection of a Network Topology for a network can not be done in isolation as it affects the choice of media and the access method used. Because it determines the strategy used in wiring a building for a network and deserves some careful study.

                

The following factors are considered while selecting a topology:

 

1. Cost

2. Reliability

3. Scalability

4. Bandwidth capacity

5. Ease of installation

6. Ease of troubleshooting

7. Delay involved in routing information from one node to another.

Types of Topologies

• While making a selection of a particular topology we consider the relative status of different devices that are to be linked.

• The nodes in a network can have following two relationships:

 

1. Peer to Peer: In this relationship, all the devices in the network have equal status in sharing the link. For example, Ring & Mesh topology.

2. Primary-Secondary: In this, one device controls the traffic and all other devices transmit through primary device. e.g. Star topology.

Basic Network Topology

The three simple Topology that are combined to form a basic Network Topology. They are, Bus Topology, Ring and Star Topology.

 

Bus Topology

 

The physical Bus Network Topology is the simplest and most widely used of the network designs. It consists of one continuous length of cable (trunk) that is shared by all the nodes in the network and a terminating resistor (terminator) at each end that absorbs the signal when it reaches the end of line. Without a terminator the electrical signal would reach the end of copper wire and bounce back, causing errors on the network.

Data communication 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. Bus Network Topology requires a multipoint connection.

All nodes on the bus topology have equal access to the trunk. This is accomplished using short drop cables or direct T-connectors. The number of devices and the length of the trunk can be easily expanded.

 

Advantages of Bus Topology

 

The advantages of physical bus topology are:

1. It uses established standards and it is relatively easy to install and the use for small networks.
2. It requires less media than other topologies.
3. Failure of one node does not affect the network functioning.

2. Cost is less as only one main cable is required and least amount of cable is required to connect computers.

4. Expansion is easier. New node can be easily added by using a connector.

Disadvantages of Bus Topology

 

The disadvantages of bus Topology are:

 

1. If the main central line fails the entire network collapses.
2. The bus networks are difficult to reconfigure, especially when the acceptable number of connections or maximum distances have been reached.
3. 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 knee.
4. Sharing a single communication channel results in slower access time.

5. In this topology, higher network traffic slows down the bus speed. Only one device transmits at a time, other devices wait for their turn. As a result there is no coordination between the devices for reservation of transmission time slots, so data collisions are frequent.

Ring Topology

The physical ring Topology is a circular loop of point-to-point links. Each device connects directly to the ring or indirectly through and interface device or drop cable. Message travel around the ring from node to node in a very organized manner. Each workstation checks the message 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.

 

• In ring topology, the various nodes are connected in form of a ring or circle (physical ring), in which data flows in a circle, from one station to another station.

• It has no beginning or end that needs to be terminated.

• In this topology, each device or node has a dedicated point to point line configuration with only two devices on either side of it.

• Signal is passed along the ring in one direction from one station to another until it reaches destination.

• Each device in ring incorporates a repeater.

• When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along.

 

• There are two kinds of ring topologies:

 

1. Single Ring

2. Dual Ring

 

•. In this topology, each device or node has a dedicated point to point line configuration with only two devices on either side of it.

• Signal is passed along the ring in one direction from one station to another until it reaches destination.

• Each device in ring incorporates a repeater.

• When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along.

 

• There are two kinds of ring topologies:

 

1. Single Ring

2. Dual Ring

                                

1. Single ring - In single ring network, a single cable is shared by all the devices and data travel only in one direction.

Each device waits for its turn and then transmits. When the data reaches its destination, another device can transmit.

2. Dual ring: This topology uses two rings to send the data, each in different direction. Thus allowing more packets to be sent over the network.

 

Advantages of Ring Topology

 

The advantages of Ring Topology are:

1. 1.They are very easy to troubleshoot because each device incorporates a repeater.
2. 2.A special internal feature called beaconing allows troubled workstations to identify themselves quickly.

3. There is no master computer on controller. Every computer has equal chance to place the data and access the token.
4. There are no collisions.
5. Data packets travel at greater speeds.
6. It is easier to locate the problems with device and cable i.e. fault isolation is simplified. If one device does not receive a signal within a specified time, it can issue an alarm. This alarm alerts the network operator to the problem and its location.

Disadvantages of Ring Topology

 

The disadvantages of ring topologies are:

1. A ring network requires more cable than a bus network.
2. A break in cable ring brings down entire network (in case of single ring).
3. Adding or removing the node disturbs the network activity.
4. In ring network, communication delay is directly proportional to the number of nodes in the network. Hence addition of new nodes in the network also increases communication delay.
5. It is considerably difficult to install and reconfigure ring Topology

1. 6.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 device has a dedicated point-to-point link to the central hub. There is no direct link between these computers and the computers can communicate via central controller only. This strategy prevents troublesome collisions and keeps the lines of communications open and free of traffic.

The routing function is performed by the central controller which centrally controls communication between any two computers by establishing a logical path between them. It means that if one computer A wants to send data to another computer B, Computer A sends the data to the controller & this controller then sends the data to computer B.

                            

This Topology, obviously, require a great deal of cabling. This design provides an excellent platform for reconfiguration and trouble-shooting. Changes to the network are as simple as plugging another segment into the hub and a break in the LAN is easy to isolate and doesn't affect the rest of the network.

 

Advantages of Star Topology

 

The benefits of star topology are:

1. It is easier to add new node or modify any existing node without disturbing network i.e. expansion is easier.

2. Addition of new node does not increase communication delay.

3. If any local computer or link fails, the entire system does not collapse. Only that link or computer is affected.

4. It is easy to find device and cable problems i.e. fault identification and isolation is easier.

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

 

Disadvantages of Star Topology

 

The disadvantages are considered as follows:

1. If the central controller or hub fails, entire system collapses.

2. Cabling cost is more as each node is connected individually to the hub.

3. Requires more cable than most topologies

4. Moderately difficult to install

 

Dual-ring Topology : The type of  Topology in which each of the nodes of the network is connected to two other nodes in the network, with two connections to each of these nodes, and with the first and last nodes being connected to each other with two connections, forming a double ring – the data flows in opposite directions around the two rings, although, generally, only one of the rings carries data during normal operation, and the two rings are independent unless there is a failure or break in one of the rings, at which time the two rings are joined to enable the flow of data to continue using a segment of the second ring to bypass the fault in the primary ring.

 

                          

• The local area network (LAN) is a network which is designed to operate over a small physical area such as an office, factory or a group of buildings. LANs are very widely used in a variety of applications.

• LANs are easy to design and troubleshoot. The personal computers and workstations in the offices are interconnected via LAN.

• The exchange of information and sharing of resources becomes easy because of LAN.

• Local Area Network technology connects people and machines within a site.

• A LAN is a form of local (limited-distance), shared packet network for computer communications.

• In LAN all the machines are connected to a single cable. Different types of. Topologies such as Bus, Ring, Star, Tree etc. are used for LANs.

• LAN uses a layered architecture and they are capable of operating at hundreds of Mbits/sec.

• A local area network (LAN) is usually a privately owned and links the devices in a single office, building or campus of upto a few kilometers in size.

• Depending on the needs of an organization and the type of technology used, a LAN can be as simple as two personal computers and a printer in someone's office or home or it can extend throughout a company and include voice, sound and video peripherals.

• LAN s are widely used to allow resources to be shared between personal computers or workstations. The resources to be shared can be hardware like a printer or softwares or data.

• A common example of a LAN found in many business organizations, links a work group of task related computers, e.g. accounting and finance PCs, administrative PCs or engineering workstations.

• One of the computer in a network can become a server serving all the remaining computers called clients. Software can be stored on the server and it can be used by the remaining clients.

• In a LAN its size can be determined by licensing restrictions on the number of users per copy of software or by restricting the number of users licensed to access the operating system.

• LAN's are also distinguished from MAN's and WAN's based on the transmission media they use and topology. In general a given LAN will use only one type of transmission medium. The most common topologies used are bus, ring and star.

The data rates for LAN range from 4 to 16 Mbps with the maximum of 100 Mbps.

 

The components used by LANs can be divided into cabling standards, hardware, and protocols. Various LAN protocols are Ethernet, Token Ring: TCP/IP, 5MB, NetBIOS and NetBeui, IPX/SPX, Fiber Distributed Data Interchange (FDDI) and Asynchronous Transfer Mode (ATM).

LAN Applications and Benefits

LANs are used almost exclusively for data communications over relatively short distances such as within an office, office building or campus environment. LANs allow multiple workstations to snare access to multiple host computers, other workstations, printers and other peripherals, and connections to other networks. LANs are also being utilized for imaging applications, as well. They are also being used for video and voice communications, although currently on a very limited basis.

 

LAN applications include communications between the workstation and host computers, other workstations, and servers. The servers may allow sharing of resources. Resources could be information, data files, e-mail, voice mail, software, hardware (hard disk, printer, fax, etc.) and other networks.

 

LAN benefits include the fact that a high-speed transmission system can be shared among multiple devices in support of large number of active terminals and a large number of active applications in the form of a multi-user, multi-tasking computer network. LAN-connected workstations realize the benefit of decentralized access to very substantial centralized processors, perhaps in the form of mainframe host computer and storage capabilities (information repositories). Additionally, current technology allows multiple LANs to be inter-networked through the use of LAN switches, routers and the like.

 

Disadvantages of LANs include concern for security of files and accounts.

LAN topologies:

Various topologies are possible for the broadcast LANs such as bus topology or ring topology.

                              

Bus topology:

• Bus topology is shown in Fig. In this topology at any instant only one computer acts as master and it is allowed to transmit (broadcast). The others are supposed to listen.

• If two or more machines want to transmit simultaneously then an arbitration mechanism has to be used for resolving the conflict.

• It is possible to have a centralized or distributed type arbitration mechanism.

• The most popular example of bus topology is Ethernet (IEEE 802.3). It has a decentralized control and it operates at 10 or 100 Mbps.

• Computers on Ethernet can transmit whenever they want. If collision of their packets takes place, then they wait for a random time and retransmit their packets.

Ring topology:

• This is another broadcast topology.

• In a ring each bit propagates around on its own without waiting for the rest of the packet to which it belongs.

• Since it is a broadcast system, some rules are essential for arbitrating the simultaneous access to the ring.

• An example of ring based LAN is IEEE 802.5 (IBM token ring) operating at 4 and 16 Mbps.

Static and dynamic broadcast networks:

• The broadcast networks are further classified into two types namely,

 

1. Static networks and

2. Dynamic networks.

 

• This classification is based on how the channel is allocated.

• In static allocation, each machine is allowed to broadcast only in its allotted time slot.

• But static allocation wastes the channel capacity when a machine does not want to transmit in its allotted time slot.

• Hence most of the systems try to allocate the channel dynamically i.e. on demand.

                         

 

WAN is the acronym for, Wide Area Network and refers to a network used to connect different equipments from remote areas. This technology connects sites that are in diverse locations. Wide Area Networks (WANs) connect larger geographic area, such as New York, Canada, or the world. The geographical limit of WAN is unlimited. Dedicated transoceanic cabling or satellite uplinks may be used to connect this type of network. Hence, a WAN may be defined as a data communications network that covers a relatively broad geographic area to connect LANs together between different cities with the help of transmission facilities provided by common carriers, such as telephone companies. WAN technologies function at the lower three layers.

Normally, network services are provided by a Common Carrier of, for example, telephone company. Users can use services on rent basis. Available services include telephone network, leased line, packet switched network, X.25, ISDN, frame relay and cell relay.

WAN is composed of a number of autonomous computers that are distributed over a large geographical area. LAN can be extended across large distances using Satellite Bridge but still this cannot accommodate many computers arbitrarily. WAN must be scalable to long distances and many computers. Therefore, network must replace shared medium with packet switches to span long distances or many computers. Each switch moves an entire packet from one connection to another. This mechanism is called packet switching. These switches are nothing but a small computer with network interfaces, memory and program dedicated to packet switching function. These packet switches may connect to computers and to other packet switches, typically high-speed connections to other packet switches, lower speed to computers. These packet switches can be linked together to form WANs. WANs need not be symmetric or have regular connections, i.e. each switch may connect to one or more other switches and one or more computers.

                          

Data delivery from one computer to another is accomplished through store and forward technology. Packet switch stores incoming packet and forwards the packet to another switch or computer that has internal memory. Therefore, this can hold a packet in queue if outgoing connection is busy.

 

• When a network spans a large distance or when the computers to be connected to each other are at widely separated locations a local area network cannot be used.

• A wide area network (WAN) must be installed. The communication between different users of "WAN" is established using leased telephone lines or satellite links and similar channels.

• It is cheaper and more efficient to use the phone network for the links.

• Most wide area networks are used for transferring large blocks of data between its users. As the data is from existing records or files, the exact time taken for this data transfer is not a critical parameter.

• Another example of WAN is an airline reservation system. Terminals are located all over the country through which the reservations can be made .

• It is important to note here that all the terminals use the same common data provided by the central reservation computer.

• Because of the large distances involved in the wide area networks, the propagation delays and variable signal travel times are major problems.

• Therefore most wide area networks are not used for time critical applications. They are more suitable for transfer of data from one user to the other which is not a time critical application. Wide area networks are basically packet switching networks.

• A WAN provides long distance transmission of data, voice image and video information over large geographical areas that may comprise a country, a continent or even the whole world.

                               

• WAN contains a collection of machines used for running user (i.e. application) programs. All the machines called hosts are connected by a communication subnet.

                          

• The function of the subnet is to carry messages from host to host. The subnet consists of two important components; transmission lines and switching elements.

• Transmission lines move bits from one machine to another. The switching elements are specialized computers used to connect two or more transmission lines. When data arrive on an incoming line, the switching element must choose an outgoing line to forward them.

• The switching elements are either called as packet switching nodes, intermediate systems, data switching exchanges or routers.

• When a packet is sent from one router to another via one or more intermediate routers, the packet is received at intermediate router. It is stored in the routers until the required output line is free and then forwarded. A subnet using this principle is called a point to point, store-forward or packet switched subnet.

• WAN's may use public, leased or private communication devices, and can spread over a wide geographical area. A WAN that is wholly owned and used by a single company is often called as an enterprise network.

• In most WANs the network contains a large number of cables or telephone lines each one connecting a pair of routers.

• If two routers which are not connected to each other via a cable want to communicate, then they have to do it indirectly via other routers.

 

Packet switching technologies such as Asynchronous Transfer Mode (ATM), Frame Relay, Switched Multimegabit Data Service (SMDS), and X.25 are used to implement WAN along with statistical multiplexing to enable devices to share these circuits.

Packet switched subnet

• When a packet is sent from one router to the other, via some intermediate routers, the packet received at each intermediate router is stored until the required output line is free.

• Once the line becomes free, the packet is forwarded.

• A subnet working on this principle is called as point to point or store and forward or packet switched subnet.

• If the packets are small and of same size they are called as cells.

Router interconnection topologies

• Fig. shows some of the possible router interconnection topologies in a point to point subnet.

                     

• The LANs have a symmetric topology while WANs have irregular topologies.

• The WAN s can also be formed using satellite or ground radio system. Satellite networks are inherently broadcast type so they are useful when the broadcast property is important.

 

Difference between WAN and LAN

 

 

With LAN additional expanses are rarely required once it is installed. With WAN, users must continue to pay a communication cost to their contracted common carrier.

 

 

 

• WAN is generally slower in transmission speed. Requesting the same level of speed as with LAN leads to a substantial increase in communication costs.
• The Satellite Bridge can extend LAN across large distances while in case of the WAN, it spans over a wide geographical area.
• LAN still can not accommodate arbitrarily many computers; WAN must be scalable to long distances and many computers
•  

  Protocol – What is Protocol?

  Written by Dinesh Thakur Category: Computer Network
   

  A protocol is a set of mutually accepted and implemented rules at both ends of the communications channel for the proper exchange of information. Protocols comprise standards which, at a basic level, include the dimensions of line setup, transmission mode, code set, and non-data exchanges of information such as error control (detection and correction).

   

  To establish a meaningful session, a certain sets of rules need to adopt by vendors of networking device. The Figure given below shows the basic configuration of a data communication system. The numbers shown in Figure indicate the following components of a network:

   

  1. Data Transmission line.

  2. Data Circuit terminating Equipment (DCE) - Modem or DSU (Digital Signal Unit)

  3. Interface between Data Terminal Equipment (DTE) and DCE

  4. Communication program on DTE

  5. Communication program on CCP (Communication Control Processor).

   

  If the interface provided between (2) and (3) is not physically compatible with either of them, the DTE and DCE cannot be connected. And, even if they can be physically connected the DTE and host cannot communicate with each other if the communication procedures used for (4) and (5) are different. Therefore, it is necessary to use the same procedure for (4) and (5). This communication procedure is called a protocol.

                               

  A protocol is required for communication between computers. To connect different types of computers from a variety of computer vendors, protocols must be first standardized. The ARPA (Advanced Research Project Agency) part of the US Defense program was the first to introduce the concept of a standardized protocol. ARPA is a resource sharing network connecting different computers at universities and laboratories in the US. The concept of the protocol and its layer structure, which will be mentioned later, emerged from the ARPA network. ARPA developed an integrated network using packet protocol and is also renowned for its development of packet switching.

  Transmission Control Procedure

  As important as it is that there should be one accepted standard that allows all types of machines to communicate. There are several different protocols in use today. These are:

   

  • Synchronous Protocol These protocols involve timing information of sender along with the data bytes. This helps receiver to remain synchronization with the sender. When the sender has no data to transmit, the sender transmits a sequence of alternating 0s sand 1s to maintain sender/receiver synchronization. This sequence of 0s and 1s is called idle flags. Data bytes are packaged into small chunks called packets including address fields and checksums. As error checking is an inherent feature of this protocol, this overcomes the major deficiency of the asynchronous protocol.

  • Asynchronous Data Link Control (DLC) Protocols Asynchronous protocols are used primarily for low-speed data communications between PCs and very small computers. Framing occurs at the byte level, with each byte surrounded by a start bit (a 0 bit) and a stop bit (a 1 bit). A parity bit often accompanies each character as well.

  Character-orientated Protocols (COP)

  Each character has its own meaning in character-orientated protocols. A character may be a data byte or a control byte during transmission. The main COP in use today is known as Bisync or binary synchronous. Each character sent is transmitted using the ASCII code. Control bytes obviously have values in ASCII of between 00 and 1F, whereas data bytes have values between 20 and 7F.

  In this type of scheme, following Figure represents a sequence of communication between the sender and receiver. This can be inferred as handshaking between the sender and receiver:

                              

  This has an acknowledgement scheme. In this case, if an acknowledgement is not received by the sender in a specified time, the sender retransmits the packet. This time is called time out. After the successful transmission of a packet, next packets are transmitted until entire message is sent. If a packet is received and contains errors, the receiver will send a negative acknowledge. This implies that the sender has to send it again. Data bytes contain data according to the ASCII code for text or simply a value between 0 and 255 for binary data. Control bytes determine the behavior of the communication link, and are used for a range of different purposes.

   

  The link between sender and receiver is half duplex.

   

  • Binary Synchronous Protocol (Bisync or BSC) Bisync was developed by IBM in 1966 as a character-oriented protocol that frames the data with control codes which apply to the entire set of data. Bisync organizes data into block of up to 512 characters, which are sent over the link sequentially (one-at-a-time) as shown in Figure. An ACK or NAK is transmitted from the receiving terminal to the transmitting device following the receipt of each block Error control is on the basis of a Block Checking Character (BCC) that is transmitted along with the data. The receiving device independently calculates the BCC and compares the two calculations.

   

  From the Figure, it can be seen that each message has three parts:

   

  • Header - This is recognized by the control characters SOH (Start of Header)

  • Text -This is recognized by the control characters STX (Start of Text block)

  • Trailer - This is recognized by the control characters ETX (End of Text block)

   

  SYN characters are used to establish synchronization between the sender and receiver. The message block follows the SYN characters.

  In this scheme, sender breaks each message into blocks of small messages for transmission. The trailer for each block consists of a block check character (BCC). Both the sender and the receiver generate distinct BCC during the transmission of message. At the end of receiving the trailer, the receiver compares its own BCC against that of the senders. If they are the same, this indicates the block has been successfully received without any error. In this case, the receiver will reply using a positive acknowledge (ACK). If the BCC of the receiver does not match that of the sender, the receiver knows an error has occurred during transmission, and will instruct the sender to retransmit the block by replying with a negative acknowledge (NACK). This is also explained with the help of Figure.

   

  Bisync scheme has one major disadvantage in distinguishing between the control characters and the same text in a binary file. If a control character like ETX occurs in the text field, the receiver would interpret this as the end of the text field and take the next character as the BCC. This is incorrect. This is overcome by using a technique called data transparency. This means preceding each control character with the Data link Escape control character (DLE). If the receiver gets a DLE code, it knows the next byte is a control code. The receiver discards the DLE control character. What happens if the sender has a DLE code as part of the text block? In this case the sender precedes it with a DLE, and the receiver discards the first and uses the second as a data byte

Advantages of Dual-ring Topology

•  
• Very orderly network where every device has access to the token and the opportunity to transmit
• Performs better than a star topology under heavy network load
• Does not require network server to manage the connectivity between the computers

 

Disadvantages of Dual-ring Topology

•  
• One malfunctioning workstation or bad port can create problems for the entire network
• Moves, adds and changes of devices can affect the network
• Much slower than an bus network under normal load.

Mesh Topology

In mesh topology, each node is connected to every other node in the network i.e. each node has a dedicated point to point link to every other node as shown. Dedicated means that the link carries the traffic only between two devices it connects.

 

In this way there exist multiple paths between two nodes of the network. In case of failure of one path, the other one can be used.

 

Advantages of Mesh Topology

 

1. It is robust as the failure of one node does not collapse the entire system. If one link fails, the entire system continues to work.

2. There is no traffic congestion problem as dedicated links are being used.

3. Dedicated links ensure faster transmission without any delay.

4. Dedicated links also ensure data privacy and security.

5. Point to point links makes fault identification and isolation easier.

                                  

Disadvantages of Mesh Topology

 

1. Connecting each device to every other device in the network makes installation and reconfiguration difficult.

2. It has high cabling cost as n (n-l)/2 links are required to connect n nodes.

Fully Connected Topology: The type of network topology in which each of the nodes of the network is connected to each of the other nodes in the network with a point-to-point link – this makes it possible for data to be simultaneously transmitted from any single node to all of the other nodes. The physical fully connected mesh topology is generally too costly and complex for practical networks, although the topology is used when there are only a small number of nodes to be interconnected.

Tree Topology

Tree or Hierarchical Topology: The type of  Topology in which a central 'root' node, the top level of the hierarchy, is connected to one or more other nodes that are one level lower in the hierarchy i.e., the second level, with a point-to-point link between each of the second level nodes and the top level central 'root' node, while each of the second level nodes that are connected to the top level central 'root' node will also have one or more other nodes that are one level lower in the hierarchy, i.e., the third level, connected to it, also with a point-to-point link, the top level central 'root' node being the only node that has no other node above it in the hierarchy – the hierarchy of the tree is symmetrical, each node in the network having a specific fixed number, f, of nodes connected to it at the next lower level in the hierarchy, the number, f, being referred to as the 'branching factor' of the hierarchical tree.

                        

Advantages:

1. Supported by several hardware and software venders.
2. It allows more devices to be attached to a single central hub and can therefore increases the distance a signal can travel between devices.
3. It allows the network to isolate and prioritize communication from different computers i.e. the computers attached to one secondary hub can be given priority over the computers attached to another secondary hub.

 

Disadvantages:

•  
• Overall length of each segment is limited by the type of cabling used.
• If the backbone line breaks, the entire segment goes down.
• More difficult to configure and wire than other topologies
• It has higher cabling cost in setting up a tree structure.

 

Hybrid Topology

 

The hybrid Topology is a type of Topology that is composed of one or more interconnections of two or more networks that are based upon different physical topologies in a single network that is composed of one or more interconnections of two or more networks that are based upon the same physical topology.

                               

When two hubs of different topologies are joined so that the devices attached to them can communicate as in figure, it is called a Star-Bus network.

                         

When two or more star topologies are linked together using a specialized hub called a MAU (Multi-utilization Access Unit), it is known as Star-Ring topology.