Introduction A network is a group of computers and other devices, such as printers and modems, connected to each other. This enables the computers to effectively share data and resources.

The concept of sharing resources over a network is called networking. The computers in a network can share data, messages, graphics, printers, fax machines, modems, and other hardware and software resources.

 

Network elements

Servers

Servers are the computers that provide the shared resources to network users. There is usually only one server in a small network, but bigger networks may have more than one server.

 

Clients

Clients are the computers that can access the shared network resources provided by a server.

 

Media

The computers in a network are connected to each other by hardware components, such as cables. These components are called the media.

 

Shared data

A server provides shared resources and data over a network. The files that are provided by the server over the network are called shared data. This shared data can be a document, a worksheet or a folder.

 

Resources

Files, printers or other items that can be used by network users are known as resources. These resources can be either hardware or software resources.

Benefits of the computer NetworkIn most organizations, a network offers benefits, such as data sharing and online communication that are not provided by a stand-alone environment (An environment in which there is several computers that are not connected to each other is called a stand-alone environment.). In this topic, you will identify the benefits of a network.

 

Data sharing

The data can be easily sharable in a network so other user can access it at any time. For example, a user working in a team wants to share some data with the other team members. In a stand-alone environment, to share the data you can verbally exchanging information or writing memos or putting the data on a floppy and copying it to the machines of other members. If it network the data can be easily made sharable in a network so that the other team members can access.

A centralized data storage system enables multiple users to access data in different locations.

 

Resource sharing

Resource sharing is also an important benefit of a computer network. For example, if there are four people in a family, each having their own computer, they will require four modems (for the Internet connection) and four printers, if they want to use the resources at the same time. A computer network, on the other hand, provides a cheaper alternative by the provision of resource sharing.

In this way, all the four computers can be interconnected, using a network, and just one modem and printer can efficiently provide the services to all four members. The facility of shared folders can also be availed by family members.

 

Centralized Software Management

One of the greatest benefits of installing a network is the fact that all of the software can be loaded on one computer (the file server). This eliminates that need to spend time and energy installing updates and tracking files on independent computers throughout the building.

 

Communication medium

A computer network can provide a powerful communication medium among people spread widely at different physical locations. It acts as a powerful communication medium when it comes to sharing information and resources.

 

Data Security and Management

In a business environment, a network allows the administrators to much better manage the company's critical data. Instead of having this data spread over dozens or even hundreds of small computers in a haphazard fashion as their users create it, data can be centralized on shared servers. This makes it easy for everyone to find the data, makes it possible for the administrators to ensure that the data is regularly backed up, and also allows for the implementation of security measures to control who can read or change various pieces of critical information.

 

Speed

Sharing and transferring files within Networks are very fast. Networks provide a very rapid method for sharing and transferring files. The sharing of data with the other users in a stand-alone environment requires physical transfer of data. This can be done by copying the data on a floppy disk or taking a printout. This requires more time and is inconvenient as compared to sharing the data over a network.

Peer-to-Peer NetworksIn a peer-to-peer network, there are no dedicated servers, and there is no hierarchy among the computers. All the computers are equal and therefore are known as peers. Each computer functions as both a client and a server.

And there is no administrator responsible for the entire network. The user at each computer determines what data on that computer is shared on the network. Below figure shows a peer-to-peer network in which each computer functions as both a client and a server.

Computers in a peer-to-peer network are called peers. In a peer-to-peer network, all computers are considered equal; they all have the same abilities to use the resources available on the network. Each computer can function both as a client and a server. Computers are not dedicated to function as servers. They use the network to share resources among the independent peers. The computer whose applications are required by the other networked computers functions as a server. The other computers function as clients. Therefore, a dedicated administrator is not assigned for network management.

A peer-to-peer network is a small group of people using a network. Peer-to-peer networks members usually perform similar tasks, which necessitates the sharing of resources. The peer-to-peer networks support 10 computers. The users in a peer-to-peer network are located in the same geographical area. Operating systems, such as Microsoft Windows 98 or Microsoft Windows XP, can be used to set up a peer-to-peer network. Additional software is not required because peer-to-peer networking is built into the systems.

Another important point of peer-to-peer networks is that the users of each computer plan and control the security of their resources. The users determine the resources on their computers, which can be shared on the network. The shared network resources, such as disk space, printers or faxes, can be used by anyone who has access to the network. This is possible only if the shared network resources are not password protected. Peer-to-peer networks have weak and intrusive security because a central server is not used to administer and secure the network. In addition, some users may not implement security.

A peer-to-peer network does not support a central login process. This implies that a user who logs on to one peer can access any shared network resource, which is not controlled by a specific password. Peer-to-peer networks are relatively simple. Because each computer functions as a client and a server, there is no need for a powerful central server

or for the other components required for a high-capacity network. Peer-to-peer networks can be less expensive than server-based networks.

Peer-to-peer networks are simple and inexpensive to install and maintain. The cost of implementing peer-to-peer networks is low because a central server is not used to administer the network. In addition, the components for a high-capacity network are not required in a peer-to-peer network.

In a peer-to-peer network, the users handle administration. This means that all the users need to be trained in how to share files, folders, and printers. In a peer-to-peer network, suddenly shutting down your computer can cause one of your colleagues to be unable to print.

Peer-to-peer networks are appropriate for environments where all the users are located in the same geographical area and the network security is not an important factor. In addition, these networks are useful when the network expansion is limited.

 

Advantages of a peer-to-peer network:

Less initial expense - No need for a dedicated server. Setup - An operating system (such as Windows XP) already in place may only

need to be reconfigured for peer-to-peer operations.

 

Disadvantages of a peer-to-peer network:

Decentralized - No Centralized server. Security - Does not provide the security available on a peer-to-peer network.

Server Based NetworksA dedicated server is one that functions only as a server and is not used as a client or workstation. Server based networks (see below Figure) have become the standard models for networking.

In a server-based network, clients rely on the services that the server provides, such as file storing and printing. Client computers are generally less powerful than server computers.

A server-based network using network operating system is that the networks are organized into domains. A domain is a collection of networks and clients that share security information. Domain security and logon permissions are controlled by special servers called domain controllers. Users cannot access the resources of servers in a domain until a domain controller has authenticated them.

In server-based networks, a network administrator centrally manages the resource security. The administrator defines and manages user access to network resources.

Another beneficial of server-based networks is central file storage. Server-based networks provide easy backup of critical data. Data backup is another useful characteristic of server based networks.

Server based networks can support a larger number of users than peer-to-peer networks. To support a large number of users, server-based networks use monitoring and network management tools. Servers must perform varied and complex tasks. (See below figure)

 

Security is often the primary reason for choosing a server-based approach to networking. In a server-based environment, one administrator who sets the policy and applies it to every user on the network can manage security. (see below figure)

 

 

Advantages of a client/server network

Centralized - Resources and data security are controlled through the server. Security - More security then Peer-to-peer network.

Flexibility - New technology can be easily integrated into system. Interoperability - All components (client /server) work together. Accessibility - Server can be accessed remotely and across multiple platforms.

 

Disadvantages of a client/server network

Expense - Requires initial investment in dedicated server. Maintenance - Large networks will require a staff to ensure efficient operation. Dependence - When server goes down, operations will cease across the network.

 

Wired Local Area Network (LAN)It is the simplest type of network in which computers are connected to each other by cables.

 

Each of the computers on the LAN is also called a node .

 

A LAN is characterized by three primary attributes:

Topology

The topology is the pattern used to connect the computers together. With a bus topology, a network cable connects each computer to the next one, forming a chain. With a star topology, each of the computers is connected to a central nexus called a hub/Switch. A ring topology is essentially a bus network with the two ends joined together. (You will see more about network topologies in Chapter 6 )

 

Medium

Four basic types of media are used in local-area networks; coaxial cable, twisted-pair wires, fiber-optic cable, And wireless.

Each medium has its advantages and disadvantages relative to cost, speed, and expandability. Coaxial cables provide high speed and low error rates. Twisted-pair wires are cheaper than coaxial cables, can sustain the speeds common to most personal computers, and are easy to install. Fiber-optic cable is the medium of choice for high-speed local-area networks. Wireless local-area networks have the advantage of expandability. (You will see more about network medium in Chapter 5.2 )

 

Protocols

The topology and the medium used on a particular network are specified by the protocol. (You will see more about protocol in Chapter 8 ).

 

LAN computer networks that usually cover a limited range, say, within the boundary of a building. A LAN computer network is two or more computers that communicate with each other through some medium.

The primary usage of local-area networks (LANs) is the sharing of hardware, software, or information, such as data files, multimedia files, or electronic mail. Resource sharing provided by local-area networks improves efficiency and reduces overhead.

There are a number of ways in which nodes can communicate over a network. The simplest is to establish a dedicated link between the transmitting and receiving stations. This technique is known as circuit switching. A better way of communicating is to use a technique known as packet switching, in which a dedicated path is not reserved between the source and the destination. Data are wrapped up in a packet and launched into the network. In this way, a node only has exclusive access to the medium while it is sending a packet. During its inactive period, other nodes can transmit. A typical packet is divided into preamble, address, control, data, and error-check fields.

The computers in a LAN are connected by using cables. This method cannot be used to connect computers that are in different locations, for example, in buildings across a town or city. Therefore, a LAN is not suitable for large businesses with offices in several locations.

Wide Area Network (WAN)A wide area network (WAN) is a telecommunications network, usually used for connecting computers, that spans a wide geographical area. WANs can by used to connect cities, states, or even countries.

An example of a WAN connection would be a company with two offices in distant cities, each with its own LAN and connected by a leased telephone line. This type of WAN is illustrated in below figure. Each end of the leased line is connected to a router and the routers are connected to individual LANs. Any computer on either of the LANs can communicate with any one of the other computers at the other end of the WAN link or with a computer on its own LAN.

 

WANs are often used by larger corporations or organizations to facilitate the exchange of data and in a wide variety of industries, corporations with facilities at multiple locations have embraced WANs. Increasingly, however, even small businesses are utilizing WANs as a way of increasing their communications capabilities.

Although WANs serve a purpose similar to that of local area networks (LANs), WANs are structured and operated quite differently. The user of a WAN usually does not own the communications lines that connect the remote computer systems; instead, the user subscribes to a service through a telecommunications provider. Unlike LANs, WANs typically do not link individual computers, but rather are used to link LANs. WANs also transmit data at slower speeds than LANs.

WANs have existed for decades, but new technologies, services, and applications have developed over the years to dramatically increase their efficacy for business. WANs were originally developed for digital leased-line services carrying only voice, rather than data. As such, they connected the private branch exchanges (PBXs) of remote offices of the same company. WANs are still used for voice services, but today they are used more frequently for data and image transmission (such as video conferencing). These added applications have spurred significant growth in WAN usage, primarily because of the surge in LAN connections to the wider networks.

WANs are either point-to-point, involving a direct connection between two sites, or operate across packet-switched networks, in which data is transmitted in packets over shared circuits. Point-to-point WAN service may involve either analog dial-up lines, in which a modem is used to connect the computer to the telephone line, or dedicated leased digital telephone lines, also known as "private lines." Analog lines, which may be either part of a public-switched telephone network or leased lines, are suitable for batch data transmissions, such as congruent order entry and point-of-sale transactions. Dedicated digital phone lines permit uninterrupted, secure data transmission at fixed costs.

Point-to-point WAN service providers include both local telephone companies and long distance carriers. Packet-switched network services are typically chosen by organizations which have low volumes of data or numerous sites, for which multiple dedicated lines would be too expensive.

Depending on the service, WANs can be used for almost any data sharing purpose for which LANs can be used. Slower transmission speeds, however, may make some applications less practical for WANs. The most basic uses of WANs are for electronic mail and file transfer, but WANs can also permit users at remote sites to access and enter data on a central site's database, such as instantaneously updating accounting records.

New types of network-based software that facilitate productivity and production tracking, such as groupware and work-flow automation software, can also be used over WANs. Using groupware, workers at dispersed locations can more easily collaborate on projects. WANs also give remote offices access to a central office's other data communications services, including the Internet.

Bus topologyA bus topology network is a network architecture in which a set of clients are connected via a shared communications line, called a bus. The bus topology is often referred to as a "linear bus" because the computers are connected in a straight line. This is the simplest and most common method of networking computers.

Below figure shows a typical bus topology. It consists of a single cable called a trunk (also called a backbone or segment) that connects all of the computers in the network in a single line.

 

Computers on a bus topology network communicate by addressing data to a particular computer and sending out that data on the cable as electronic signals. Network data in the form of electronic signals is sent to all the computers on the network. Only the computer whose address matches the address encoded in the original signal accepts the information. All other computers reject the data. Figure shows a message being sent from 0020af151d8b to 02608c133456.

 

Only one computer at a time can send messages. Because only one computer at a time can send data on a bus network, the number of computers attached to the bus will affect network performance. The more computers there are on a bus, the more computers will be waiting to put data on the bus and, consequently, the slower the network will be.

There is no standard way to measure the impact of a given number of computers on the speed of any given network. The effect on performance is not related solely to the number of computers.

The following is a list of factors that in addition to the number of networked computers will affect the performance of a network:

Hardware capabilities of computers on the network Total number of queued commands waiting to be executed Types of applications (client-server or file system sharing, for example) being run

on the network Types of cable used on the network Distances between computers on the network

 

Computers on a bus either transmit data to other computers on the network or listen for data from other computers on the network. They are not responsible for moving data from one computer to the next. Consequently, if one computer fails, it does not affect the rest of the network. Because the data, or electronic signal, is sent to the entire network, it travels from one end of the cable to the other.

If the signal is allowed to continue uninterrupted, it will keep bouncing back and forth along the cable and prevent other computers from sending signals. Therefore, the signal must be stopped after it has had a chance to reach the proper destination address. To stop the signal from bouncing, a component called a terminator is placed at each end of the cable to absorb free signals. Absorbing the signal clears the cable so that other computers can send data. Both ends of each cable segment on the network must be plugged into something. For example, a cable end can be plugged into a computer or a connector to extend the cable length. Any open cable ends not plugged into something must be terminated to prevent signal bounce. below Figure shows a properly terminated bus topology network.

 

 

Advantages and Disadvantages of a Bus Network

Advantages

1. Easy to implement and extend 2. Well suited for temporary networks (quick setup) 3. Typically the cheapest topology to implement 4. Faster than a ring network. 5. If any node on the bus network fails, the bus its self is not effected. 6. Requires less cable than a Star network.

Disadvantages

1. Difficult to administer/troubleshoot 2. Limited cable length and number of stations 3. A cable break can disable the entire network 4. Maintenance costs may be higher in the long run 5. Performance degrades as additional computers are added or on heavy traffic 6. Low security (all computers on the bus can see all data transmissions) 7. One virus in the network will affect all of them (but not as badly as a star or ring

network) 8. Proper termination is required.(loop must be in closed path)

Ring topologyA ring network is a topology of computer networks where each node is connected to two other nodes, so as to create a ring. The ring topology connects computers on a single circle of cable. Unlike the bus topology, there are no terminated ends. The signals travel around the loop in one direction and pass through each computer, which can act as a repeater to boost the signal and send it on to the next computer. Below Figure shows a typical ring topology with one server and four workstations. The failure of one computer can have an impact on the entire network.

 

One method of transmitting data around a ring is called token passing . (A token is a special series of bits that travels around a token-ring network. Each network has only one token.) The token is passed from computer to computer until it gets to a computer that has data to send. Below figure shows a token ring topology with the token. The sending computer modifies the token, puts an electronic address on the data, and sends it around the ring.

 

The data passes by each computer until it finds the one with an address that matches the address on the data. The receiving computer returns a message to the sending computer indicating that the data has been received. After verification, the sending computer creates a new token and releases it on the network. The token circulates within the ring until a workstation needs it to send data. It might seem that token passing would take a long time, but the token actually travels at roughly the speed of light. A token can circle a ring 200 meters (656 feet) in diameter about 477,376 times per second.

Ring networks tend to be inefficient when compared to Star networks because data must travel through more points before reaching its destination. For example, if a given ring network has eight computers on it, to get from computer one to computer four, data must travel from computer one, through computers two and three, and to its destination at computer four. It could also go from computer one through eight, seven, six, and five until reaching four, but this method is slower because it travels through more computers.

Ring networks also carry the disadvantage that if one of the nodes in the network breaks down then the entire network will break down with it as it requires a full circle in order to function.

 

Advantages and Disadvantages of a Ring Network

Advantages

1. The transmission of data is relatively simple as packets travel in one direction only.

 

Disadvantages

1. Data packets must pass through every computer between the sender and recipient Therefore this makes it slower.

2. If any of the nodes fail then the ring is broken and data cannot be transmitted successfully.

3. It is difficult to troubleshoot the ring.

Star topology Star networks are one of the most common computer network topologies . In its simplest form, a star network consists of one central switch , hub or computer which acts as a router to transmit messages.

In the star topology, cable segments from each computer are connected to a centralized component called a hub . Below Figure shows four computers and a hub connected in a star topology. Signals are transmitted from the sending computer through the hub to all computers on the network. This topology originated in the early days of computing when computers were connected to a centralized mainframe computer.

 

The star network offers the advantage of centralized resources and management. However, because each computer is connected to a central point, this topology requires a great deal of cable in a large network installation. Also, if the central point fails, the entire network goes down. If one computer or the cable that connects it to the hub fails on

a star network, only the failed computer will not be able to send or receive network data. The rest of the network continues to function normally.

Another characteristic of the star topology is that it is easy to modify. Computers can be added or removed from the network without disturbing the functioning of the network. The star topology supports the expansion of networks. This is done by placing another hub where a computer can be located. This enables several more computers or hubs to be connected to the main hub.

A disadvantage of the star topology is that the cost of cabling is higher in a network based on this topology because all the network cables need to be extended to one central point.

 

Advantages and Disadvantages of a Ring Network

Advantages

1. Easy to implement and extend, even in large networks 2. Well suited for temporary networks (quick setup) 3. The failure of a non-central node will not have major effects on the functionality

of the network. 4. Reliable market proven system 5. No problems with collisions of Data since each station have its own cable to the

server/hub. 6. Security can be implemented in the hub/switch. 7. The centre of a star i.e. the hub or switch is best place to find network faults. 8. It is easy to modify and add new computers to a star network without disturbing

the rest of the network.

 

Disadvantages

1. Depending on the transmission media, length limitations may be imposed from the central location used

2. Failure of the central node can disable the entire network 3. Limited cable length and number of stations 4. Maintenance costs may be higher in the long run

Mesh topologyA mesh topology network offers superior redundancy and reliability. In a mesh topology, each computer is connected to every other computer by separate cabling. This

configuration provides redundant paths throughout the network so that if one cable fails, another will take over the traffic. While ease of troubleshooting and increased reliability is definite pluses, these networks are expensive to install because they use a lot of cabling. Often, a mesh topology will be used in conjunction with other topologies to form a hybrid topology.

 

Mesh networks are self-healing: the network can still operate even when a node breaks down or a connection goes bad. As a result, a very reliable network is formed. This concept is applicable to wireless networks, wired networks, and software interaction.

A mesh network is a networking technique which allows inexpensive peer network nodes to supply back haul services to other nodes in the same network. It effectively extends a network by sharing access to higher cost network infrastructure.

 

Advantages and Disadvantages of a mesh network

Advantages

1. Provides redundant paths between devices.2. The network can be expanded without disruption to current users.

 

Disadvantages

1. Requires more cable than the other LAN topologies.2. Complicated implementation.

Selecting a TopologyThere are many factors to consider when deciding which topology best suits the needs of an organization. Below table provides some guidelines for selecting a topology.

Topology Advantages Disadvantages

Bus

Use of cable is economical.

Media is inexpensive and easy to work with.

System is simple and reliable.

Bus is easy to extend.

Network can slow down in heavy traffic.

Problems are difficult to isolate.  

Cable break can affect many users.

Ring

System provides equal access for all computers.

Performance is even despite many users.

Failure of one computer can impact the rest of the network.

Problems are hard to isolate.

Network reconfiguration disrupts operation.

Star

Modifying system and adding new computers is easy.

Centralized monitoring and management are possible.

Failure of one computer does not affect the rest of the network.

If the centralized point fails, the network fails.

Mesh

System provides increased redundancy and reliability as well as ease of troubleshooting.

System is expensive to install because it uses a lot of cabling.