Topics covering in this Tutorial

• LANS, WANS and the Internet
• Uses and benefits of networks
• Network Components
• Server based and peer-to-peer networks

LANs, WANs and the Internet

In its most basic form, a network is two or more computer systems which are linked together by some form of transmission medium that enables them to share information. It does not matter whether the network contains two or thousands of machines; the concept is essentially the same. A network will provide services to its users. These services may include access to shared files and folders, network printing and database applications.

To attempt to clarify the size and nature of individual networks, the industry has developed terms that broadly define the scope of the network.





Uses and Benefits of a LAN

Shared Data

One of the principal benefits of networking is the ability to share data. Whether the data is a word processing document or a corporate database system, the ability to share the data is essential. Networks enable multiple users to access the same disk or disk system. A single copy of a file can be held on a central server rather than duplicating the file on each workstation. Facilities such as file and record locking ensure it is impossible for two users to simultaneously access the same piece of data.

Shared Peripherals

Networks allow users to share devices such as printers, scanners, modems, fax systems and CD-ROMs. Sharing can provide several benefits:

• Fewer devices have to be purchased and maintained. The devices that are purchased can be of a higher specification and sharing this equipment between many users ensures it is used more efficiently
• Providing shared modems and fax systems allows system administrators to monitor and control the use of the facilities

Shared Applications

Some network applications can be configured with the program files located on a central server and just small numbers of files loaded onto the user's workstation or personal directory. Shared software can reduce administration time as upgrades can be implemented on the server with minimal changes required for each user. It also allows a system administrator to implement a standard configuration that reduces support costs and may simplify the control of software licenses. Its disadvantages lie in the increased network traffic generated and the inability to work when the server is down.

Electronic messaging

Electronic mail (email) is one of the most common network applications. The messaging server acts as the central repository for messages. Users can submit messages to another user who is not currently connected to the network and the message waits at the server until the user connects to view his or her messages. The client messaging software is used to compose and address the message.

Widely used LAN-based messaging systems include Microsoft Exchange, Lotus Notes and Novell GroupWise.

Workgroup applications

Workgroup applications are designed to allow two or more users to improve their productivity by communicating and sharing information across a network. Often these applications are based on messaging systems that have been enhanced to provide extra facilities. The most common example is scheduling applications. These enable users to view the appointment books of other users and schedule meetings. Examples of these include; Microsoft Outlook, Microsoft Schedule+ and Lotus Organizer.

Client-server applications

Older network applications were designed so that most of the processing effort was performed by the client machine. These applications are described as client-based. For example, in a database application, the server acted as a repository for the database (a shared file containing the data), while the client-based application performs all the queries and operations on the data. In this situation, the server is a passive participant.

A number of systems have been developed where both client and server are active participants. In a client-server system, the server performs most of the data manipulation and provides the client with the information requested. The server is typically a more powerful machine than the client computer. The reason for this is because it holds data locally and does not have to request information across the network. As a result, client-server systems generally provide better performance. The client machines can be of a lower specification because they are only required to make requests and display the results.

In essence, a client-server based system is one where the application's components are distributed amongst both clients and servers.

Examples of client-server systems include; SQL Server (pronounced "sequel server"), Oracle and Microsoft Exchange Server.

Resilience

Local area networks can be used to provide system resilience. Centralized tape backup systems ensure data can be recovered after a disaster. Disk arrays using RAID technology allow the system to continue operating when a disk fails. For example, a mirrored hard drive duplicates data onto two hard drives, with the 'good' hard drive continuing to operate until the other can be replaced. Replication of data between servers allows them to act as a backup for each other, and modern cluster technology provides grouping of servers so that the others can take over in the event of failure. The addition of another server to a cluster improves performance when the load becomes too great for the existing configuration.

Networks are especially vulnerable to virus infection, but have the advantage that a centralized virus policy can be implemented to protect all servers and workstations. By ensuring all servers and workstations hold an up-to-date version of the virus software, an administrator can minimize the risk to data. Virus software vendors provide software that automatically downloads the latest copy of the virus database to a workstation when an update is required.

Security

Local area networks offer varying levels of security. Simple network systems may use a password to control access to resources, while the more sophisticated systems control access by a user name. User names are an important part of network security and many network systems deny access until the user has provided a valid user name and password.

Network Components

Despite the variations in size and design all networks have certain components in common. The key elements include the transmission medium, the clients and the servers (or hosts) that hold the shared data.

Transmission Media

Information must be transferred using some form of media. Typically, this takes the form of a cable but 'wireless' media using technologies such as infrared, microwave and radio can provide the same function.

Client

Network clients are the computers on the network that allow users to request the shared resources on the servers. Clients are often referred to as workstations.