Communications networks differ in geographical size. Networks may be constructed within a building or across several buildings. Networks may also be citywide and even international, using both cable and air connections. There are three major network types: LAN (local area networks), MAN (metropolitan area networks), and WAN (wide area networks).

A computer communications network contained in a small area such as a commercial building is known as a local area network (LAN). A local area network's linkages usually are accomplished with either telephone, coaxial, or fiber-optic cables. Often, LANs use a bus configuration. The major benefit of a local area network is that it can help to reduce costs by allowing people and microcomputers to share expensive resources. Also, local area networks enable end users to participate in office automation technology. In addition, LANs may be connected to other networks by means of a network gateway. Network designers can choose the method in which messages are controlled in a local area network. Two basic methodologies are the token-passing approach and the contention-based approach. The token- passing approach allows the designers of a network to achieve a degree of centralized control. A group of data bits, or "token," is passed from one network node to another. A node can only send a message when it is in possession of the token.

In the contention-based approach, a node that wishes to send a message first listens to determine if another node is currently sending a message. If not, the node attempts to send its message. However, the lack of centralized control can result in a collision - two nodes attempting to send messages simultaneously. Consequently, contention-based approach is usually not suitable for networks with a large amount of communications activity.

Why Use an LAN?

There are two basic reasons for developing an LAN: information sharing and resource sharing.

• Information sharing: This refers to having users who access the same data files, exchange information via electronic mail, or search the Internet for information. The main benefit of information sharing is improved decision making, which makes it generally more important than resource sharing.

• Resource sharing: It refers to one computer sharing a hardware device (e.g., a printer) or a software package with other computers on the network. The main benefit of resource sharing is cost savings.

Types of LANs

By the categories, there are three common types of LANs. Dedicated server LANs account for more than 70 percent of all installed LANs.

• Dedicated Server Networks
  
• Peer-to-Peer Networks
  
• Zero-Slot LANs
  

• Dedicated Server Networks: A dedicated server LAN can connect with almost any other network, can handle very large databases, have a dedicated network server, and uses sophisticated LAN software. Moreover, high-end dedicated server LANs can be easily interconnected to form enterprise-wide networks or, Sometimes, replace the host mainframe central computer. Generally speaking, the dedicated server is a powerful microcomputer.
  Three software components must work together and with the network hardware to enable communications: the network operating system (NOS) in the dedicated server, the network communication software that interconnects the server to the user computers, and the application software that runs on the server and client computers. Four common types of dedicated server LANs are file servers, database servers, print servers, and communication servers.

• Peer-to-Peer Networks: This network is a local area network that allows all users access to data on all workstations. In this networks, any computer can perform as both a client and a server. Each computer on the network shares its resource such as hard disk and printer with any other computer on the same network.
  This network is usually slower, has less capability, supports a limited number of computers, provides less sophisticated software, and is more difficult to manage than dedicated server LANS. However, this LAN uses cheaper computers and programs. Examples of peer-to-peer LANs include Artisoft's LANtastic, Novell's NetWare Lite, and Windows for Workgroups.

• Zero-Slot LANs: This LAN operates like peer-to-peer LAN, but offers limited, simple abilities such as sharing files and printers one another, transfer files, and transmit e- mail. It is inexpensive. It does not require a network interface circuit card. Its adapter plug can be plugged into a serial or parallel port. This network usually can handle up to 30 computers.

LAN Components

There are five basic components to an LAN.

• Server
  
• Client Computer
  
• NIC (Network Interface Card)
  
• Network Cables/Hubs
  
• NOS (Network Operating System)
  

• Server: A server is a computer in a network that shared by multiple users. There are many kinds of servers in dedicated server networks. For example, file servers, printer servers, and communication servers are they.

• Client Computer: In a communications network, the client computer is the requesting machine from the supplying machine, server.

• NIC: The Network Interface Card (NIC) is a printed circuit board that plugs into a network server or client computer. It enables the computer to be physically connected to the network cable, which provides the physical layer connection among the computers in the network. The NIC performs the electronic functions of the access method, or data link protocol, such as Ethernet, Token Ring and LocalTalk.

• Network Cables/Hubs: Network cable physically connects each computer to the other computers in the network. The selection of an LAN topology can be influenced greatly by the type of cable.
  • Network Cable: Most LANs use a combination of unshielded twisted pair (UTP) wires, shielded twisted pair (STP), coaxial cable, and fiber optic cable. The advantage of the shielded and unshielded twisted pair wires is in the low cost. Coaxial cable is also commonly used and physically larger and heavier than twisted pairs. The fiber optic cable is thin and light. Its high capacity makes it perfect for backbone networks. It is beginning to use fiber optic cable for LANs.

  • Network Hubs: A network hub is called in many different names such as concentrator, multistation access unit, transceiver, or repeater. It serves two purposes. First, they provide an easy way to connect network cables. Second, hubs act as repeaters or amplifiers.

  • Wireless LANs: Wireless LAN is an alternative method of cabling a local area network. They use the same protocols as other LANS, but they deliver data and information through the air rather than through physical cable. Wireless LAN offers an alternative for an old building where wiring is difficult and expensive. It provides new capability for mobile computing for laptop computers. Its disadvantages are the noise and insecurity.

• NOS: The NOS (network operating system) is the control program that resides in a server or workstation within an LAN. This controls the network. The NOS handles the requests for data from all the users on the networks. It is responsible for data link layers and the network layers. It must interact with the application programs and the computer's own operating system.
  One of the well-known NOS for microcomputer is a NetWare. This is an NOS from Novell, Inc. This runs on 286 and higher personal computers and supports DOS, OS/2 and Macintosh operating system. It also supports many LAN access methods such as Ethernet, Token Ring, ARCNET and Starlan.

Two Most Common LANs

The two most commonly used LANs are Ethernet and Token Ring local area networks.

Ethernet

The Ethernet is a local area network developed by Xerox, Digital (DEC) and Intel and is the most popular LAN in the world, accounting for almost 50 percent of all LANs.

• Topology: Ethernet uses a bus topology. A major circuit running the length on the network connects all computers. When the Ethernet uses a central hub, the topology of it looks like a star (physically) from the outside, but it is really a bus (logically).

• Types of Ethernet:
  • 10Base5: The original Ethernet specification was a 10Mbps data rate using Baseband signaling on thick coaxial cable, called "Thick Ethernet (5)."

  • 10Base2: Today, thin coaxial cable is rapidly replacing the original thick coax because it is considerably cheaper and easier to work with. The 10Base2 standard is often called "Thin Ethernet (2)."

  • 10BaseT: This is the most commonly used type of Ethernet. The name means 10 million bits per second, Baseband, and the "T" means it uses twisted pair wiring. The extremely low cost of 10BaseT made Ethernet very inexpensive and the most popular type of LAN.

  • 10Broad36: It means 10 Mbps, broadband, with a maximum distance of 3600 meters. This standard is most commonly used in backbone networks.

Token Ring

This is a local area network developed by IBM that uses a special twisted wire and the token passing access method. Token Ring LANs are the second most popular type of LAN, with almost 40 percent of all LANs worldwide.

• Topology: A Token Ring network uses a ring topology. All messages pass to each computer in turn. Computers receive all messages, but only process those addressed to themselves. They deliver the rest of messages to the next computer in the network. When central hubs are used, it also looks like a star configuration (physically), but it really is a ring network (logically).

• Media Access Control: Token Ring network uses a token passing technology. A computer with a message to deliver waits until it receives free token. The computer then changes the free token into a busy token, attaches its message to it, and retransmits it on the circuit to the next computer.

• Types of Token Ring: There are two common types of Token Ring networks. The Token Ring developed first was Token Ring-4, transmitting data at 4Mbps over twisted wire. The newer Token Ring network is Token Ring- 16, delivering information at 16Mbps over high quality twisted wire.

MANs

The next larger network than LAN may be the Metropolitan Area Networks (MANs). MAN usually spans a geographical area that usually encompasses a city or county area. It interconnects various buildings or other facilities within this citywide area. For example, linkages can be established between two commercial buildings. A more recent use of MAN technology has been the rapid development of cellular phone systems.

A wide area network (WAN) is one that operates over a vast distance (e.g., nationwide). Its nodes may span cities, states, or national boundaries. This network interconnects computers, LANS, BNs, MANS, and other data transmission facilities. Typically, WAN will employ communications circuits such as long- distance telephone wires, microwaves and satellites. FOR example, nationwide automated teller machines used in banking represent a common application of a wide area network.

NetBIOS

NETwork Basic Input/Output System is a commonly used transmission protocol for PC LANs. This is an extension to DOS, which examines all functions and direct to DOS. It provides a program called "redirector" that resides on top of DOS.