A network is defined as a group of two or more computers linked together for the purpose of communicating and sharing information and other resources, such as printers and applications. Most networks are constructed around a cable connection that links the computers, however, modern wireless networks that use radio wave or infrared connections are also becoming quite prevalent. These connections permit the computers to communicate via the wires in the cable, radio wave or infrared signal. For a network to function it must provide connections, communications, and services.

. Connections are defined by the hardware or physical components that are required to connect a computer to the network. This includes the network medium, which refers to the hardware that physically connects one computer to another, i.e., the network cable or a wireless connection; and the network interface, which refers to the hardware that attaches a computer to the network medium and is usually a network interface card (NIC).

. Communications refers to the network protocols that are used to establish the rules governing network communication between the networked computers. protocols allow computers running different operating systems and software to communicate with each.

. Services define the resources, such as files or printers, that a computer shares with the rest of the networked computers.

Network Definitions

Computer networks can be classified and defined according to geographical area that the network covers. There are four network definitions: a Local Area Network (), a Campus Area Network (CAN), a Metropolitan Area Network (MAN), and a Wide Area Network (). There are three additional network definitions, namely the Internet, an intranet and an Internetwork. These network definitions are discussed in Table 1.2.

Definition Description
Local Area Network (LAN) A LAN is defined as a network that is contained within a closed environment and does not exceed a distance of 1.25 mile (2 km). Computers and peripherals on a LAN are typically joined by a network cable or by a wireless network connection. A LAN that consists of wireless
connections is referred to as a Wireless LAN (WLAN).
Campus Area Network (CAN) A CAN is limited to a single geographical area but may exceed the size of a LAN
Metropolitan Area Network (MAN) A MAN is defined as a network that covers the geographical area of a city that is less than 100 miles.
Wide Area Network (WAN) A WAN is defined as a network that exceeds 1.25 miles.
A WAN often consists of a number of LANs that have been joined together. A CAN and a MAN is also a WAN. WANs typically connected numerous LANs through the internet via telephone lines, T1 lines, Integrated Services Digital Network (ISDN) lines, radio waves, cable or satellite links.
Internet The Internet is a world wide web of networks that are based on the TCP/IP protocol and is not own by a single company or organization.
Intranet An intranet uses that same technology as the Internet but is owned and managed by a company or organization. A LAN or a WAN s usually an intranet.
Inter network An inter network consists of a number of networks that are joined by routers. The Internet is the largest example of an inter network.

TABLE 1.2: Network Definitions

Of these network definitions, the most common are the Internet, the LAN and the WAN.

Types of Networks

These network definitions can be divided into two types of networks, based on how information is stored on the network, how network security is handled, and how the computers on the network interact. These two types are: Peer-To-Peer (P2P) Networks and Server/Client Networks. The latter is often also called Server networks.

. On a Peer-To-Peer (P2P) Network, there is no hierarchy of computers; instead each computer acts as either a server which shares its data or services with other computers, or as a client which uses data or services on another computer. Furthermore, each user establishes the security on their own computers and determines which of their resources are made available to other users. These networks are typically limited to between 15 and 20 computers. Microsoft Windows for Workgroups, Windows 95, Windows 98, Windows ME, Windows NT Workstation, Windows 2000, Novell’s NetWare, UNIX, and Linux are some operating systems that support peer-to-peer networking.

. A Server/Client Network consists of one or more dedicated computers configured as servers. This server manages access to all shared files and peripherals. The server runs the network operating system (NOS) manages security and administers access to resources. The client computers or workstations connect to the network and use the available resources. Among the most common network operating systems are Microsoft’s Windows NT Server 4, Windows 2000 Server, and Novell’s NetWare. Before the release of Windows NT, most dedicated servers worked only as hosts. Windows NT allows these servers to operate as an individual workstation as well.

Network Topologies

The layout of a LAN design is called its topology. There are three basic types of topologies: the star topology, the bus topology, and the ring topology. Hybrid combinations of these topologies also exist.

. In a network based on the star topology, all computers and devices are connected to a centrally located hub or switch. The hub or switch collects and distributes the flow of data within the network. When a hub is used, data from the sending host are sent to the hub and are then transmitted to all hosts on the network except the sending host. Switches can be thought of as intelligent hubs. When switches are used rather than hubs, data from the sending host are sent to the switch which transmits the data to the intended recipient rather than to all hosts on the network.

FIGURE: The Star Topology

. In a network based on the bus topology, all computers and devices are connected in series to a single linear cable called a trunk. The trunk is also known as a backbone or a segment. Both ends of the trunk must be terminated to stop the signal from bouncing back up the cable. Because a bus network does not have a central point, it is more difficult to troubleshoot than a star network. Furthermore, a break or problem at any point along the bus can cause the entire network to go down.

FIGURE: The Bus Topology

. In a network based on a ring topology, all computers and devices are connected to cable that forms a closed loop. On such networks there are no terminating ends; therefore, if one computer fails, the entire network will go down. Each computer on such a network acts like a repeater and boosts the signal before sending it to the next station. This type of network transmits data by passing a “token” around the network. If the token is free of data, a computer waiting to send data grabs it, attaches the data and the electronic address to the token, and sends it on its way. When the token reaches its destination computer, the data is removed and the token is sent on. Hence this type of network is commonly called a token ring network. Of these three network topologies, the star topology is the most predominant network type and is based on the Ethernet standard.

FIGURE: The Ring Topology

Network Technologies

Various network technologies can be used to establish network connections, including Ethernet, Fiber Distribution Data Interface (FDDI), Copper Distribution Data Interface (CDDI), Token Ring, and Asynchronous Transfer Mode (ATM). Of these, Ethernet is the most popular choice in installed networks because of its low cost, availability, and scalability to higher bandwidths.

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