Local Area Networks

A local area network (LAN)connects two or more communicating devices within 2,000 feet (usually within the same building), so that every user device on the network has the potential to communicate with every other device. A LAN allows a large number of users to share corporate resources (such as storage devices, printers, programs, and data files) and integrates a wide range of functions into a single system. In an office, a LAN can give users fast and efficient access to a common bank of information while also allowing the office to pool resources such as printers and facsimile machines. A well-constructed LAN can also eliminate the need to circulate paper documents by distributing electronic memos and other material to each worker’s terminal.

LANs come in an assortment of topologies. The topologyof a network is the physical layout and connectivity of a network. Specific protocols, or rules of communications, are often used on specific topologies, but the two concepts are different. Topology refers to the ways the channels connect the nodes, whereas protocol refers to the rules by which data communications take place over these channels. There are five basic network topologies: star, bus, ring, hierarchical, and hybrid. Figure 1 illustrates these different types.

Each topology has strengths and weaknesses. When systems developers choose a topology, they should consider such performance issues as delay, speed, reliability, and the network’s ability to continue through, or recover after, failure in any device or connection to the network. The company should also consider such physical constraints as the maximum transmission speed of the circuit, the distances between nodes, the circuit’s susceptibility to errors, and the overall system costs.

Figure 1. The five main network typologies.

LAN technology.The LAN file server is a repository of various software and data files for the network. The server determines who gets access to what and in what sequence. Servers may be powerful microcomputers with large, fast-access hard drives, or they may be workstations, minicomputers, or mainframes. The server typically

houses the LAN’s network operating system, which manages the server and routes and manages communications on the network.

The network gateway connects the LAN to public networks or other corporate networks so that the LAN can exchange information with networks external to it. A gatewayis a communications processor that can connect dissimilar networks by translating from one set of protocols to another. A bridge connects two networks of the same type. A router routes messages through several connected LANs or to a wide area network.

A LAN consists of cabling or wireless technology linking individual devices, network interface cards (special adapters serving as interfaces to the cable), and software to control LAN activities. The LAN network interface card specifies the data transmission rate, the size of message units, the addressing information attached to each message, and network topology.

LANs employ a baseband or a broadband channel technology. In baseband LANs, the entire capacity of the cable is used to transmit a single digitally coded signal. In broadband LANs, the capacity of the cable is divided into separate frequencies to permit it to carry several signals at the same time.

Wireless local area networks (WLANs). WLAN technologiesprovide LAN connectivity over short distances, typically limited to less than 150 meters, and usually within one building.

Bluetooth technology. Bluetoothis a wireless technology that allows digital devices such as computers, printers, keyboards, cell phones, and Palm Pilots to communicate with each other via low-power radio frequencies. Bluetooth can also form a home network by linking devices like lights, televisions, the furnace and air conditioning, and the garage door. Bluetooth is not line-of-sight, which means that transmissions may occur around corners, through walls, and through briefcases. Problems with Bluetooth include security, transmission speed (Bluetooth maximum

transmission speed is 720 Kbps), and cost. IT’s About Business Box 6.3 provides an example of a Bluetooth application.

Wide Area Networks

Although most businesses have to transmit data throughout a LAN, most also have to send and receive data beyond the confines of the local area network. This is accomplished by connecting to one or more wide area networks. Wide area networks (WANs)are long-haul, broadband (analog) networks covering wide geographic areas. They generally are provided by common carriers. WANs include regional networks such as telephone companies or international networks such as global communications services providers. They usually have very-large-capacity circuits, with many communications processors that make it possible to use these circuits efficiently. WANs may combine switched and dedicated lines, microwave, and satellite communications. Some WANs are commercial, regulated networks, while others are privately owned, usually by large businesses that can afford the costs. Some WANs, however, are “public” in terms of their management, resources, and access. One such public WAN is the Internet, the foundation of the worldwide information superhighway. WANs can use any of the five basic types of network topologies, but they most generally use the star topology in order to more tightly control the network. A common WAN spanning the continental United States may have a dozen or more “hubs” that form a very complex star or group of stars.

Value-added networks. Value-added networks (VANs)are types of WANs. They are private, data-only networks that are managed by outside third parties and used by multiple organizations to provide economies in the cost of service and in network management. VANs can add message storage, tracking, and relay services as well as teleconferencing services, thus enabling their users to more closely tailor communications capabilities to specific business needs. VANs offer value through the telecommunications and computing services these networks provide to subscribers. Customers do not have to invest in network hardware and software or perform their own error checking, editing, routing, and protocol conversion. Subscribers realize savings in line charges and transmission costs because the costs of using the network are shared by many users.

Virtual private networks.A virtual private network (VPN)is a WAN operated by a common carrier. VPNs allow an organization to leverage the robust, shared communications infrastructure of the Internet to hook up with remote users, branch offices, and business partners worldwide, without paying the distance-sensitive fees that carriers charge for conventional network links. VPNs provide a gateway between a corporate LAN and the Internet, and they allow access to a corporate network’s e-mail, shared files, or intranet, via an Internet connection. A VPN server handles the security, such as authentication, permitting access from the Internet to an intranet. The data travels over the Internet in encrypted form. VPNs are particularly effective for extranets, because they allow the use of the Internet among business partners instead of using a more expensive VAN. VPNs are also especially important for international business, where long-distance calls or VANs remain very expensive.