Networks

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In its simplest form, networking takes place between two devices that are directly connected. However, it is often impractical for devices to be directly connected, such as when devices are far apart or when more than two devices want to communicate. The solution is to attach each device to a communication network. Communication networks can be categorized on the basis of architecture and techniques to transfer data:

1. broadcast networks, where a transmission from any device is broadcast and received by all other stations;
2. circuit-switched networks, where a dedicated connection is established between devices on a network across switching nodes within the network;
3. packet-switched networks, where data are sent in smaller units, called "packets," from node to node within a network from source to destination.

The basic configuration, or topology, of a network is the geometric representation of all the links and nodes of a network. A link is the physical communication path that transfers data from one device to another. A node isa network-addressable device. There are five basic topologies: mesh, star, tree, bus, and ring. In a mesh topology, every node has a dedicated point-to-point link to every other node, which requires n (n1)/2 links to connect n nodes. For example, a network with 5 nodes would need 10 links to connect the nodes. In a star topology, each node has a dedicated point-to-point link to a central hub. If one node wants to send data to another, it sends to the hub, which then relays the data to the destination node. A tree topology occurs when multiple star topologies are connected together such that not every node is directly connected to a central hub. In a bus topology, one long cable connects all nodes in the network; in a ring topology, each node has a dedicated point-to-point connection to the nodes on either side in a physical ring such that a signal from a source travels around the ring to the destination and back to the source.

Network devices use signals in the form of electromagnetic energy to represent data. Electromagnetic energy, a combination of electrical and magnetic fields vibrating in relation to each other, includes electrical current, radio waves, and visible light. Unguided, or wireless, media transport electromagnetic waves without using a physical conductor to guide the wave. Instead, signals are broadcast through media, such as air or water, and are thus available to any device capable of receiving them. Guided, or wired, communications direct electromagnetic waves within the physical limitations of a conductor, which may be metallic wire, a hollow tube waveguide, or optical fiber.

Optical fiber uses light as a transmission medium. Light is electromagnetic energy at a specific range of frequencies (430 to 750 terahertz) whose speed depends on the density of the medium through which it is traveling. Theoretically, rays of light injected into strands of pure glass at specific angles will experience total internal reflection, meaning that no loss of energy occurs when light travels down the strand. In practice, some attenuation (loss of energy) and dispersion (mixing of frequencies) does occur because of impure glass and injected light signals at multiple frequencies; however, the range of frequencies, and thus data rates, that can be supported is dramatically higher than is possible with copper cables.

Copper cabling that accepts and transports signals in the form of electrical current comes in four different types:

1. unshielded, which is used most commonly in telephone systems;
2. twisted pair, which consists of two copper conductors surrounded by an insulating material and wrapped around each other to reduce significantly the impact of noise;
3. shielded twisted pair, which has a metal foil encasing each twisted pair;
4. coaxial cable, which carries signals of higher-frequency ranges because of its different construction (a central core conductor enclosed in an insulating sheath that is encased in an outer foil, or braid, that is protected by a plastic cover).

Asynchronous Transfer Mode (ATM) is a packet-switched technology where all the packets are the same size, referred to as "cells." Asynchronous meansthat the cells are independent of each other with potentially different gaps between them. The fixed cell size of 53 bytes allows ATM to have traffic characteristics such as increased switching speed and predictably decreased delay/cell loss, which is preferable for the convergence of real-time voice and video with data.

Other important networking techniques include Ethernet and frame relay. Ethernet is a standard for network devices communicating over a bus topology. Any device wishing to transmit will listen to the bus to determine whether the bus is clear; if the bus is clear, transmission can commence. If a collision between signals from different devices occurs, transmission stops and the process is repeated. Frame relay is a packet-switching protocol with no error correction that is appropriate for fiber optic links with their corresponding low error rates.

A local area network (LAN) is usually privately owned and connects nodes within a single office or building designed to share hardware, such as a printer; software, such as an application program; or data. A wide area network (WAN) provides long-distance transmission over large geographic areas that may constitute a nation, a continent, or even the whole world. A WAN that is wholly owned by a single company is referred to as an "enterprise network," but WANs may buy or lease network capacity from other companies. A metropolitan area network (MAN) is designed to extend over an entire campus or city. A MAN may be a single network, as with cable television, or a series of interconnected LANs.

Two other concepts that are relevant are the Internet and intranets. The Transmission Control Protocol/Internetworking Protocol (TCP/IP) is a set of protocols (or protocol suite) that defines how all transmissions are exchanged across the Internet. The Internet itself is a network of networks connected with the TCP/IP protocol suite connecting more than 200 million devices worldwide in virtually all populated countries. Conversely, an intranet is an organizational network of private addresses not directly accessible from the Internet.

Internet; Intranet; Telecommunications; World Wide Web.

Comer, Douglas E. The Internet Book: Everything You Need to Know about Computer Networking and How the Internet Works, 3rd ed. Englewood Cliffs, NJ: Prentice Hall, 2000.

Forouzan, Behrouz. Introduction to Data Communications and Network, 2nd ed. New York: McGraw-Hill, 2000.

Halsall, Fred. Data Communications, Computer Networks and Open Systems, 4th ed. New York: Addison-Wesley, 1996.

Stallings, William. Data and Computer Communications, 6th ed. New York: Macmillan, 2000.

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