3. Media

The medium used to transmit information limits the speed of the network, the effective distance between computers, and the network topology. Copper wires and coaxial cable provide transmission speeds of a few thousand bits per second for long distances and about 100 million bits per second for short distances. (A million bits is equal to one megabit, and one megabit per second is abbreviated Mbps.) Optical fibers carry 100 million to 40 billion bits of information per second over long distances. (A billion bits is equal to one gigabit, and a billion bits per second is abbreviated Gbps.)

Wireless networks, often used to connect mobile, or laptop, computers, send information using infrared or radio-frequency transmitters. Infrared wireless local area networks (LANs) work only within a room, while wireless LANs based on radio-frequency transmissions can penetrate most walls. Wireless LANs using Wi-Fi technology have capacities of around 54 Mbps and operate at distances up to a few hundred meters. Wireless communications for wide area networks (WANs) use cellular radio telephone networks, satellite transmissions, or dedicated equipment to provide regional or global coverage. Although transmission speeds continue to improve, today’s wide area cellular networks run at speeds ranging from 14 to 230 kilobits per second. (A kilobit is equal to 1,000 bits, and one kilobit per second is abbreviated Kbps.) Some networks use a home’s existing telephone and power lines to connect multiple machines. HomePNA networks, which use phone lines, can transmit data as fast as 128 Mbps, and similar speeds are available on Power Line or HomePlug networks.

4. Topology

Common topologies used to arrange computers in a network are point-to-point, bus, star, ring, and mesh. Point-to-point topology is the simplest, consisting of two connected computers. The bus topology is composed of a single link connected to many computers. All computers on this common connection receive all signals transmitted by any attached computer. The star topology connects many computers to a common hub computer. This hub can be passive, repeating any input to all computers similar to the bus topology, or it can be active, selectively switching inputs to specific destination computers. The ring topology uses multiple links to form a circle of computers. Each link carries information in one direction. Information moves around the ring in sequence from its source to its destination. On a mesh network, topology can actually change on the fly. No central device oversees a mesh network, and no set route is used to pass data back and forth between computers. Instead, each computer includes everything it needs to serve as a relay point for sending information to any other computer on the network. Thus, if any one computer is damaged or temporarily unavailable, information is dynamically rerouted to other computers—a process known as self-healing. see Computer Architecture.

LANs commonly use bus, star, or ring topologies. WANs, which connect distant equipment across the country or internationally, often use special leased telephone lines as point-to-point links.