Users must have access to communications media to communicate between com puttingr devices. A communications medium is a link (a connection) that allows computers in different locations to be connected. When communications take place between distant computers, a combination of media may be used, some of which the user may never see. Communications media are broadly classified as either wired or wireless.
Wired Communications Media
While many computers, devices, and networks now use wireless technologies, many others continue to use wired technologies as a means of communicating The medium chosen depends mainly on user requirements relating to availability cost, speed, and other factors
Twisted-Pair cable, one of the older types of communications media, was originally developed for telephone networks. Early versions consisted of wires wrapped (twisted) around one another to reduce noise. Today such cables used with computer networks typically consist of two parallel copper wires, each individually wrapped in plastic and bound together by another plas tic casing. The pairs are often bundled in packs of hundreds or thousands, buried in underground electrical conduits (pipes), and run to various locations, such as buildings and rooms, where they can be connected to standard phone jacks.
Twisted-pair cable can be used to connect computers in networks for transmit ting data over relatively short distances. Millions of home computer owners use this medium with a modem because the cable is already in place. The advantages of twisted-pair cable are its availability and low price. To ensure more accurate transmissions over long distances, repeater stations may be positioned along the way to refresh (strengthen) the communication signals.
Coaxial Cable Coaxial cable is commonly used for cable television connections, in telephone networks, and in some computer networks. The cable consists of an insulated center wire grounded by a shield of braided wire. Coaxial cable is more expensive than twisted-pair, but is less susceptible to interference and can carry much more data. Baseband coaxial cable, often used in computer networks, is about s inch thick and has a single channel for trans- mitting digital signals at about 10 Mbps. Broadband coaxial cable has several channels, each of which can carry about 10 Mbps. Broadband is used for cable television transmissions.
Millions of cable television subscribers already have cable installed in their homes and offices. By adding a cable modem, television subscribers can take advantage of this communications medium to receive much faster data transmis sion speeds than twisted-pair cable can offer. According to a June 2009- conducted by the Pew Internet and American Life Project, cable modems are a preferred medium for broadband connections to the Internet, with 41 percent of broadband users connecting via cable modems. To use a cable modem, a device called a splitter must be installed. One part of the splitter connects to the televi sion cable, and the other part connects to the cable modem. survey
Fiber-Optic Cable A twisted-pair cable and a coaxial cable both contain cop- per conductors and transmit electrical signals-streams of electrons. Instead of copper, a fiber-optic cable uses a string of glass to transmit photons-beams of light. the construction of a single fiber-optic cable. However, a fiber-optic cable typically consists of hundreds of clear fiberglass or plastic fibers (threads), each approximately the same thickness as a human hair. Data is converted into beams of light by a laser device and transmitted as light pulses. Billions of bits can be transmitted per second. At the receiving end, optical detec tors convert the transmitted light pulses into electrical pulses that computing devices can read. The advantages of using fiber-optic cables include:
⚫ faster transmission speeds (up to 1 trillion bits per second)
⚫ higher data transmission volumes
⚫ minimal interference
⚫ greater security longer cable life
Fiber-optic cable is expensive and difficult to work with, but the advantages of using the technology outweigh the disadvantages. The most important advantage of fiber-optic cables is that they are a very high bandwidth (broadband) medium, and therefore they have become the medium of choice for many local area net- works. However, when data must be sent to distant computers or to networks using analog media, the sending computer or the network's host computer uses a modem to convert the data into analog form.
Integrated Services Digital Network (ISDN) Lines In some locations a special digital telephone line, called Integrated Services Digital Network (ISDN) line, is available. This type of line can be used to dial into the Internet and transmit and receive information at very high speeds, ranging from 64 Kbps to 128 Kbps. Using an ISDN line requires a special ISDN modem. Monthly fees for ISDN lines are higher than for regular phone lines, adding to a user's communications costs. With the widespread adoption of high-speed modems and DSL at a relatively low cost, the growth of ISDN line usage remains flat.
Digital Subscriber Line (DSL) Digital Subscriber Line (DSL) technology uses existing copper phone lines and new optimized switched connections to achieve faster telecommunications speeds than traditional dial-up phone access. DSL separates voice and data into discrete channels so that users can still make phone calls while connected to the Internet via a DSL modem. DSL is considered broadband technology, as connection speeds range from 144 Kbps to 1.56
Mbps. Currently cable modems provide the strongest competition to DSL lines According to a June 2009 survey by the Pew Internet and American Life 33 percent of households with broadband connections to the Internet used DSL However, DSL technology is not available in all locations because there is a physical limitation on how far away from a telephone company office a DSL line can reach.
T Lines The generic term T line refers to any of several types of digital high-speed long-distance telephone lines developed by Bell Labs that are capable of carrying multiple types of signals across the line, including both voice and data. T lines can carry data at very high speeds, but they are expensive. Their high cost typically limits their use to large companies and organizations.
Unlike a standard dial-up telephone line that can carry only a single signal, T lines use multiplexing, thereby making it possible for multiple signals to share a single telephone line. Two popular types of digital T lines are TI lines and T3 lines, both of which can carry voice and data. There are no T2 lines.