Web Summary

The electromagnetic spectrum consists of fields of electrical energy and magnetic energy, which travel in waves. In the middle is the radio frequency spectrum, fields of electrical and magnetic energy that carry communications signals, which vary according to frequency, or repeating waves. A range of frequencies is called a band or bandwidth. The wider the band, the faster data can be transmitted. Broadband connections are very high speed.

A communications channel is the path over which information travels in a telecommunications system. Channels may be wired or wireless.

Three types of wired channels are the following. (1) Twisted-pair wire, or standard telephone wire, consists of two strands of insulated copper wire twisted around each other; it is used for both voice and data transmission. (2) Coaxial cable consists of insulated copper wire wrapped in other materials; it is better than twisted-pair for resisting noise. (3) Fiber-optic cable consists of thin strands of glass or plastic that transmit beams of light rather than electricity; it is very fast and noise-resistant.

Four types of wireless channels are the following. (1) Infrared transmission sends data via infrared-light waves, as in some wireless mouses. (2) Broadcast radio sends data over long distances, as between states. (3) Microwave radio transmits voice and data via superhigh-frequency radio waves, as between hilltops. (4) Communications satellites are microwave relay stations that orbit the earth, occupying low, medium, or high (geostationary) earth orbits.

Types of long-distance wireless communications may be one-way or two-way. One-way is exemplified by (1) the Global Positioning System (GPS), timed radio signals sent by satellites that can be used to identify earth locations, and by (2) pagers, radio receivers that receive data from special radio transmitters. Two-way is exemplified by (1) two-way pagers; (2) analog cellular phones, designed for communicating by voice through a system of cells, each 8 miles or less in diameter and served by a transmitter-receiving tower; (3) digital wireless services, which support digital cellphones and personal digital assistants, using a network of cell towers to send voice communications and data over the airwaves in digital form; and (4) broadband wireless digital services, which are able to quickly transmit video, still pictures, and music.

Short-range wireless communication standards include: (1) Bluetooth, a short-range wireless digital standard aimed at linking cellphones, PDAs, computers, and peripherals up to distances of 30 feet; (2) WiFi, a short-range wireless digital standard aimed at helping machines inside offices to communicate at high speeds and share Internet connections at distances up to 300 feet; and (3) HomeRF, a separate, incompatible standard designed to network up to 10 PCs and peripherals as far as 150 feet apart.

Compression is a method of removing repetitive elements from a file so that it requires less time to transmit; at the receiving end, the file is decompressed -- the repeated patterns are restored. Two methods of compression are lossless and lossy. Lossless compression uses mathematical techniques to replace repetitive patterns of bits with a kind of coded summary; on decompression, the bits are restored, so that the data is the same as what went in -- important in database and similar computer data. Lossy compression permanently discards some data during compression; it is often used for graphics files and sound files. Two compression standards are JPEG, for still images, and MPEG, for moving images.

Types of networks are as follows. A wide area network (WAN) covers a wide geographical area, such as a country. A metropolitan area network (MAN) covers a city or suburb. A local area network (LAN) covers a limited area such as an office or a building. Most large networks have a host computer, a mainframe or midsize central computer to control the network. Any device attached to a network is called a node. MANs and LANs may be connected to the Internet by a high-speed network called a backbone.

Two types of LANs are client/server and peer-to-peer. A client/server LAN consists of microcomputers requesting data (clients) and powerful computers supplying data (servers). A file server, for example, stores programs and data files; other servers are database server, printer server, Web server, and mail server. In a peer-to-peer LAN, there is no server; microcomputers on a network communicate with each other directly . Several standard components of a LAN are the connecting or cabling system, microcomputers with network interface cards, network operating system (Novel NetWare, Microsoft Windows NT/2000, Unix, or Linux), and other shared devices (printers, storage devices). Other components are a router, a special computer that directs communicating messages when networks are tied together; a bridge, an interface used to connect the same types of networks; and a gateway, an interface permitting communication between dissimilar networks.

Organizations now use two variant networks that use the Internet's infrastructure and standards. One is an intranet, an organization's internal private network for employee use. The other is an extranet, for selected suppliers and other strategic parties as well as employees. Security for such networks is maintained through a firewall, a system of hardware and software that blocks unauthorized users inside and outside the organization.

To protect children against access to controversial material, parents may employ blocking software that screens objectionable material based on keywords; browsers that contain built-in ratings for Internet and World Wide Web use; and the V-chip, which allows the screening of TV programs high in violence, sex, and the like.

Privacy, the right of people not to reveal information about themselves, is under pressure from information technology. Web cookies, files stored on a user's hard drive when he or she visits a Web site, allow Web site operators to track user movements online. Medical records and office e-mail are other areas in which there may be privacy intrusions.