From the Analog to the Digital Age. Computers use digital signals, which present information in a binary way. Most other systems, such as traditional telephones and TV, use analog signals, which continuously vary in strength of quality. A modem converts digital signals into analog signals, so that computer signals can be sent over phone lines.
The Practical Uses of Communications. There are many forms of connectivity, or communications connections. (1) Videoconferencing, the linking of people through TV, video, and sound, by using point-to-point or multipoint arrangements, is useful for long-distance meetings. (2) Workgroup computing, in which microcomputer networks enable workers to cooperate on projects, allows people to work on the same information at the same time. (3) Telecomputing, working at home with telecommunications, can increase productivity and decreases costs for employers and employees. (4) Virtual offices, nonpermanent, mobile offices that run with telecommunications and computers, add workplace flexibility. (5) Home networks enable households to link and share all kinds of peripheral devices and internet services. (6) Smart television consists of digital television (DTV) (DTV), high-definition television (HDTV), and standard-definition television (SDTV). DTV uses a digital signal, or series of 0s and 1s. The high-resolution type of DTV is HDTV. An alternate DTV is standard-definition television (SDTV), which allows broadcasters to transmit more information within the HDTV bandwidth so that they can broadcast six channels instead of one.
Communications Channels. The following factors affect how data is transmitted.
The electromagnetic spectrum consists of fields of electrical energy and magnetic energy, which travel in waves. A part of the electromagnetic spectrum is the radio frequency spectrum, fields of electrical and magnetic energy that carry communications signals, which vary according to frequency. A range or a band of frequencies that a transmission can carry in a given period of time is called 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. (2) Broadcast radio sends data over long distances, between states, regions, and countries. (3) Microwave radio transmits voice and data as superhigh-frequency radio waves. (4) Communications satellites are microwave relay stations in orbit around the earth. They occupy one of the three zones in space: GEO, MEO, or LEO.
Types of long-distance wireless communications may be one-way or two-way. Examples of one-way communication are (1) Global Positioning Satellite (GPS), which consists of satellites that can be used to identify earth locations identified by GPS signal receivers, and (2) pagers, radio receivers that receive data from special radio transmitters. Examples of two-way communication are (1) two-way pagers; (2) analog cellphones, 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, supports digital cellphones and personal digital assistants, uses 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 transmit data at high speed, provide web access, and display color video and still pictures and play music.
Short-range wireless communication standards include (1) Bluetooth, a short-range wireless digital standard aimed at linking cell phones, PDAs, computers, and peripherals up to distances of 30 feet, and (2) WiFi, a short-range wireless digital standard aimed at helping portable computers and handheld wireless devices to communicate at high speeds and share internet connections at distances up to 300 feet. It connects through access points to a kind of local area network known as the Ethernet.
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 usually 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. When they are decompressed, the bits are restored, so that the data is the same as what went in—important in computer data, databases, spreadsheets, and word processing files. 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.
Factors Affecting How Data is Transmitted. Several factors affect how data is transmitted. (1) Line configurations are the methods whereby communications lines are connected. A point-to-point line directly connects the sending and receiving devices, whereas a multipoint line is a single line that interconnects several communications devices to one computer. (2) Serial transmission transmits the bits sequentially; parallel data transmission transmits bits through separate lines simultaneously. (3) Data can flow in three ways: simplex (one way); half-duplex (in both directions but not at the same time); and full-duplex (in both directions simultaneously). (4) The transmission mode can be either asynchronous or synchronous. In asynchronous transmission, data is sent one byte at a time, with a "start" bit and a "stop" bit. With synchronous transmission, data is transmitted in blocks, with a start and a stop bit pattern to delineate each block. (5) In circuit switching, the transmitter has full use of the circuit until all the data has been transmitted and the circuit is terminated. In packet switching, electronic messages are divided into packets for transmission over a wide area network to their destination, through the most expedient route. (6) The efficiency of data transmission can be increased by transmitting multiple signals over a single communications channel, a process known as multiplexing. Multiplexing devices include multiplexers, concentrators, and front-end processors. (7) A protocol is a set of conventions that control the exchange of data between hardware and/or software components in a communications network. (8) OSI is an international standard that defines seven layers of protocols for worldwide computer communications.
Networks. A communications network is a system of interconnected computers, phones, or other communications devices that can share applications and data. Networks enable people to share peripheral devices, programs, and data; provide better communications; help to secure information; and provide access to numerous databases.
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 that request data (clients) and powerful computers that provide data (servers). A file server, for example, stores programs and data files; other servers are database server, print 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 connection or the cabling system, microcomputers with network interface cards (NIC), 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 helps to communicate messages when networks are tied together; a bridge, an interface used to connect the same types of networks; a gateway, an interface that allows communication between dissimilar networks; and a hub, a common connection point for devices in a network.
Networks can be laid out in three different topologies, or shapes: star, ring, and bus. In a star network, all devices are connected to a central server. In a ring network, all devices are connected in a bus network. In a bus network, all devices are connected to a common channel.
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. In order to minimize communication costs, some companies have established VPN (Virtual private Networks), private networks that use a public network to connect remote sites.
The Future of Communications. Some of the new developments in communications are (1) Satellite-based systems consisting global high-speed satellite networks that will permit users to exchange a broader range of data, including internet pages and videophone calls, anywhere in the world. (2) 4G (Fourth Generation) wireless technology is currently being researched by Hewlett-Packard and a Japanese telecommunications company even before 3G radio spectrum is available in the U.S. (3) Photonics, the science of sending data bits by means of light pulses carried on glass fibers, would enable more light signals to be carried on fiber-optic lines. (4) In grid computing, the grid refers to the linking of many servers into a single system for the purpose of breaking down complex computing tasks and doing work previously possible only with supercomputers. Geologists use grid networks in a limited way to simulate the effects of earthquakes, and biochemists use it to simulate virus attacks on the human body.
Cyberethics. Three important issues of cyberethics are as follows. (1)To protect children against access to controversial material, parents may employ blocking software that screens objectionable material based on keywords; browsers may contain built-in ratings for internet and web use; and the V-chip, which allows the screening of TV programs high in violence and sex.
