Encyclopedia of Computer Science and Technology

data communications 133Hellerstein, Joseph S., and Michael Stonebreaker, eds. Readingsin Database Systems. 4th ed. Cambridge, Mass.: MIT Press,2005.Hoffer, Jeffrey A., Mary Prescott, and Fred McFadden. ModernDatabase Management. 8th ed. Upper Saddle River, N.J.: PrenticeHall, 2006.Powell, Gavin. Beginning XML Databases. Indianapolis: Wrox,2006.“Web Programming: Databases.” Available online. URL: http://www.webreference.com/programming/databases.html.Accessed July 8, 2007.Williams, Hugh E., and David Lane. Web Database Applicationswith PHP and MySQL. 2nd ed. Sebastapol, Calif.: O’ReillyMedia, 2004.data communicationsBroadly speaking, data communications is the transfer ofdata between computers and their users. At its most abstractlevel, data communications requires two or more computers,a device to turn data into electronic signals (and backagain), and a transmission medium. Telephone lines, fiberoptic cable, network (Ethernet) cable, video cable, radio(wireless), or other kinds of links can be used. Finally, theremust be software that can manage the flow of data.Until recently, the modem was the main device usedto connect personal computers to information services orModern data communications can be thought of as a series of layers,from the actual physical connection (such as a cable) at the“bottom” to the operations of software such as Web browsers or e-mail programs at the highest level.networks (see modem). In general, data being sent over acommunications link must be sent one bit at a time (this iscalled serial transmission, and is why an external modem isconnected to a computer’s serial port). However most phonecables and other links are multiplexed, meaning that theycarry many channels (with many streams of data bits) atthe same time.To properly recognize data in a bit stream coming over alink, the transmission system must use some method of flowcontrol and have some way to detect errors (see error correction).Typically, the data is sent as groups or “frames”of bits. The frame includes a checksum that is verified bythe receiver. If the expected and actual sums don’t match,the recipient sends a “negative acknowledgment” messageto the sender, which will retransmit the data. In the originalsystem, the sender waited until the recipient acknowledgedeach frame before sending the next, but modern protocolsallow the sender to keep sending while the frames beingreceived are waiting to be checked.The actual transmission of data over a line can be consideredto be the lowest level of the data communicationsscheme. Above that is packaging of data as used and interpretedby software. Unless two computers are directly connected,the data is sent over a network, either a local areanetwork (LAN) or a wide-area network such as the globalInternet. A network consists of interconnected nodes thatinclude switches or routers that direct data to its destination(see network). Networks such as the Internet usepacket-switching: Data is sent as individual packets thatcontain a “chunk” of data, an address, and an indicationof where the data fits within the message as a whole. Thepackets are routed at the routers using software that triesto find the fastest link to the destination. When the packetsarrive at the destination, they are reassembled into theoriginal message.ApplicationsData communications are the basis both for networks andfor the proper functioning of servers that provide servicessuch as World Wide Web pages, electronic mail,online databases, and multimedia content (such as audioand streaming video). While Web page design and e-commerceare the “bright lights” that give cyberspace its character,data communications are like the plumbing withoutwhich computers cannot work together. The growingdemand for data communications, particularly broadbandservices such as DSL and cable modems, translates into asteady demand for engineers and technicians specializingin the maintenance and growth of this infrastructure (seebroadband).Besides keeping up with the exploding demand formore and faster data communications, the biggest challengefor data communications in the early 21st centuryis the integration of so many disparate methods of communications.A user may be using an ordinary phoneline (19th-century technology) to connect to the Internet,while the phone company switches might be a mixtureof 1970s or later technology. The same user mightgo to the workplace and use fast Ethernet cables over a