Encyclopedia of Computer Science and Technology

cache 69particularly for writing software to run on Web servers andbrowsers (see Java). For an alternative approach to creatinga somewhat more “streamlined” C-type language, see c#.Further Reading“C++ Archive.” Available online. URL: http://www.austinlinks.com/CPlusPlus/. Accessed May 24, 2007.“Complete C++ Language Tutorial.” Available online. URL:http://www.cplusplus.com/_doc/tutorial/. Accessed May24, 2007.Prata, Stephen. C++ Primer Plus. 5th ed. Indianapolis: SAMS,2004.Stroustrup, Bjarne. “A History of C++: 1979–1991.” In History ofProgramming Languages II, edited by Thomas J. Bergin, Jr.,and Richard G. Gibson, Jr. New York: ACM Press; Reading,Mass.: Addison-Wesley, 1996, 699–755.———. The C++ Programming Language. Special 3rd ed. Reading,Mass.: Addison-Wesley, 2000.cable modemOne of the most popular ways to connect people to theInternet takes advantage of the cable TV infrastructure thatalready exists in most communities. (For another pervasivealternative, using telephone lines, see dsl.)Cable systems offer high-speed access (see broadband)up to about 6 megabits/second (Mb/s), at least 20 timesfaster than an ordinary telephone modem and generallysuitable for receiving today’s multimedia offerings, includingstreaming video. (Upload speeds are usually “throttled”to 384 kb/s or fewer.)In a typical installation, a splitter is used to separate thesignal used for cable TV from the one used for data transmission.The data cable is then connected to the modem.The modem can then either be connected directly to a computerusing a standard Ethernet “Cat 5” cable, or connectedto a switch (or more commonly, a router) that will in turnprovide the Internet connection to computers on the localnetwork. (If the cable modem is connected directly to acomputer, additional security against intrusions should alsobe provided. See firewall.)A typical cable TV system has from 60 channels to severalhundred, most of which are used for TV programming.A few channels are dedicated to providing Internet service.Users in a given division of the cable network (typically asmall neighborhood) thus share a fixed pool of bandwidth,which can reduce speed at times of high usage. The cablesystem can adjust by reallocating channels from TV to dataor by adding new channels.DOCSIS (Data Over Cable Service Interface Specification)is the industry standard for cable modems in NorthAmerica.Marketing ConsiderationsAs of 2007 there were about 30 million households in NorthAmerica with cable Internet service. Monthly service feesare $40–$60, though cable providers generally try to bundletheir cable TV and Internet services. Increasingly theyare also offering telephone service over the cable network,using voice over Internet protocol (see voip).In turn, telephone companies compete with cable companiesby offering DSL Internet access. Although “traditional”DSL is generally somewhat slower than cablemodems, Verizon in 2005 announced a new, much fasterfiber-based form of DSL called fios, with speeds of up to15 Mb/s (see also fiber optics). And just as cable companiescan now offer phone service over the Internet, phonecompanies can offer video content, potentially competingwith cable TV services. (Verizon has announced its ownInternet-based television network, IPTV.) In general there islikely to be increased competition and more (if sometimesperplexing) choices for consumers.Further ReadingCable Industry Insider. Available online. URL: http://www.lightreading.com/cdn/.Accessed May 10, 2007.Cable Modem Information Network. Available online. URL: http://www.cable-modem.net/. Accessed May 10, 2007.Dominick, Joseph R., Barry L. Sherman, and Fritz J. Messere.Broadcasting, Cable, the Internet and Beyond: An Introductionto Electronic Media. 6th ed. New York: McGraw-Hill, 2007.Dutta-Roy, Amitava. Cable Modem: Technology and Applications.New York: Wiley-Interscience, 2007.cacheA basic problem in computer design is how to optimizethe fetching of instructions or data so that it will be readywhen the processor (CPU) needs it. One common solutionis to use a cache. A cache is an area of relatively fast-accessmemory into which data can be stored in anticipation of itsbeing needed for processing. Caches are used mainly in twocontexts: the processor cache and the disk cache.CPU CacheThe use of a processor cache is advantageous becauseinstructions and data can be fetched more quickly fromthe cache (static memory chips next to or within the CPU)than they can be retrieved from the main memory (usuallydynamic RAM). An algorithm analyzes the instructionscurrently being executed by the processor and triesto anticipate what instructions and data are likely to beneeded in the near future. (For example, if the instructionscall for a possible branch to one of two sets of instructions,the cache will load the set that has been used mostoften or most recently. Since many programs loop overand over again through the same instructions until somecondition is met, the cache’s prediction will be right mostof the time.)These predicted instructions and data are transferredfrom main memory to the cache while the processor isstill executing the earlier instructions. If the cache’s predictionwas correct, when it is time to fetch these instructionsand data they are already waiting in the high-speed cachememory. The result is an effective increase in the CPU’sspeed despite there being no increase in clock rate (the rateat which the processor can cycle through instructions).The effectiveness of a processor cache depends on twothings: the mix of instructions and data being processed and