Encyclopedia of Computer Science and Technology

local area network 283edu/mc/LispNotes/RecursiveListProcessingAlgorithmsIn-Lisp.pdf. Accessed August 13, 2007.“Linked List Basics [In C/C++]” Available online. URL: http://cslibrary.stanford.edu/103/LinkedListBasics.pdf. AccessedAugust 13, 2007.Seibel, Peter. “They Called It LISP for a Reason: List Processing.”Available online. URL: http://www.gigamonkeys.com/book/they-called-it-lisp-for-a-reason-list-processing.html. AccessedAugust 13, 2007.local area network (LAN)Starting in the 1980s, many organizations sought to connecttheir employees’ desktop computers so they could sharecentral databases, share or back up files, communicate viae-mail, and collaborate on projects. A system that linkscomputers within a single office or home, or a larger areasuch as a building or campus, is called a local area network(LAN). (Larger networks linking branches of an organizationthroughout the country or world are called wide areanetworks, or WANs. See network.)A Token Ring network connects the machines in a “chain” aroundwhich messages called tokens travel. Any PC can “grab” a passingtoken and attach data and the address of another PC to it. Each PCin turn watches for tokens that are addressed to it.The Star network configuration uses a central hub to which eachPC is attached. To extend the network (such as into other offices),the hubs can be connected to one another so they function asswitches. When a token arrives that is addressed to one of its PCs,the hub will route it to the appropriate machine.Hardware ArchitectureThere are two basic ways to connect computers in a LAN.The first, called Ethernet, was developed by a project at theXerox Palo Alto Research Center (PARC) led by Robert Metcalfe.Ethernet uses a single cable line called a bus to whichall participating computers are connected. Each data packetis received by all computers, but processed only by the oneit is addressed to. Before sending a packet, a computer firstchecks to make sure the line is free. Sometimes, due to thetime delay before a packet is received by all computers,another computer may think the line is free and start transmitting.The resulting collision is resolved by having bothcomputers stop and wait varying times before resending.Because connecting all computers to a single bus line isimpractical in larger installations, Ethernet networks arefrequently extended to multiple offices by connecting a busin each office to a switch, creating a subnetwork or segment(this is sometimes called a star topology). The switches arethen connected to a main bus. Packets are first routed tothe switch for the segment containing the destination computer.The switch then dispatches the packet to the destinationcomputer. Another advantage of this switched Ethernetsystem is that more-expensive, high-bandwidth cable canbe used to connect the switches to move the packets morequickly over greater distances, while less-expensive cablingcan be used to connect each computer to its local switch.An alternative way to arrange a LAN is called tokenring. Instead of the computers being connect to a bus thatends in a terminator, they are connected in a circle wherethe last computer is connected to the first. Interference isprevented by using a special packet called the token. Likethe use of a “talking stick” in a tribal council, only thecomputer holding the token can transmit at a given time.After transmitting, the computer puts the token back intocirculation so it can be grabbed by the next computer thatwants to send data.LAN SoftwareNaturally there must be software to manage the transmissionand reception of data packets. The structure of a packet(sometimes called a frame) has been standardized with apreamble, source and destination addresses, the data itself,