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LAN Tutorial

15 December 1994 EP 1110-3-7 SECTION 2. LAN TUTORIAL This section defines LAN concepts, terminology, and components. All information is in compliance with International Standards Organization (ISO), International Electrotechnical Commission (IEC), Institute of Electrical and Electronics Engineers (IEEE), and American National Standards Institute (ANSI) standards. WHAT IS A NETWORK With the development of personal computers (PC) in the 1980s, more and more computer oriented operations were inte
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  15 December 1994 EP 1110-3-7NETWORK TUTORIAL2-1 SECTION 2. LAN TUTORIAL This section defines LAN concepts, terminology, andcomponents. All information is in compliance withInternational Standards Organization (ISO), InternationalElectrotechnical Commission (IEC), Institute of Electricaland Electronics Engineers (IEEE), and American NationalStandards Institute (ANSI) standards.WHAT IS AWith the development of personal computers (PC) in theNETWORK1980s, more and more computer oriented operations wereinterconnecting terminals, desktop computers, printers,micro and minicomputers, mainframes, and otherinformation processing equipment. Interconnection quicklygrew in popularity because it enhanced the communicationcapabilities and lowered the cost of user/computerarrangements The organization of communications channelsmaking this interconnection possible, shown in figure 2-1,was termed a computer network.  EP 1110-3-715 December 19942-2NETWORK TUTORIALWHAT IS A LANA LAN is a computer network that operates within a limitedgeographic area, such as an office, a building, or a smallcluster of buildings. It operates at speeds of 1 megabits persecond (Mbps) to over 100 Mbps. LANs are generallyprivately owned networks, which are controlled by theirowners.WHAT IS A WANA wide area network (WAN) is a computer network thatspans a large geographical area, for example, states,countries, and continents. Figure 2-2 shows that a WAN isused to interconnect distant computers or LANs, which aretoo far apart to interconnect using a single device.WANs typically use a switching technology for datatransmission and operate at speeds of 56 kilobits persecond (kbps) or above. WANs use public communicationsservices including digital data service (DDS) lines, T-1 orfractional T-1 circuits, T-3 circuits, and X.25 public datanetworks.  15 December 1994 EP 1110-3-7NETWORK TUTORIAL2-3BENEFITS OF A LANs provide resource sharing, improve reliability, reduceLANcosts, increase worker productivity, and enhance groupcomputer management.Resource sharing enables LAN users to share softwareresources such as files and applications and hardwareresources such as printers and plotters. Use of a LANmeans that fewer printers, plotters, and other sharedequipment must be purchased. Software licenses do nothave to be purchased for each LAN user; only a singlenetwork or group license has to be acquired.Improved reliability is provided by multiple access toresources. Critical files and applications can reside on morethan one computer. If one computer goes down, the LANuser can still access resources from another computer.Cost savings are realized as a result of distributedprocessing. When a mainframe computer is the sole sourceof processing power, users who are not collocated requiredirect communication lines, and leased public servicecommunications cost money. In addition, distributedprocessing on smaller computers provides a betterprice/performance ratio than processing on a centralmainframe.LANs increase worker productivity and therefore decreasepersonnel costs. They enable information sharing anddecreased response times and eliminate or ease repetitiveoperations.In terms of group computer management, LANs makecentralized LAN user assistance groups a practical optionfor LAN user support. As a result of global network access,support groups are able to improve LAN user efficiency anddecrease equipment down time.OPEN SYSTEMSUntil the late 1980s, LAN vendors designed and producedhardware and software that functioned well on its own, butcould not be used with the products of a different or thirdparty vendor. LAN administrators who purchased a single  EP 1110-3-715 December 19942-4NETWORK TUTORIALvendor solution found themselves in a dilemma when itcame to upgrading or purchasing additional hardware andsoftware for their LANs. This was especially true if theycould not achieve a desired level of operability using theequipment or architecture offered by the single vendor. Anadministrator had to decide whether to spend a considerablesum of money to upgrade the network to an open system, orgo with a less acceptable solution and still be restricted byproprietary network architecture.The OSI referenceIn 1983, the ISO proposed the Open SystemsmodelInterconnection (OSI) reference model. This model definesan international set of standard protocols for communicationbetween different network architectures. These standard,vendor independent protocols promote worldwideconnectivity and interoperability between applicationprocesses in networks. The model was designed anddeveloped as a guideline so that hardware, applicationsoftware, and network services would interoperate betweenvendors. Currently, OSI is supported by all majorgovernments and principle computer/network manufacturersthroughout the world. The OSI layers The OSI reference model organizes LAN connectivity intoseven definable pieces or layers (a protocol stack). Withrespect to the purpose of each layer, ISO mandatedprotocols define how networking hardware and software is tohandle data and transfer it across a network.Interoperability, the purpose for defining a standard protocolmodel, exists when there is compatibility between theprotocol stack of one workstation or peripheral device andthat of another. Each individual layer is able to communicatewith the respective layer of a receiving station orintermediate translation device as long as the OSI referencemodel is supported. Figure 2-3 shows the protocol stacks oftwo stations attached to the same LAN and shows the dataflow between the sending and receiving applicationprocesses.
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