PROCEEDINGS OF THE IEEE, VOL. 66, NO. 11, NOVEMBER 1978 1497 An Introduction to Local Area Networks DAVID D. CLARK, MEMBER, IEEE, KENNETH T. POGRAN, MEMBER, IEEE, AND DAVID p. WED Invited Paper A-t-Within 8 msbicled Per SUCh 8S 8 &I&buildin& or 8 Smd munication serves to distinguish networks, as discussed in this duskrofbuiIdbg,highsp&d@eaterthanlMbit/s)&taf . * paper, from otherarrangements of datacommunication hndhble8t.enrllfnctioaoftfreColtotob~coarp~~loag- holaervicefranrta~iffedcanmonctrdet... LOcrlare81letwmksuse hardware. this low-cor$ high-apeed t CSprbiliv 8S the bgis for 8 &pneral-purpaedata Mernetwak. There .re two basic issues m B. Relationship of Local Area Networks to Long-Haul loal area netwak desi@. Fit, how should the hadware R.lizing the Network netwak be orgmized to provi& mhbk high-speed canmuuiatim at ~WIl~y!WiththehWCdofthenWh#UtD~~8bility, I) The Evolution of Networking: Local area networks share care is required to keep #e amdated hardware cats ~dinglya kinship with both long-haul packet communication networks low. Secand, Atprotocols should be wdfa the apemt~anof the and with 1/0 bus structures of digital computer systems; their wtwak? While mrny protocd probkm6 are wmmm to leal -8 structure and protocols are rooted in packet communication, netwadrs d loaghml networtr atcb as the ARPANET, new protocds while their hardware technology derives from both networks 8re requid to explat the extended clprbilitier of leal area network Thbpipaddrerrer&edetWobrdcirrpea ItduoctxMi&athem0er- and computer busses.Local area networks arose out of the cainectioa of loal area netwaka and long4d netwab rnd plgenk continuing evolution of packet communication networks and 8 c.& study which descrii m detail the host computer interfice hd- computer hardware technology. Packet communication tech- ware required for a typid locd are8 networt niques have become well known and widely understood in the nine years since development of the ARPANETwas begun. I. INTRODUCTION Meanwhile, computer hardware has come down in price dra- S ITS NAME IMPLIES, alocal area network is a data matically, giving rise to environments where, within a sin& communication network, typically a packet communi- building or a small cluster of buildings, there may be one or A cation network, limited in geographic scope.’ A local more large mainframe computers along with a numberof mini- area network generally provides high-bandwidth communica- computers, microprocessor systems, andother intelligent tion over inexpensive transmission media. This paper discusses devices containing microprocessors. Local areanetworks what local area networksare, their structures, the sorts of evolved to meet the growing demand for high data rate, low- protocols that are used with them, and their applications. It cost communication among these machines. also discusses the relationship of local area networks to long- 2) Geographic Scope;Economic and TechnicalConsidera- haul networks and computer system 1/0 buses, as well as the tions: Fig. 1 illustrates the geographic scope spanned by long- impact of these networks on the field of computer communi- haul packet networks, local area networks, and computer sys- cations today. tem busses.Long-haul packetnetworks typically span A. Components of a Local Area Network distances ranging from meters’ to tens of thousands of kilometers (for intercontinental packet networks); bus struc- Like any other data communication network, a local area tures used in computer systems range from those of micro- network is composed of three basic hardwareelements: a processor systems, which can be short as 1-10 cm, to those transmission medium, oftentwisted pair, coaxial cable, or as used in large-scale multiprocessorsystems, which can be fiber optics; a mechanism for control of transmission over the as much 100 m in length. As Fig. 1 indicates, local area net- medium; and an interface to the network for the host comput- as works span distancesfrom several metersthrough several ers or other devices-the nodes of thenetwork-that are kilometers in length. connected to the network. In addition, local area networks The firstlocal area networks evolved inenvironments in share with long-haul packet communication networks afourth basic element: a set of software protocob, implemented in the which the distances to be spanned by the network were within host computers or other devices connected to the networks, the.range of inexpensive high-speed digital communication which control the transmission of information from one host technologies. Today, the relationship has been turned around, so that the distance range of local area networks is governed or device to another via the hardware elementsof the network. These software protocols function at various levels, from low- by the distance over which these inexpensive techniques can level packettrtlnsport protocols to high-level application be used. The result is high-data-rate networks in which the protocols, and are an integral part of both local area networks cost of transmission and the cost of control of transmission is very low compared to the costs associated with traditional and their close relatives, long-haul packet communication net- works. This combined hardware-software approach to com- ’Long-haul packet communication network technology has been used in environments that could be served more effectively, and at Lower Manuscript received April 12,1978;revised June 30,1978. cost, by local area networktechnology. This local area use of long- The authors are with Laboratory for Computer Science, Massachu- haul network technology, indicated by the shaded area of Fig. 1, is sep Institute of Technology, Cambridge, MA 02 139. due to the fact that long-haul packet communication technology has Conventional Packet communications networks, not limited in geo- been available commercially for several years, while local area network graphic scope, are referred to in this paper as “long-haul” networks. technology has not. 0018-9219/78/1100-1497$00.7S 0 1978 IEEE Authorized licensed use limited to: IEEE Xplore. Downloaded on December 15, 2008 at 13:06 from IEEE Xplore. Restrictions apply. 1498 PROCEEDINGS OF THE IEEE, VOL. 66, NO. 11, NOVEMBER 1978 possible to use such a central file system, accessed over the local areanetwork, for swapping or paging-an application made especially attractive by the fact that the cost of local area network interface hardware for a typical minicomputer Local Area Networks can beless thanthe cost of a”floppy disk” driveand its associated controller. The high bandwidth of local area networks can be exploited to simplify the control structure of communication protocols ~ Computer System I/D Busses by removing any motivation to minimize the lengthof control or overhead information in a packet. Fields of packet headers in local area networks can be arranged to simplify the process- inginvolved in creating or interpreting the packetheader, I I I I I I I I I using as many bits as are necessary. There is little need to use 16-4 ob1 .& d.1 I Ib Ido lobo 164 km “shorthand” techniques often found in the protocols of long- Fig. 1. Geographicrange of computer communication networks and haul networks which necessitate additional table lookups by 1/0 buses. The shadedarea of the long-haul network bar indicates the receiver of a message. Simplicity also extends to other the distance range for whichthat technology has been used in the past, aspects of local area network protocols, such schemes for but which could be better served, in both cost and performance, by as emerging local area network technology. allocation of network bandwidth, flow control, and error de- tection and correction. It should be emphasized that local area networks are not data communication networks, providing some unique oppor- an off-the-shelf, plug-in panacea for all local area computer tunities conventional longhaul networks do notafford. communication needs. For the distance range over which they For long-haul networks, the cost of communication is high. operate, the technology of local area networksholds the Wide-band common carrier circuits, satellite circuits, and promise of doing forcomputer communications what the privatemicrowave links are expensive. Longhaul packet hardwareinnovations of the last five years have done for communicationnetworks commonly employ moderately computingpower: they can bringdown the cost of high- expensive(i.e., 50 000-dollar)minicomputers as packet bandwidth communication andmake possible newapplications. switches to manage and route traffic flow to make the most But they cannot by themselves solve the “software problem,’’ effective use of the network communication links, delivery of for with low-cost hardware, thecosts of software development packets to their proper destinations. The geographic charac- will dominate the cost of any system development using local teristics of local area networks yield economic and techno- area network technology. logical considerations that are quite different. Inexpensive, privately owned transmission media can be used. For example, C. Interconnection with Other Networks simple twisted pair can support point-topoint communication While some local area networks now in use or under con- in the 1-10-Mbit/srange over distances onthe order of a struction are “stand-alone” networks, not connected to other kilometer between repeaters. Coaxial cable, such as low-loss networks, the trend is toward interconnection of local area CATV cable, can support either point-to-point