DTJ Number 3 September 1987 - Digital Technical Journals

The Extended Local Area Network Architecture and LANBridge 100• In addition to the valid technological andenvironmental reasons fo r choosing a particularLAN technology, some customers hadbased their choices upon faulty assumptions.This was particularly noted in discussions onthe delay variance of token-based systems invarious normal recovery modes.• The importance of performance-monitoring ·and serviceability features were emphasizedalmost universally by customers.At this point it was clear that the original projectgoal of investigating only broadband technologywas too narrow. Using broadband technologyalone could not satisfy the broad requirements ofthe environments identified by the team . Therefore,the team expanded its scope to encompassthe larger problem of providing a wide variety ofservices (terminal-to-computer, computer-tocomputer,and real-time) in the three environments(office, campus, and factory) .It was also clear that there were two fundamentalapproaches to providing those services.First, the team could attempt to develop a LANarchitecture, or enhance an existing one, that· could cope with the wide range of nodes, distances, media, performance, and cost constraints. Second, the team could attempt todevelop a mechanism for interconnecting thevarious LAN technologies.LAN Tecbnology Alternatives· The team decided first to evaluate a variety ofsuitable media access methods. Each alternativeand the conclusions reached by the team aresummarized below.Carrier Sense Multiple Access with CollisionDetection (CSMAjCDj 2This was the alternative most familiar to the teammembers, since Digital was currently buildingproducts utilizing CSMAfCD for both basebandand broadband media. The performance ofCSMA/CD does not degrade rapidly as a functioof the number of connected nodes (see Figure1). However, its extent (maximum signal propagationpath length), transmission rate, and minimumpacket size are not independent because offi nite propagation delays .3·4 Therefore , toincrease the physical extent of CSMA/CD LAN,the minimum packet size or the transmission rateor both must be decreased to ensure that thereare no undetected collisions.w 10 2...J-