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Chapter 9
Local Area Networks: Connectivity and Internetworking

Just as computer networks have grown across continents and oceans to interconnect major computing facilities around the world, they are now growing down corridors and between buildings to interconnect minicomputers in offices and laboratories.

Robert M. Metcalfe and David R. Boggs, Xerox Palo Alto Research Center, Ethernet, 1976

Once upon a time, computer networks consisted of mainframes in glass houses. Input was in the form of punch cards read by card readers and output was in the form of printed results via local printers. Local terminals were mostly used for control and programming input. All processing was on a batch basis, rather than being interactive. In other words, the input was transmitted in a batch from a card reader over a short circuit to the processor, the processor processed the program in a batch, and the output to the printer was in a batch. The first true mainframe was the IBM 360, introduced in 1964 [9-1]. As the first computer capable of both scientific and business computing, it went full circle, hence the designation 360 [degrees] [9-2]. During the author’s pursuit of several degrees (academic) at The University of Texas at Austin, his programs (which seldom ran successfully) were processed in batch mode on a CDC 6600 computer which cost roughly $6 million, occupied a huge room, required tons of air conditioning, and had enough large cabinets to house the population of a small nation.

Over time, input to the mainframe was extended to multiple users at dumb terminals that connected to ports on the mainframe through terminal controllers, or cluster controllers. Controllers essentially act as traffic concentrators to allow multiple slow terminal inputs to share one of a limited number of very expensive ports on the host computer. As time ticked away, Remote Job Entry (RJE) was developed to allow the remote entry of data over a WAN, with the clusters of dumb terminals and terminal controllers residing at remote locations.

In parallel to the development of data networking, the computers, themselves, began to change. Computers became more powerful as processor speeds increased with the development of ever faster microprocessors on silicon chips. Memory became more available as chip technology and hard drive technology improved. Additionally, computers became smaller and less expensive, to the point that the typical desktop PC is equivalent to an early mainframe that would have filled a moderate-size office building! (The PC, by the way also is more powerful and infinitely easier to use.) The PC was legitimized by the introduction of the IBM PC in 1983. By 1993, it was estimated that 75% of professionals had a PC or workstation on the desktop.

It was only logical that all of this computing power and storage capability on all of these desktops would lead to a need to network those devices within the workplace. It has been estimated that 80% of data transfer is confined to the workplace, while only 20% travels across the WAN. Whether or not that figure is accurate, it is clear that PC users need to share access to hosts, databases, printers, and WANs. LANs provide a solution to that requirement.

LAN technology was first conceived by Robert M. Metcalfe and his associates at the Xerox Palo Alto Research Center (Xerox PARC). That concept was originally known as the Altos Aloha Network, as it connected Altos computers through a concept based on the University of Hawaii’s AlohaNet packet radio system technology. In a memo written on May 22, 1973 this magical network became known as ethernet, from luminiferous ether, [9-3] the nonexistent omnipresent passive medium once thought to support the transmission of electromagnetic energy through a vacuum. Xerox commercialized the technology and renamed it The Xerox Wire. When Digital Equipment Corporation (DEC), Intel and Xerox standardized the technology in 1979, they reverted to the name Ethernet. Ethernet quickly became a de facto standard. Ethernet and LANs were officially recognized in February, 1980, when the IEEE established Project 802 at the request of members. In December, 1982, the first standard was published and circulated.

Ethernet, clearly, still is the most popular LAN standard. According to Metcalfe’s estimates [9-4], in 1994 there were 50 million ethernet-connected computers, five million of which were on 10 Mbps networks. Further, 500,000 Ethernet networks were TCP/IP registered and 50,000 were connected to the Internet. No doubt those numbers have increased considerably in the past few years.

This discussion of the basic concepts of LANs and LAN internetworking serves as the launching pad for discussion of the network technologies of the future. In this chapter, the definition, origin, and evolution of LANs and their application will be addressed. Dimensions of LANs to be explored include media alternatives, physical and logical topology, baseband versus broadband, media access control, and standards and standards bodies. Bridges, routers, hubs, switches, and gateways will be defined and illustrated, as will LAN operating systems. This chapter will conclude with a discussion of LAN internetworking, remote LAN access, and recent developments in the realm of high-speed LANs.

LANs Defined

A LAN is a form of local (limited-distance), shared packet network for computer communications. LANs interconnect computers and peripherals over a common medium in order that users might share access to host computers, databases, files, applications, and peripherals. LANs offer raw bandwidth of 1 Mbps to 100 Mbps or more, although actual throughput often is much less. LANs are limited to a maximum distance of only a few miles or kilometers, although they may be extended through the use of bridges, routers, and other devices. Data are transmitted in packet format, with packet sizes ranging up to 1500B+. For the most part, LAN specifications are the province of the IEEE, although ANSI and other standards bodies increasingly are involved.


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