Determinaci´ on del tama˜ no de la muestra

The project team first proceeded to investigate the user environments in which these networks would be utilized. There were three types of environments ofconcern to the project: the busi­ ness office, the university campus, and the fac­ tory. Clearly, assumptions about these environ­ ments were not mutually exclusive, but the names evoke the problems to be solved in each one. The next step was to gather more input on customers' requirements, applications, and physical environments.

Some information had already been collected by team members on previous visits to customer sites, including a heavy-manufacturing facility

Digital Tecbnk4ljournal

and a university campus . This i nformation bel ped the team to construct a refined set of questions to be asked on visits to other cus­ tomers. Subsequently, the team visited two more universities and several commerCial sites where continuous process monitoring and control, and research were performed. The team also exam­ ined one of Digital's sites that represented an extensive office environment.

The team discovered several generalized types of message traffic that were characteristic of the applications studied. These types were terminal­ to-computer, computer-to-computer, and real­ time traffic. 1 Unfortunately, most customers were unable to deliver actual network loads and traffic matrices for their environments. There­ fore, the team had to derive models for those generalized types of traffic, using previous mea­ surements of internal workloads and some edu­ cated assumptions. These models were subse­ quently used to evaluate several architectures offered by the team as candidates to meet the project's goals.

The environmental model for each traffic type shows particular characteristics. The terminal­ to-computer model has a large number of termi­ nals, all needing access to a small or moderate number of host computers. Although the aggre­ gate throughput is small, the traffic is bursty. In addition , the cost to connect each terminal device to the network must be small (i.e., not large compared with the cost of an inexpensive terminal).

Computer-to-computer traffic needs full logi­ cal connectivity and has higher throughput (up to several megabits per second per computer) Table 1 Definitions of Environments

Environment Extent

Office Less than

3 kilometers Campus Less than

25 kilometers

Factory Less than

8 kilometers Digital TecbnlcalJournal No. 3 September 1986 Number of Stations Less than 1 30 Less than 1 0,000 Less than 2200

than the previous model. The traffic for this model is also bursty. Fu

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_I hermore, the project team thought that, as workstations and personal computers became comm

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n, this class of traffic would soon become mu

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h more widespread than terminal-to-compute

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traffic.

The real-time environment is characterized by a large number of devic.

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I s (thousands) whose requirements to communicate are quite hierar- chically structured. The a

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plied load for a real­ time environment is more accurately modeled by deterministic arrivals. M

d

reover, most applica­ tions in this environment

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xpect the variance of the access latency to be l

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w in the IAN.

The team next defined

h

ominal environments for an office, a campus, an

a

a factory. These defi­ nitions are summarized in

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Table 1 .

In these definitions, harsh and benign environ­ ments refer to the enviro

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mental characteristics in which the IAN needs to

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operate. For example,

in a harsh environment one might expect a wide range of operating tempe

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ures or the presence of strong electromagnetic

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fields.

Added to the definitions were a number of

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facts that customers stressed or that were of general use to the projec

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. These facts were as follows:

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.

. . .

• Many customers had a variety of standard and

nonstandard higher-le

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protocols running on their LANs. Clearl

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, any solution had to take those existing protocols into account.

• Despite using nonsta

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dard protocols, cus- 1

tomers generally implemented their LANs _I with subsystems comp

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iant with a standard, such as one of the IEEE

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802 'tandards.

Physical

Environments

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Frequency of Station Movement

Benign · Benign within a building Harsh between buildings Harsh

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Occasional Possibly frequent Rare 55 New Products

The Extended Local Area Network Architecture and LANBridge 100

• In addition to the valid technological and

environmental reasons for choosing a particu­

lar LAN technology, some customers had

based their choices upon faulty assumptions. This was particularly noted in discussions on the delay variance of token-based systems in various normal recovery modes.

• The importance of performance-monitoring ·

and serviceability features were emphasized almost universally by customers.

At this point it was clear that the original project goal of investigating only broadband technology was too narrow. Using broadband technology alone could not satisfy the broad requirements of the environments identified by the team . There­ fore, the team expanded its scope to encompass the larger problem of providing a wide variety of services (terminal-to-computer, computer-to­ computer, and real-time) in the three environ­ ments (office, campus, and factory) .

It was also clear that there were two funda­ mental approaches to providing those services.

First, the team could attempt to develop a LAN

architecture, or enhance an existing one, that · could cope with the wide range of nodes, dis­

tances , media, performance, and cost con­ straints . Second, the team could attempt to develop a mechanism for interconnecting the

various LAN technologies.

In document PARA ESTUDIANTES DE CIENCIAS (página 147-152)