Boggs D.R.Measured capacity of an Ethernet.Myths and reality.1988.pdf

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SEPTEMBER 1988

WRL

Research Report 88/4

Measured Capacity

of an Ethernet:

Myths and Reality

David R. Boggs

Jeffrey C. Mogul

Christopher A. Kent

digi tal

Western Research Laboratory

100 Hamilton Avenue Palo Alto, California 94301 USA

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Measured Capacity of an Ethernet:

Myths and Reality

David R. Boggs

Jeffrey C. Mogul

Christopher A. Kent

September, 1988

digi tal

Western Research Laboratory

100 Hamilton Avenue Palo Alto, California 94301 USA

Abstract

Ethernet, a 10 Mbit/sec CSMA/CD network, is one of the most successful

LAN technologies. Considerable confusion exists as to the actual capacity of

an Ethernet, especially since some theoretical studies have examined operat-

ing regimes that are not characteristic of actual networks. Based on

measurements of an actual implementation, we show that for a wide class of

applications, Ethernet is capable of carrying its nominal bandwidth of useful

traffic, and allocates the bandwidth fairly. We discuss how implementations

can achieve this performance, describe some problems that have arisen in

existing implementations, and suggest ways to avoid future problems.

This paper was originally published in

Proceedings of the SIGCOMM ’88 Symposium on

Communications Architectures and Protocols,

ACM SIGCOMM, Stanford, California, August

1988.

1988 Association for Computing Machinery

Permission to copy without fee all or part of this material is granted provided that the copies

are not made or distributed for direct commercial advantage, the ACM copyright notice and the

title of the publication and its date appear, and notice is given that copying is by permission of

the Association for Computing Machinery. To copy otherwise, or to republish, requires a fee

and/or specific permission.

1. Introduction

Local Area Networks (LANs) have become indispensable in the past few years. Many LAN

technologies have been designed, and more than a few of these have made it to market;

Ethernet

[8] is one of the most successful. There are many factors that influence a choice be-

tween different technologies, including availability, acceptance as a standard, cost per station and

per installation, and ease of maintenance and administration. All of these are difficult or impos-

sible to quantify accurately. Performance characteristics, on the other hand, are easier to quan-

tify, and can thus serve as the basis for religious debates even though the potential performance

of a LAN technology may have very little to do with how useful it is.

Considerable confusion exists as to the actual capacity of an Ethernet. This capacity can be

determined either by measurement or by analysis. Measurements of intrinsic Ethernet perfor-

mance (its performance in the limiting case) are fairly meaningless because, at least until

recently, most feasible experiments could only measure the performance of host implementations

and interfaces, not of the Ethernet per se. If no experiment can ‘‘see’’ the intrinsic performance

of the network, then applications probably cannot see it either, and intrinsic performance there-

fore does not matter. Analyses, on the other hand, have tended to concentrate on the intrinsic

performance; first, because software and interface design is too hard to analyze, and second,

because Ethernet performance is ‘‘interesting’’ only at very high loads. Therefore, most

theoretical studies have examined operating regimes that are not characteristic of actual net-

works, and their results are also of limited utility in comparing network technologies.

Ethernet works in practice, but allegedly not in theory: some people have sufficiently misun-

derstood the existing studies of Ethernet performance so as to create a surprisingly resilient

mythology. One myth is that an Ethernet is saturated at an offered load of 37%; this is an incor-

rect reading of the theoretical studies, and is easily disproved in practice. This paper is an at-

tempt to dispel such myths.

We first summarize the theoretical studies relevant to Ethernet, and attempt to extract the im-

portant lessons from them. Then, based on measurements of actual implementations, we show

that for a wide class of applications, Ethernet is capable of carrying its nominal bandwidth of

useful traffic, and allocates the bandwidth fairly. We then discuss how implementations can

achieve this performance, describe some problems that have arisen in existing implementations,

Boggs D.R.Measured capacity of an Ethernet.Myths and reality.1988.pdf

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