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Chapter 1

Introduction

In this report we present a system for measuring the performance of wide area computer

networks. The system is used to test network devices (switches, routers) and various net-

work topologies that will be part of the ATLAS experiment at CERN.

This experiment (at the CERN Large Hadron Collider) will generate a huge amount of data

that will have to be processed and ﬁltered in real time. There are proposals to distribute

the necessary computing power in diﬀerent remote loc ations that will be connected by

high speed networks. In order to transfer the data generated by the experiment, very high

performance networking will be needed – speeds of the order of Gigabit/sec will be common.

We want to measure diﬀerent parameters of high speed networks and estimate the impact

of their performance on the data processing system.

The most important measurements that are needed are the one-way transfer latency, the

throughput and the packet loss. We want to stress here that the one-way latency, as opposed

to half of the Round Trip Time (RTT), is important because the routing over the Internet is

not symmetric – a data stream between two points can travel on one route in one direction

and on a completely diﬀerent route in the opposite direction.

The method of performing this type of latency measurement will be presented in detail in

this report. We shall see that it requires synchronized clocks in remote locations and we’ll

show how this is obtained using the Global Positioning System.

The following section will present several testing systems or architectures that are available

on the market. We shall speak also about the diﬃculties that appear in long distance

measurements.

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