This paper presents some of the conclusions of our studies on ATM and Fast Ethernet in the ATLAS
level-2 trigger Pilot Project. We describe the general concept and principles of our data
collection and event building scheme that could be transposed to various experiments in high
energy and nuclear physics. To validate the approach in view of ATLAS High Level Triggers, we
assembled a testbed composed of up to 48 computers linked by a 7.5 Gbit/s ATM switch. This
modular switch is used as a single entity or is split into several smaller interconnected
switches. This allows studying how to construct a large network from smaller units.
Alternatively, the ATM network can be replaced by Fast Ethernet. We detail the operation of the
system and present series of performance measurements made with event building traffic pattern.
We extrapolate these results to show how today's commercial networking components could be used
to build a 1000-port network adequate for ATLAS needs. Finally, we list the benefits and the
limits of ATM for our application and discuss an alternative approach based on Fast/Gigabit Ethernet.
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