ATM Based Event Building andPC Based Level 3 Trigger at

Baseline event building and Level 3 trigger processing at the CDF experiment at Fermilab is speciied to process about 300 events/s with an average event size of about 150 KB. First, the event must be assembled from fragments originating from approximately 16 readout sources, with individual average fragment sizes ranging from 12 to 16 KB. Then the Level 3 processor-based trigger performs reconstruction and ltering tasks requiring in excess of 45000 MIPS of CPU power on the assembled events. We present a distributed, scalable architecture using commodity components: VME-based CPU modules for the readout, an ATM switch for event buildling, and Pentium-based PC's running Linux for event processing. Pentium-based PC's are also used for event distribution throughout and collection from the Level 3 processors via multiple 100 Mbps Ethernets. We report on preliminary studies conducted at CDF with a small-scale prototype. This architecture is also a possible candidate for the CMS experiment at LHC.