Performance of the L3 second level trigger implemented for the LEP II with the SGS Thomson C104 packet switch

The L3 experiment is one of the four experiments collecting data at LEP. For the LEP phase II, the second level trigger has been upgraded to a network of 28 ST T9000 transputers and 2 ST C104 asynchronous packet switches interconnected by IEEE1355 links. It collects trigger data at each LEP crossing (22 μs), builds-up the trigger data block, processes it and rejects online the background events in a few milliseconds. The L3 data acquisition has been running with this system since July 1995. In the data-taking environment and for different hardware and software implementations, the event building throughput rate has been measured. A bandwidth of 5.9 Mbytes per second per link has been measured in a configuration with 1.2 sources and one processing unit connected with 2 links. The expected global throughput of 70 Mbytes per second has been measured in a farm of 6 processing units. While varying the number of sources and destinations, we didn´t observe any significant bandwidth loss. Nevertheless performance relies strongly on some software implementation choices, which are presented and discussed