Distributed SCI-based Data Acquisition Systems constructed from SCI bridges and SCI switches

The new standard IEEE std. 1596-1992, Scalable Coherence Interface (SCI)-- provides novel possibilities to build data acquisition systems for large and very high rate experiments in High Energy Physics (HEP). The RD24 project at CERN started 2 years ago to investigate applications of SCI to data acquisition at the Large Hadron Collider (LHC). The participants come from CERN, universities and computer companies. As part of the RD24 project, simulation of large SCI-based data acquisition systems is performed by a simulator written in the object-oriented language MODSIM II. A typical ring structure is sensitive to hardware failures and limited by its peak load. Early simulations showed that large SCI rings are not scalable. An SCI bridge model, which is in principle two SCI node chips connected back-to-back, is presented here and used as a building block for a simulated system. Several rings connected by SCI bridges can offer high performance, but large SCI-based systems containing thousands of nodes with many bridges will not be able to utilize the potential bandwidth. A long path between nodes leading through bridges also introduces long latencies. We present an SCI switch model, which provides a direct connection between any two rings that are connected to the switch. Such an SCI switch will significantly improve system performance and give a lower latency. We compare the simulation results and show the differences between SCI systems based on pure SCI switches and those based on SCI bridges, in particular the different performances and latencies under various load and design parameters such as the depth of input/output FIFOs, switch (bridge) delays etc. We investigate self-routing SCI multistage DAQ systems with optimized configuration, which is uniquely routed. The goal of this paper is to investigate the difference between SCI switch- and SCI bridge-based systems, and to study some of the design criteria for the SCI switch element to form the interconnection of large scale SCI-based data acquisition systems.