Department of Informatics Technische Universität München Informatik X: Rechnertechnik und Rechnerorganisation / Parallelrechnerarchitektur Prof. Dr. Arndt Bode , Prof. Dr. Hans Michael Gerndt

The SCINET project (Scalable-Coherent-Interface Network-project) is sponsered by and conducted together with the Max-Planck-Institute for Plasma Physics in Garching near Munich. Its goal is to investigate the applicability of SCI for data-acquisition systems of large-scale fusion-reactor experiments. In plasma-physical fusion-devices, hydrogen ions may fuse to helium ions, povided that they can be kept long enough and dense enough at very high temperatures (typically >> 1 Mio. degrees), thereby delivering energy according to E = mc**2. To control the plasma confinement and to get information about its physical behaviour, a high-speed and high-volume data-acquisition system is needed for the online and offline monitoring and evaluation of measured plasma and fusion-device data. State-of-the-art experiments produce approx. 100 MBytes of measurement values during a 10 second experimental period. Future plasma devices will deliver one to two orders of magnitude more data in the same time interval, while additionally operating continously in a steady-state mode. This imposes high requirements on the real-time behaviour, i.e. the transmission latency, as well as on the bandwidth of the underlying communication network that is part of the data-acquisition system. A low and guaranteed latency is very important for the closed-loop feed-back systems of the fusion device that keep the burning plasma hot, stable and away from every physical material. Potential candidates for the communication network of such a future data-acquisition system are FDDI, Gigabit-Ethernet, Gigabit-ATM and SCI. From those, SCI seems to be especially attractive because of its forward progress guarantee, prioritized bandwidth-allocation and extremely low latency. For the same reasons, also other physical experiments such as the "Large Hadron Collider" at CERN are considering SCI as their high-end communication medium.

In SCINET, it is investigated how data-acquisition computers can be efficiently connected with each other and with their sensors and actuators, what topological structure the network should have, and which bandwidth and latency can be expected. To get this mandatory data, in the SCINET project a software was developed that allows to simulate topological extensions of Banyan-type networks with sizes of up to 512x512 ports. Additionally, a test-stand was established to verify the simulation results, that comprises of two PentiumPCs, an SCI switch and two PCI/SCI interface boards from the Dolphin company. A copper cable-based test transmission-line was established, that showed 51 MB/s throughput and 2.8 mikro sec. latency.

With the SCI software simulator, it was revealed that the retry-transmission traffic on an SCI ring has heavy impact on the available bandwidth. Additionally, the simulator gave hints on how to reduce it. Questions to be investigated in detail by means of the simulator are the minimization of the retry traffic, the maximization of the SCI switches' throughput, and the optimization of the underlying network topology with respect to performance, costs and fault-tolerance. The latter issues imply that networks with adaptive routing are considered too. Currently, the Omega-, Flip-, Generalized-Cube and Indirect Binary n-Cube types of Banyan networks with wiring base 2 and 4 can be simulated, as well as three novel topologies that allow for adaptive routing. In the future, the test-stand will be upgraded to glass fiber transmission with full switch inclusion. Furthermore, the interplay between the transmission's performance and the higher software levels will be analysed. On the simulator side, a tool for the graphical representation of the network's topology is currently developed. Finally, by many simulation runs valuable information will be gathered to decide which network topology suits best for this specific type of application.

For more information on SCINET, please contact Dr. Harald Richter.

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