Towers, racks, and high-speed link 293Draft Document for Review October 18, 2004 5486Towr.fmHigh-speed linkHSL fabricHSL loops provide redundancy to all attached towers. In addition, theimplementation of HSL and OS/400 provide data flow balancing across the loopby assigning communication paths during an initial program load (IPL) tooptimize loop throughput based upon loop and tower configurations.HSL loops can be either copper or optical. Optical provides longer distance, butoffers a lower data rate. With iSeries Models 825, 870, and 890, and with I/O towers, such as the#5094 PCI-X Expansion Tower, #0595/#5095 PCI-X Expansion Tower, and#0588/#5088 PCI-X Expansion Unit, a slightly different copper HSL port isused—HSL-2. There is no change to the optical HSL loop on Models 825, 870, and 890.Considerations for loop configurations include: Rack mounted Model 520 system units can only attach to HSL-2/RIO-Gcables 2.5m or longer and can only attach to SPCN cables 3m or longer.Shorter cable lengths do not allow the system unit to be pulled out from therack far enough for concurrent maintenance. iSeries Models 825, 870, 890 with V5R2 or V5R3 do not supportHSL-2/RIO-G even with RIO-G nodes attached. A link between two RIO-Gnodes runs at HSL-2/RIO-G speed only. The speed allowed by the two nodes of a link with Eserver i5 serversattached are:– 1 GB/sec for RIO-G to RIO-G– 500 MB/sec for RIO to RIO-GDue to the high bandwidth of HSL, you should see comparable performance,whether using copper or optical HSL, even though optical runs at a slower speed.However, if you have intensive I/O bandwidth requirements (for example, largesystem data mining), you may experience some performance degradation withoptical HSL. The recommendation is to use less than the allowed maximumnumber of I/O towers on an optical fiber HSL loop to optimize performance.HSL was initially implemented at OS/400 V5R1 using copper interconnectcables. These cables allow for high-speed and high-quality parallel data transfer.In August 2001, technology was introduced using optical fibres and opticaladapters to interconnect the Central Electronic Complex (CEC) and the towers ofthe iSeries server.
