Overhead cabling

Historically, cabling (power or signal) for computer systems is done within a rack or under a raised floor. There is growing customer interest in using nonraised floors, requiring some or all of the cabling to be overhead.

This information describes the considerations that are associated with overhead cabling of IBM® 19-inch racks (7014-T00, 7014-T42, 7014-B42, 0551, or 0553) and 24-inch racks ordered with IBM Power Systems servers.

As with all electronic devices, cables and power cords can act as antennas, emitting electromagnetic energy. The levels that are released are small (less than a mobile phone) and are not dangerous to people, but can interfere with other electronic devices. For example, a mobile phone's emission is measured in volts per meter while a Power Systems cable emission is measured in microvolts per meter. However, small amounts of electromagnetic emissions can add up as several cables emit more electromagnetic energy than a single cable.

Laying cables on a concrete floor helps to reduce emissions as the floor absorbs some of the energy. Placing cables under a raised floor also helps to reduce emissions. However, when you suspend cables in the air, this type of cable layout eliminates the emission reductions that are provided by a concrete subfloor, a raised floor, or both.

IBM supported configurations for servers and I/O units in racks meet both industry and IBM tests requirements for electromagnetic compatibility (EMC). These tests are completed by laying cables on the floor. To support overhead cabling, extra testing is required by placing the cables overhead and retesting selected configurations. Many overhead cabling configurations have not been tested and are not supported by IBM. Therefore, overhead cabling with a Power Systems server in IBM 19-inch and 24-inch racks is generally an unsupported configuration.

Generally, using overhead cabling is not a problem. The potential for causing interference with any device outside the data center is quite small. However, the only way to know if a configuration would be problematic is to test it and see if there are interference problems in the data center. If the overhead cabling is causing a problem, it can affect a wireless device in the data center (for example, a wireless temperature or humidity probe that is trying to report data at a constant rate, but which is sending wrong or intermittent data instead). The interference might be on a two-way radio that has more background noise than expected. Or, the interference might be poor reception in a radio or television.

It is possible that the emissions from overhead cabling can cause problems with another computer or storage equipment in the data center, but it is unlikely.

Different actions are available that you can take to mitigate emissions when you use overhead cabling. Note, while helpful, using these mitigation techniques does not mean that you have an IBM supported configuration because IBM has not conducted extensive testing for your specific configuration. The mitigations might address all the problems, but until tested and certified, the system is not officially supported by IBM.

Filters reduce emissions over a specific frequency range. Different types of cables emit differently based on their composition, length, signals that are carried, and where they are attached. A fiber optic cable does not emit radio frequency (RF) like a metallic cable unless the fiber optic cable has a metallic shield. Power cords, Small Computer System Interface (SCSI), serial-attached SCSI (SAS), copper Fibre Channel, and system power control network (SPCN) cables typically have modest emissions. InfiniBand cables are significant emitters when compared to power cords. Unshielded Ethernet cables are probably the most significant emitters. Longer metallic cables can be thought of as bigger antennas and emit more. Shorter cables emit less. If you have multiple 19-inch or 24-inch racks and need to run cables from one rack to the other, you can reduce emissions by keeping the cables in the rack instead of going over the top and into the other rack.

Using overhead cables with your Power Systems servers does not void your IBM warranty or maintenance agreement.¹ However, if IBM service and support believes that a problem might be related to the use of overhead cabling, then IBM service and support has the right to suspend the warranty or maintenance agreement until the system is put back into compliance. Therefore, overhead cabling options should be discussed with your local IBM service and support organization before you implement a new cabling scheme.

Cables can always be routed out the bottom of the product in accordance with the product installation instructions. After the cables are routed to exit the bottom of the product, they can be externally routed upward to the overhead cabling trays by using appropriate cable management techniques.

¹The customer must not cut or drill any additional holes or openings into the product enclosure under any circumstances. Similar to EMC testing, IBM must meet industry and internal standards for product safety. These requirements are not only for the safety of IBM customers, but also for service personnel. Any alteration of the physical structure of an enclosure voids the product safety certifications that are received for the product.