air filtration to protect equipment from dust. Large server rooms will often group rack cabinets together so that racks on both sides of an aisle are either front-facing or rear-facing, which simplifies cooling by supplying cool air to the front of the racks and collecting hot air from the rear of the racks. These aisles may themselves be enclosed into a cold air containment tunnel so that cooling air does not travel to other parts of the building where it is not needed or mixes with hot air, making it less efficient. Raised or false floor cooling in server rooms can serve a similar purpose; they permit cooling airflow to equipment through the underfloor space to the underside of enclosed rack cabinets. A difficulty with forced air fan cooling in rack equipment is that fans can fail due to age or dust. The fans themselves can be difficult to replace. In the case of network equipment, it may be necessary to unplug 50 or more cables from the device, remove the device from the rack, and then disassemble the device chassis to replace the fans. However, some rack equipment has been designed to make fan replacement easy, using quick-change fan trays that can be accessed without removing the cabling or the device from the rack, and in some cases without turning off the device so that operation is uninterrupted during replacement. Specifications The formal standards for a rack are available from the following: Electronic Industries Alliance EIA-310-D, Cabinets, Racks, Panels, and Associated Equipment, dated September 1992. (Latest Standard Now REV E 1996) Consumer Electronics Association CEA-310-E design requirements for Cabinets, Panels, Racks and Subracks., dated December 14, 2005 International Electrotechnical Commission - Multiple documents are available in French and English versions. IEC 60297 Mechanical structures for electronic equipment - Dimensions of mechanical structures of the 482,6 mm (19 in) series IEC 60297-1 Replaced by IEC 60297-3-100 IEC 60297-2 Replaced by IEC 60297-3-100 IEC 60297-3-100 Part 3-100: Basic dimensions of front panels, subracks, chassis, racks and cabinets IEC 60297-3-101 Part 3-101: Subracks and associated plug-in units IEC 60297-3-102 Part 3-102: Injector/extractor handle IEC 60297-3-103 Part 3-103: Keying and alignment pin IEC 60297-3-104 Part 3-104: Connector dependent interface dimensions of subracks and plug-in units IEC 60297-3-105 Part 3-105: Dimensions and design aspects for 1U chassis IEC 60297-4 Replaced by IEC 60297-3-102 IEC 60297-5 Multiple documents, -100, 101, 102, ... 107, replaced by