West, Jill; Dean, Tamara; Andrews, Jean (2015). ”Structured Cabling and Networking Elements”. Network+ Guide to Networks (Seventh). Boston, MA: Engage Learning. Sid. 169. ISBN 9781305480865. Läst 9 december 2019. ”Racks are measured in rack units (RU or U) with the industry standard being 42U tall — about 6 feet.”