In the public switched telephone network (PSTN) hierarchy, a Class 4 office that also serves as a Class 5 office.The separate functions are provided through logical and physical partitioning within the switch. See also Class 4, Class 5, and PSTN.
"Class 4/5." YourDictionary, n.d. Web. 24 May 2018. <http://www.yourdictionary.com/class-4-5>.
Class 4/5. (n.d.). Retrieved May 24th, 2018, from http://www.yourdictionary.com/class-4-5