en it would then route the call to a class 4 office. If the first three digits dialed by the caller contained a 1 or a 0 in the middle-digit sition, then the class 5 office knew to expect 10 digits and to treat the first three digits as an area code. As a general rule, all calls with an area code were routed directly the class 4 office.4 The class 4 office would then use its translations to determine if the call should be sent directly to a class 5 office, to another class 4 office, or up to a class 3 office. Toll switches, classes 1 through 4, prior to the breakup of the Bell System had no re- sponsibility other than routing calls and passing signaling information back and forth. The AMA messages were created by the class 5 office where the call originated. Superscript(3) single office code supports up to 10,000 numbers. Therefore, larger offices would have more than one office code. There also were instances where very small offices would share an office code. Other mechanisms are used today, although not consistently, to identify if the office is to ex- pect 7 or 10 digits. Hence the restrictions on the use of 1 and 0 largely have been dropped in the North American numbering plan. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. The PSTN and Telephone Switching Key Telephony Concepts The breakup of the Bell System created major architectural changes and new re- sponsibilities for the class 4 office. First, all class 5 offices were given to the local ex- change carriers (LECs). AT&T, in its new role as an interexchange carrier (IXC), re- tained the class 4 offices and above. Since there were many small IXCs, as well as many class 5 offices, it was deemed unreasonable for each IXC to have to connect to every class 5 office. Therefore, the concept of an access tandem office was created. 5 The cess tandem office was essentially a class 4 offic