Also known as a backbone switch and a core switch, a tandem switch is a high-capacity switch positioned in the physical core, or backbone, of a Public Switched Telephone Network (PSTN), where it serves to interconnect edge switches, or Central Office (CO) switches. In the traditional PSTN hierarchy, a tandem might be a Class 1 regional toll center, Class 2 sectional toll center, Class 3 primary toll center, or Class 4 tandem toll center. An access tandem switch serves to connect local exchange carriers (LECs), i.e., local telephone companies, to the interexchange carriers (IXCs), i.e., long distance carriers, over dedicated interoffice trunks, known as access trunks. In a contemporary PSTN, a tandem switch commonly is a hybrid Class 4/5, functioning as both a tandem and a CO (Class 5). See also IXC, LEC, PSTN, and switch.
A telephone central office switch that links telco end offices together and does not connect to the customer directly. Also called a "Class 4 switch" or "TDM switch," a tandem switch is a computer that is specialized for TDM-based, circuit-switched telephone calls. Tandem switches are typically from Lucent and Nortel Networks (see ESS and DMS). In the past, most of the call recording and billing was handled by tandem switches, also called "toll/tandem switches." Subsequently, such services were taken over by end office switches. Sector and Access Tandems A sector tandem switch connects end offices for intraLATA traffic, while an access tandem switch provides the connection between end offices and the POPs for interexchange carriers (IXCs). In the past, Class 4 tandem switches dealt only with high-speed, four-wire T1, T3 and OC-3 connections in contrast to two-wire local lines on Class 5 end office switches. Subsequently, all switches were made to support four-wire lines. See end office switch and LATA.