PSTN- Making a Call (Public Switched Telephone Network)

Common Switching Terms PBX (Private Branch Exchange) CO (Central Office) or end office Tandem Interexchange Switch (Long Distance Switch) PIC Code

PBX (Private Branch Exchange) A private telephone system switch used by medium and large companies. The switch is connected to the local public telephone network and performs in-house routing and switching. A PBX uses technology similar to that used by a central office switch, but on a smaller scale. The acronym PBX originally stood for “plug board exchange”.

PBX

CO (Central Office) A Central Office is provided by the RBOC or LEC, which provides copper from the CO to the customers minimum point of entry (MPOE), also known as demarcation point (DMARC). The connection between the CO and MPOE is called the “last mile” or “local loop”.

Central Office

Tandem A local exchange carrier (LEC) facility used to collect switched traffic from multiple COs. Tandems are used to provide intraLATA, long distance and local interoffice calling (CO to CO). There are eight Tandems in the Bay Area: San Francisco (2), Santa Clara (2), Sunnyvale, Oakland (2), Santa Rosa

Tandem’s Relationship to COs

Interexchange Switch Interexchange (IXC) providers, more commonly referred to as long distance carriers, have point-of-presence (POP) with the LEC at the Tandem. When the call is handed to the IXC, the call than is routed to the IXC switch. In high call volume areas, IXCs have their POP at the LECs end office. This relationship is also called “end office trunking”.

Types of Calls Local: within 16 miles (zones 1,2,&3) IntraLATA: 16+ miles. Covers area codes 650, 415, 408, 510, 925, 831 and 707 Intrastate: In-state outside the LATA Interstate: Calls outside the state International: Calls outside the USA Toll Free: 800, 888, 877 and 866 calls Calling Cards: Same as travel cards

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