PSTN switches interconnect business PBXs and public and private telephones. Large PSTN switches are located at COs, which provide circuits throughout the telephony network. PSTN switches are deployed in hierarchies to provide resiliency and redundancy to the PSTN network and avoid a single point of failure.
PSTN signaling traditionally supported only basic features such as caller ID and direct inward dialing. Modern PSTN switches now support, on a fee basis, many traditional PBX services, including conferencing, forwarding, call holding, and voice mail.
Modern PSTN service providers offer competitive services to differentiate themselves and generate additional revenue. These PSTN services include the following:
■ Centrex: Centrex is a set of specialized business solutions (primarily, but not exclusively, for voice service) in which the service provider owns and operates the equipment that provides both call control and service logic functions; therefore, the equipment is located on the service provider's premises.
■ Voice virtual private networks (VPN): Voice VPNs interconnect corporate voice traffic among multiple locations over the PSTN. PBXs are connected to the PSTN instead of directly over tie trunks. The PSTN service provider provides call routing among locations, and all PBX features are carried transparently across the PSTN.
■ Voice mail: Voice mail is an optional service that lets PSTN customers divert their incoming PSTN calls to a voice mailbox when they are unable to answer their telephones, such as when the line is busy or they are unavailable. Alternatively, all calls can be diverted to the voice mailbox.
■ Call center: A call center is a place of doing business by telephone, combined with a centralized database that uses an automatic call distribution (ACD) system. Call centers require live agents to accept and handle calls.
■ Interactive voice response: Interactive voice response (IVR) systems allow callers to exchange information over the telephone without an intermediary live agent. The caller and the IVR system interact using a combination of spoken messages and dual-tone multifrequency (DTMF) touch-tone telephone pad buttons.
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