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Collaboration, Voice & AV · Foundational

Public Switched Telephone Network (PSTN)

The worldwide circuit-switched telephone network operated by carriers, reached from enterprise systems through gateways or SIP trunks.

Definition

The PSTN is the aggregate of national and international carrier telephone networks, historically built on dedicated 64 kbps circuits, TDM switching and SS7 signaling. Enterprise call control reaches it through analog lines, ISDN PRI circuits or, increasingly, IP-based SIP trunks from a service provider. Calls leaving the enterprise to the PSTN are often called off-net calls.

Example

A user dials 9-1-555-123-4567 and CUCM routes the call through a CUBE SIP trunk to the carrier PSTN.

Exam tip

Know that the PSTN path is the fallback for many failover designs (AAR, SRST, CFUR), so expect questions on routing calls off-net when the WAN fails.

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