Solutions Fast Track | PSTN Architecture

PSTN: What Is It, and How Does It Work?

• Outside plant represents cabling, interfaces, and other infrastructure outside of the CO. F1 feeds the SAI and F2 typically connects directly to the subscriber drop cable. SONET or SDH rings sometimes are used to feed an SAI or provide connectivity between COs.
• Frequency Division Multiplexing (FDM), an analog technology, was replaced with the more efficient and secure Time Division Multiplexing (TDM). TDM is used in various implementations throughout the communications industry.
• The SS7 stack is similar to the OSI network model and through SIGTRAN standards can also operate over IP. Security measures should be taken with all PSTN signaling protocols.

PSTN Call Flow

• Digital switches still need sophisticated circuit detection capabilities for call processing.
• When making SIP-based calls through an ISP, one’s call may terminate several hundred miles away and not at the local central office.
• SIP-to-SIP based calls happen entirely over IP networks. SIP-to-PSTN calls start on an IP network and then get handed off to the PSTN.

PSTN Protocol Security

• Use extreme caution around pre-SS7 signaling protocols like ITU-T 5 or R2 (also known as CCITT 5 or R2 because of their toll fraud potential.
• Even with SS7, take precautions to limit potential damage from fraudulent or corrupted data that could arrive in your SS7 network from other carriers.
• Do not use ANI or any other ISUP or QSIG information for authentication.