Basic Telephony Signaling

Many corporations find it advantageous to operate their own voice networks, and they do so by connecting dedicated links between Private Branch eXchanges (PBXs) for inter-office communication, or by using Virtual Private Networks (VPNs) for voice. Originally, PBXs were connected to the Public Switched Telephone Network (PSTN) for voice services, or they were interconnected using analog tie-lines to transfer voice. When the need for more voice trunks and the technology matured, analog tie-lines were replaced with higher-speed digital facilities capable of accessing sophisticated and feature-rich networks. This chapter analyzes the signaling techniques that traverse analog and digital facilities in corporate and interexchange networks.

This chapter also discusses channel-associated signaling (CAS) systems, such as Bell System MF, Consultative Committee for International Telegraph and Telephone (CCITT) No. 5, R1, and R2, and it reviews how these CAS systems operate.

It also describes access protocols, such as Integrated Services Digital Network (ISDN), Q Signaling (QSIG), and Digital Private Network Signaling System (DPNSS). These protocols deliver PBX signaling through a network to distant PBXs. Private ISDN networks use the PSTN for connectivity and services. QSIG is an inter-PBX signaling system similar to ISDN that enables corporate PBXs to connect, thus creating a private voice network. DPNSS is an ISDN-type protocol that enables PBX connectivity; however, it is not as widely used as ISDN and QSIG.

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