Section 4.8. Key Issues: Circuit-Switched Telephony

4.8. Key Issues: Circuit-Switched Telephony

In this chapter, the structure of traditional voice networks has been dissected. In a nutshell :

• The PSTN provides global interconnectivity and calling features for its subscribers

• SS7 is the signaling network that runs alongside the PSTN

• Businesses use PBXs to build their own private voice networks that interface with the PSTN using trunks.

• PBXs use a data-multiplexing technology called TDM to provide connections to digital phones and other switches

• A single voice call across the PSTN or a PBX requires 64 kbps of bandwidth

• Analog, one-pair phones use FXS electrical signaling and transmit voice as an analog waveform signal

• Digital, two-pair phones use digital signaling and transmit voice as a slice in the TDM bit stream

• Extension numbering and dial-plan are key steps in PBX design

• The Erlang B table can help you decide how many trunks a phone system needs

It's important to understand the structure of the PSTN, PBXs, and the connections between them. But the network and its devicesthe structureare merely a means to a much more meaningful end: telephony as a set of applications. Human interaction is the deliverable of telephony, and the network can't deliver without programmed applications running on it.

In the next chapter, we'll examine some of the most important legacy telephony applications: intercom, call-waiting, hold, call transfer, conference call, caller ID, call park, and others.