This chapter reviewed the history and operation of legacy telephony systems. Various flavors of telephone switches were discussed, including CO switches (which make up the PSTN), PBXs, and key systems. Both PBXs and key systems are privately owned telephone switches. Larger businesses use PBXs to support anywhere from 20 to 20,000 phones. Smaller businesses needing to support only 30 to 40 phones may opt for a key system.
Placing an end-to-end phone call involves different types of signaling, including supervisor signaling (which indicates to the CO switch that a phone has gone off-hook), information signaling (which provides information to the caller about the state of a call, such as a busy signal or ring back), and address signaling (which uses pulse dialing or DTMF dialing to communicate dialed digits to a telephone switch).
You also learned that migrating from a traditional PBX-centric (or key system-centric) telephony environment to a VoIP network offers several advantages. Cost savings is a primary motivator. Examples of these cost savings include reduced equipment and staffing expenses, in addition to the elimination of recurring monthly charges for dedicated voice circuits used to interconnect privately owned telephone switches. The advantages of VoIP, however, extend far beyond cost savings. A VoIP network lays the foundation for enhanced services that are not possible with legacy telephony solutions.