Or "How Grandma Mabel Learned to Love Voice over IP."
What do a 19th-century Missouri undertaker and SIP have in common? The common thread is their ability to connect two parties without a third party intervening. That is, they both serve an intermediary role between caller and callee, signaling the call and providing a pathway for its sound signals.
In 1891, a Missouri undertaker, Almon B. Strowger, was granted a patent for an electromechanical device called a stepper switch. The stepper switch allowed the calling party to control whom he would connect to without the need for an operator. Today, that control mechanism might be analogized to a phone number. But the important point is that Strowger's invention made it possible to connect phone calls without a telephone operator's intervention.
SIP promotes the ability to call a party without the need for a SIP gateway or gatekeeper, as long as you know the recipient's sip@ address, called a SIP Uniform Resource Indicator (URI). To pay homage to the pioneering Mr. Strowger, we have included this hack to connect a rotary phone, such as a 1920 Western Electric candlestick phone (Figure 5-5), to a VoIP network.
First, let me give you a quick overview of how a rotary phone works. The rotary phone provides signaling to the central office by establishing a flow of current and then interrupting the flow momentarily to signal a pulse. The number of pulses produced in a given time period represents ...