One interesting article to reas:
Evolution of the Line Card: Part 1
http://www.commsdesign.com/showArticle. ... =207404327"Voice communication has evolved considerably since the first successful recorded voice transmission over wires occurred on March 10th, 1876."
"he first commercial step-by-step central office was opened in 1893. What we all recall from the first days of telecom is the phone operator sitting in front of rows and rows of plug boards, connecting the conductors from one calling party to another."
"Over the years, significant progress has been made in the way calls are placed, the way central offices look, and the amount of traffic that can be handled. Much of this is due to the line card, whose fundamental components include:
* Tip Ring interface circuit
* CODEC (analog to digital and digital to analog)
* Call control and voice supervision state machine
* Power supply and power supply control
* Ringing generator
* DTMF generation and detection circuit
* PCM (Pulse Code Modulation) ability: Digital representation of an analog signal where the magnitude of the signal is sampled regularly at uniform intervals then quantized a series of symbols in a digital (usually binary) code."
"The next challenges driving the evolution of the line card are rooted in the telecom carriers need to compete with cable companies deploying voice over Internet Protocol (VoIP), as well as Analog Terminal Adapters. This competition has driven innovation, adding features to the line card such as broadband DSL for data and video. In addition, as call densities continue to increase, there is the challenge of moving voice traffic from one place to the other."
