The VoIP adapter is a CO to the telephone user connected at an FXS interface of a VoIP box. The VoIP adapter has to emulate the characteristics of a PSTN CO. The front-end SLIC of a VoIP adapter is programmed for required voltages, line conditions, REN drive, impedances, gain/loss, and diagnostic features. Some parameters of call progress tones, DTMF, and call feature generation are programmed through a processor attached to the SLIC and CODEC. Many of these parameters are country-dependent variations that have to be taken care of in the VoIP adapter, and these features and parameters run into several pages in local PSTN standards.
As represented in Fig. 17.2, the alternating current (AC) terminating impedance can be modeled through resistors and a capacitor as an Rs, (Rs + Cs) or Rs + (Rp || Cs) that takes care of multiple countries. The values vary for different countries. As an example, France has 270Ω + (750Ω || 150 nF), France (TBR21) is 275Ω + (780Ω || 115 nF), and the United States/Korea/Japan uses 600 Ω, or 600 Ω + 1μF as an equivalent telephone impedance. More country-specific details are available at references [URL (Cisco-impedance), URL (Silab-DAA), URL (Si3015), URL (WinSLAC), URL (Microtronix-470C)]. Refer to the country-specific standards for the exact and complete information, as some discrepancy exists among the published literature on this subject.