In PSTN, communication from the originating CO or DLC to the destination DLC is synchronous digital communication. This process has been well established over the last several decades. PSTN transmission follows some common standards such as ITU and IEEE and country-specific standards like TR-57 and TIA standards [TR-NWT-000057 (1993), TIA/EIA-470C (2003), IEEE-STD-743 (1995)]. PSTN manufacturers use TR-57 in North America to specify the signaling and transmission requirements for DLC. The FXS interface on VoIP adapters works similar to DLC or analog CO providing the TIP-RING interface. Assuming other favorable conditions for VoIP voice quality, as explained in Chapter 20, ensuring TR-57 transmission characteristics helps in improving voice quality. TR-57 is used in North America, and the current standard is GR-57 [GR-57-CORE (2001)]. Several other standards are followed in different countries. For example, Japan makes use of the NTT PSTN specification [URL (NTT-E)], and France uses the France Telecom STI series [FT ITS-1 (2007)] of specifications. Most of these country specifications are closely related to each other with only a few deviations like end-to-end losses, transmission filter characteristics, impedance, and so on. TR-57 transmission specifications can also be followed in achieving voice quality in VoIP systems. In VoIP, some signaling events as call progress tones used in establishing end-to-end calls may go through longer ...

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