VoIP Voice and Fax Signal Processing

1.1 PSTN CO AND DLC

In this section, an overview of central offices and DLCs is presented. An analog CO provides services to the user, and in recent times, analog central offices were replaced with a combination of digital CO and DLC.

1.1.1 Analog CO

Analog telephony requires a two-wire analog TIP-RING interface. Several years back, the PSTN CO was directly providing several pairs of analog lines to the closest junction box, and from there, individual TIP-RING wire pairs were being distributed to the subscriber [URL (IEC-DLC)]. When a subscriber is far (5000 to 15,000 feet) from the CO, long-distance analog lines can create distortions and signal attenuation. To counter this problem, bigger diameter wires and compensating loading coils were used with analog CO. Sometimes the line voltage is increased at the CO to cater for voltage drop over long lines. For a coverage area of a few miles in diameter, this approach may still be used. Overall, using long lines from the CO is a costly effort, deployment may not scale up, and voice quality degrades. Additionally, fax and modem calls may operate at a lower speed on long analog lines. For a growing customer base, an analog CO may not scale up properly. A VoIP adapter providing telephone service to the end user closely resembles the analog CO by providing telephone interfaces like PSTN DLC or CO and by allowing voice calls through an Internet connection.

1.1.2 Digital CO and DLC

In recent years, PSTN systems migrated to ITU-T-G.711 A-law/μ-law ...