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Training: Examining Cable Telephony, Part 1
By the NCTI

This month’s installment begins a series on cable telephony service. The material is adapted from a lesson in NCTI’s new Digital Installer Course. © NCTI

A cable telephony system interfaces with the public switched telephone network (PSTN) to provide phone service to the customer premises. Because the PSTN uses circuit-switching requiring a fully dedicated connection, pauses between bits (telephone conversations) go unused. Although only one person can speak at a time on the telephone, the full bandwidth for a two-way call is required during the entire call.

Internet protocol (IP) packet-switching maximizes the available bandwidth by sending video, voice or data signals that are broken into packets to the same destination address. These packets can travel over any available circuits and return to the destination address. Therefore, IP doesn’t require a dedicated connection. Voice-over-IP (VoIP) enables a service provider to use IP over a private network to transmit and receive voice signals. A VoIP user can access VoIP with either a personal computer (PC) or a telephone.

Introducing cable telephony service

An understanding of the functions, powering, ranging and specifications of the network interface unit (NIU) also is helpful while installing and troubleshooting the cable telephony service at the customer premises.

Examining a cable telephony system

A cable telephony system interfacing with the PSTN consists of a host digital terminal (HDT), the element manager (EM) software, a local digital switch (LDS), the cable company’s hybrid fiber/coax (HFC) network, and the NIU. Their relationships are shown in Figure 1.

Explaining NIU functions

The NIU provides an analog telephone signal for up to four telephone lines. (See Figure 2.) The NIU is both an active interface device and the demarcation point between the HFC network and the customer premises wiring.

Network powering is one method of powering the NIU. In this case, power is provided to the NIU via a power-distributing tap and network powered drop cable through the coaxial cable input port of the NIU.

NIU ranging synchronizes the NIU modem with the HDT modem to establish communications.

Next month’s installment will continue with an examination of telephone inside wiring.

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