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Chapter 21
Hybrid Fiber/Coaxial Networks

Bill Koerner

Hybrid Fiber/Coaxial (HFC) cable networks can deliver interactive services such as telephony and high-speed data services. HFC networks use fiber transmitters, fiber nodes, RF amplifiers, and taps to distribute signals to subscribers and set-top boxes (or cable modems) to return signals to the cable company. This chapter explains HFC network management system requirements.

INTRODUCTION

Telecommunications reform, the Internet, and high-speed data services have revolutionized the definition of communication services. Services rich in multimedia content have the highest demand and require the highest bandwidth to live up to customer expectations.

HFC networks currently offer the best network alternative to deliver these services, but traditionally have never been managed like their telco counterparts. This chapter examines the requirements for managing the HFC cable network, its relationship to the services being offered, and ideas for and challenges of managing the network.

OVERVIEW OF HFC NETWORKS

When cable TV came about in the 1940s, the intent was to provide TV signals to those who could not receive them from the standard antenna. As more people signed up for cable service, the cable operators simply extended the cable to reach the new customers.


Exhibit 1.  Tree and Branch Architecture.

Unfortunately, as they extended the cable, the signal also became weaker, because signal loss is directly proportional to the length of the cable. To overcome the losses, cable operators inserted amplifiers in the transmission network, thus allowing the cable operator to now provide service to more customers. Unfortunately, once again, the amplifiers also added noise and distortions to the original signals; thus there is a theoretical limit to the number of customers the cable operators could service.

Throughout the 1980s, amplifier manufacturers improved not only the noise and distortion characteristics, but also the frequency range. It was now possible to extend the transmission networks farther and offer more channels. Exhibit 1 shows a typical example of what came to be known as the “tree and branch architecture.”

Advances in fiber optic technology made it possible to use laser transmitters, receivers, and fiber optic cable to distribute the cable TV signal to neighborhoods, and then use the existing RF network to distribute the signals to the subscribers. Thus was born the HFC cable network.

The HFC network allowed the cable operator to reduce the number of RF amplifiers needed, which increased the quality of the signal provided and overall network reliability. With the RF network reduced to just a few amplifiers, it was also now easier to support higher frequencies (i.e., more channels). Most current HFC designs support up to 750 MHz of bandwidth, which roughly translates to 110 channels. Exhibit 2 shows an example of a HFC network.


Exhibit 2.  HFC Network Architecture.

REQUIREMENTS FOR MANAGING HFC NETWORKS

Since the early days of cable TV, the intent has mostly been to deliver analog television signals to subscribers. Higher frequencies meant that cable operators could offer more channels, thus the cable network explosion during the 1980s and 1990s.

With the newly available bandwidth, several cable operators discovered that they could offer other services that generated additional revenue. Digital Music Express, a CD-like audio service, allowed the cable operator to offer 30 digital radio channels. Pay-per-view allowed the cable operator to offer newly released movies and sports events to subscribers by programming (authorizing) the set-top box for subscribers that ordered the additional service.

Achieving Interactivity

Until this point, most cable networks only worked in one direction, called the downstream path. Basically, the cable signals are broadcast to all set-top boxes, and the set-top controls what the customer is authorized to use.

Some cable plants also have, or are being designed for, a return path, or an upstream path (see Exhibit 3). This design is what makes the HFC network interactive — the ability to pass data either way — and is setting the stage for many types of services, ranging from telephony to high-speed data services, energy management, and Internet access.


Exhibit 3.  HFC Network with Impairments.

Most return paths are designed to work in the 5 MHz to 40 MHz frequency range (mostly because that spectrum is not being used for other channels). The return path, however, offers some serious challenges for reliable operation of two-way services. Examples of common problems encountered (for both upstream and return path) with a HFC network are also shown in Exhibit 3.


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