14.1 Introduction

This chapter covers a number of topics of interest to those using return plant or needing to understand the issues involved in the operation of return plant. You may wish to review Chapters 9 through 11, which cover relevant topics in distribution systems. This chapter builds on the material in those chapters.

Return, or upstream, signal levels must be managed differently from downstream signal levels, and this topic is covered in some depth. Return laser characteristics are covered, along with methods being used to determine the proper level at which to operate return lasers. Finally, noise on the return path is dealt with. Several methods of analyzing the problem are presented, as are several alternative countermeasures.

Limited two-way cable plant has been built since the mid- to late 1970s. As early as 1972, the FCC required that plant be built to be two-way ready. It was not until the mid-1990s, however, that the technology began to enjoy adequate applications to justify widespread deployment. Previously, the only applications were provision of a return path for impulse-pay-per-view (IPPV) set top terminals, some of which used an RF return path, and a few status-monitoring applications. Return analog video was and is practiced, but most links involve trunk runs only and are limited to local backhaul applications. Beginning in the mid-1990s, these applications were joined by HFC-based high-speed data transmission and telephony. More status monitoring came to be deployed in the plant though its required data rates remain modest. Interactive video continues to be a possible future application, though as of this writing, it has not enjoyed widespread acceptance.

The enabling technology that made the widespread use of the return path possible was the introduction of smaller nodal architecture in place of the older tree-and-branch architecture. This made feasible the control of noise buildup and also made it possible to realize adequate bandwidth to support marketable services. Thus, by the mid-1990s, the technology and applications had both arrived to make return plant practical and economically justifiable.

14.2 The Two-Way Node

In earlier fiber-optic deployment, the device that mounted on strand (or in a pedestal), and that received optical signals from the headend and converted them to electrical form, was called a receiver. As two-way services came to be, a return optical transmitter was added to the optical receiver. Use of the term receiver to describe the device became confusing, so the device...