TWO-WAY VOICE AND DATA OVER CATV SYSTEMS BASED ON THE DOCSIS 2.0 SPECIFICATION
of the set-top box in 2005 dollars should not exceed $350. With AM fiber to the home,
the cost target will probably be exceeded.
The reason that the driving factor is the set-top box is the multiplier effect. In this case
we would be working with multipliers of, say, 500 (subscribers) by
>$350. For a total
CATV network, we would be working with 100,000 or more customers. Given the two-
way and digital options, both highly desirable, the set-top box may exceed $1200 (2004
dollars), even with mass production. The amount is excessive to justify the business case.
Impairments Peculiar to Upstream Service
More Thermal Noise Upstream than Downstream
. Figure 17.17 shows
a hypothetical layout of amplifiers in a CATV distribution system for two-way opera-
tion. In the downstream direction, broadband amplifiers point outward, down trunks, and
out distribution cables. In the upstream direction, the broadband amplifiers point inward
toward the headend and all their thermal noise accumulates and concentrates at the head-
end. This can account for 3- to 20-dB additional noise upstream at the headend where the
upstream demodulation of voice and data signals takes place. Fortunately, the signal-to-
noise ratio requirements for good performance of data and voice are much less stringent
than for video, which compensates to a certain extent for this additional noise.
. This noise source is peculiar to a CATV system. It basically
derives from the residence/office YV sets that terminate the system. Parts 15.31 and 15.35
of the FCC Rules and Regulations govern such unintentional radiators. These rules have
not been rigidly enforced.
One problem is that the 75-
impedance match between the coaxial cable and the
TV set is poor. Thus not only all radiating devices in the TV set, but other radiating
devices nearby in residences and office buildings couple back through the TV set into the
CATV system in the upstream direction. This type of noise is predominant in the lower
frequencies, that band from 5 to 30 MHz that carries the upstream signals. As frequency
increases, ingress noise intensity decreases. Fiber-optic links in HFC configurations can
provide considerable isolation.
Other Upstream Noise Contributors
. The following are additional noise
contributors for the upstream path:
Relative intensity noise (RIN) contributed by the optical node return path laser(s)
Intermodulation distortion contributed by broadband amplifiers and the return
Shot noise and receiver noise within the optical detector at the headend
Multireflections within the HFC plant
TWO-WAY VOICE AND DATA OVER CATV SYSTEMS BASED
ON THE DOCSIS 2.0 SPECIFICATION
Data over Cable Service Interface Specification (DOCSIS) is a family of specifications
that permits CATV systems to have two-way capability: voice, data, and broadcast-type
television downstream and data and voice upstream.