Background:
The HFC network has the potential to offer tremendous bandwidth in the downstream or forward direction from the
head end or hub to the customer. Depending upon the extent of the cable plant upgrade, the available bandwidth could be as
much as from 54 to 860 MHz however downstream channel bandwidths are determined by the individual country's video broadcast
standards.
The historical broadcast video channel assignments limit the upstream or reverse direction from the customer to the spectrum
between 5 to 42 MHz for North America and 5 to 65MHz for Europe. The upstream spectrum which supports return path
connectivity is frequently impaired by ingress of interfering signals such as radio citizen band (CB), and other legitimate and non-
legitimate RF emissions.
The greatest challenge for the cable operator is to realize sufficient usable upstream bandwidth to achieve the systems throughput
requirements for data or other services. The limited upstream bandwidth must often be shared with other services, ranging from
impulse pay-per-view (IPPV), telemetry, and alarm gathering information from the active elements in the cable plant, as well as
having to compete with interfering signals that radiate into the lower frequency range.
Ingress noise:
A term assigned by cable operators to describe any interference that is coupled into the
forward or return path cable plant via an external source. Sometimes defined as a frequency selective
impairment, Ingress can be broadly categorized as follows;
Narrow band ingress: The predominant coupling mechanism for ingress noise is a poorly shielded drop
coaxial cable that is acting more like an antenna then a drop cable. AM modulated carriers, amateur
band and maritime radio transmission generate unwanted signal frequencies at varying amplitudes
according to the propagation conditions.
Location specific ingress: Electronic equipment in the subscriber premises can pass strong signal
carriers back into the cable system and these can interfere with reverse signals such as cable modems.
Cable network architecture will determine ingress sources e.g. aerial cabling versus underground
cabling
Common Path Distortion (CPD):
Produced by poor contacts in the cable distribution network - these
contacts create a diode or rectifier effect which produces potentially harmful 2nd and 3rd order intermodulation
distortion (IMD) products or beats. These beats will occur every 6, 7 or 8 MHz in the reverse path depending
on the channel plan used. Although the magnitude of these beats are small, they increase at a node when
several reverse paths are combined.
Impulse Noise:
Typically generated by man-made devices and sometimes by nature. Man-made sources
include power switching, electrical motors, engine ignitions, and air conditioning systems. Sources in nature
are generally lightning. Laser clipping in the upstream path is another impulse noise source.
CX100/110/120/150/180 series e-Manual D07-00-002 Rev B01
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