Specifications

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Cisco Cable Modem Termination System Feature Guide
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Chapter 3 Spectrum Management for the Cisco Cable Modem Termination System
Feature Overview
Frequency Management Policy
Spectrum management applies a common frequency-management policy to a set of upstream ports to
ensure that data is delivered reliably over the cable plant. Cable plant operators must make noise
measurements and determine the cable plant’s spectrum management policy. Different modulation
schemes, upstream frequency techniques, and symbol rates are used based on the cable plant
characteristics and the cable interface line card in the chassis.
Noise Impairments
Upstream noise impairments such as signal degradation on cable networks can negatively affect service
to subscribers. Two-way digital data signals are more susceptible than one-way signals to stresses in the
condition of the HFC network. Degradation in video signal quality might not be noticeable in one-way
cable TV service, but when two-way digital signals share the network with video signals, digital signals
can be hampered by:
• Impulse and electrical signal ingress—Noise can enter the network from electrical sources within a
residence or from high-voltage lines that run near CATV cabling. Two types of ingress noise include
broadband and narrowband. Broadband noise is generally of lower frequency (below 10 MHz) and
results in harmonic rolloff. Narrowband noise is a more significant interference source. Cable
equipment and infrastructure often pick up noise from amateur radio transmissions, citizen band
radios, or high-power shortwave broadcast signals. Implement a signal leakage maintenance
program to locate and repair areas of signal ingress.
• Amplifier noise—Amplifiers add noise to the HFC network that typically goes unnoticed in video
signals, but degrades digital data signals if amplifiers are improperly configured. The larger the
network, the higher the probability of amplifier noise affecting signals.
• Noise funneling—The upstream data path to the headend is susceptible to interference from the
entire network. All upstream noise ultimately ends up at the headend because the cumulative nature
of noise becomes concentrated at the headend. As a network serviced by a single RF receiver
increases in size, the probability of noise funneling also increases.
• Variable transmit levels—Temperature affects signal loss over coaxial cable. This can cause
variations of 6 to 10 dB per year.
• Clipping—The lasers in fiber-optic transmitters can stop transmitting light when input levels are
excessive. Excessive input levels introduce bit errors in both the upstream and downstream
transmissions. If a laser is overdriven as briefly as a fraction of a second, clipping can occur.
To adjust your return amplifiers and lasers, follow rigorous plant maintenance procedures documented
in the NTSC Supplement on Upstream Transport Issues or appropriate cable plant standard. Also refer
to the hardware installation guide that ships with your CMTS.
Spectrum Groups and Frequency Hopping
Cisco recommends that CMTS administrators configure upstream frequency hopping to counteract
long-term, narrowband noise. Cisco routers support a combination of guided frequency hopping and
time-scheduled frequency hopping.
The frequency hop to proactively avoid noise ingress is sometimes called frequency agility. Frequency
agility is configured and activated using spectrum groups. Spectrum management supports the creation
of as many as 32 cable spectrum groups, allowing multiple upstream ports in a single spectrum group,
and specifications of configured channel-widths for upstream segments. Each spectrum group defines the
table of frequencies to be used in a specific frequency plan. Upstream frequencies can be fixed, single
subband, or multiple subbands.