Noise in the
upstream transmission line, that is from the consumer to the service provider,
can degrade data transmission from the subscriber’s home. If the noise
impairment is of substantial duration, it may cause the cable modem to
temporarily lose communication with the headend facility. As a contingency
plan, the multiple service operators (MSOs) can reserve multiple channels or
upstream frequencies for their subscribers. If one channel suffers too much
interference, the CMTS requests that the cable modems “hop” to another channel.
frequency hopping capability, Cisco CMTS routers contain a spectrum manager
that continuously monitors the noise in unused upstream channels. If the CNR
(CNiR) reaches an unacceptable level on a particular channel, the spectrum
manager automatically assigns a new upstream channel to the cable modem using
Cisco CMTS routers
support the following techniques for upstream frequency hopping when the
frequency band in use is not clean:
hopping—In guided frequency hopping (also known as blind hopping), the spectrum
manager automatically assigns a new upstream channel frequency when a
configurable threshold of station maintenance (keepalive) messages fails.
Failed station maintenance messages represent an impairment of the upstream
channel due to noise, plant, or equipment failure. Explicit frequency subbands
and associated input power levels are assigned in a spectrum group in guided
frequency hopping—Frequency reassignment is scheduled by the time of day or by
a specific day of the week.
and time-scheduled frequency hopping.
is not effective against broadband noise phenomena such as impulse noise.
guided hopping techniques are independent concepts:
The spectrum is
controlled by a script, not a frequency table.
spectrum is time-scheduled as an option.
hopping frequency is selected from the available spectrum at the current time.
You can configure
and activate frequency hopping by using spectrum groups. You can create up to
40 cable spectrum groups, each containing multiple upstream ports. The
configured channel width is used for each upstream frequency.
After you have
created one or more spectrum groups for your cable network, you can add
characteristics to them, providing you with more definitive control over
frequency usage and frequency hopping.
You can configure
hopping thresholds. For example, the frequency hop threshold percentage method
prevents a single failing cable modem from affecting service to other working
cable modems. As long as a high enough threshold is configured, the system does
not hop endlessly due to a single cable modem failing to respond to 90 percent
of its station maintenance (keepalive) messages.
You can also
configure the minimum period between frequency hops, with a default setting of
30 seconds. If the destination channel is expected to be impaired, you can
reduce the minimum period between frequency hops to a small value, such as 10
seconds. This allows the frequency hop to continue more rapidly until a clear
channel is found. If excessive frequency hop is an issue, you can increase the
minimum period between hops.
different techniques of frequency hopping, see the
Creating and Configuring Spectrum Groups.
management is not supported for one-way (telco return) cable modems, because
spectrum management capabilities focus on the upstream path over an HFC
spectrum-band is changed, the spectrum management does not rearrange the
frequency for each US channel if the previous frequency belongs to the range of
new spectrum-band, which means that the US frequency will not be changed; if
the previous frequceny is out of range of new spectrum-band, those US channels
will not get frequencies.
You can specify
upstream channel frequency reassignment based on a configured time of every day
or of a specific day of the week. If your cable plant has an upstream noise
characteristic on a weekly cycle, use time-scheduled spectrum allocation. With
a time-scheduled policy, a single frequency becomes valid at any given time.