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.
hopping is called “guided” because the frequency hopping uses the frequencies
that are specified in the spectrum group, which can be either a set of discrete
frequencies or a band. The cable interface line cards that support guided
frequency hopping do not have a “look-ahead” mechanism that would allow them to
determine the quality of the new frequency or band ahead of time, which is why
previous documents referred to this as blind hopping. Because of this, though,
the cable interface does not need to perform any search on the new potential
frequencies, so the switching time between frequencies is only approximately 20
You can specify
some rules the system uses when hopping to another frequency when the frequency
band in use is not clean. You can assign explicit frequency subbands and
associated input power levels in a spectrum group. All cable modems then on the
upstream port migrate to the next frequency with an assigned input power level.
The number of lost station management messages exceeding a configured threshold
can initiate an upstream channel frequency reassignment. For example, you can
specify a frequency hop based on lost station management messages that exceed a
threshold. The default threshold may be 10 to 20 percent depending on the Cisco
IOS release. The frequency change occurs rapidly without data loss and with
Take care to
reduce the spectrum allocation when it is used with small channel widths.
Otherwise, there will be a large number of upstream channel slots. For example,
if the allocation is from 20.0 to 28.0 MHz and an upstream port has its channel
width set to 0.2 MHz, there are 40 possible slots for that channel width.
Guided frequency hopping can require a long time to find the clean slot,
because it tries each available slot, one at a time, for several seconds during
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.