When a cable modem
reports a downstream channel impairment via a CM-STATUS message, the cable
modem can no longer reliably receive data on that channel. The Cisco CMTS must
not use that channel to transmit data to the cable modem. The Cisco CMTS uses
the following three options to prevent the use of the impaired channel(s):
- Option 1—Suspend the RF
channel(s) from the wideband interface used by that cable modem.
- Option 2—Move the default
downstream service flow from its wideband interface to its primary channel
interface (modular or cable).
- Option 3—Move all the
downstream service flows (primary and unicast secondary service flows) from its
wideband interface to its primary channel interface (modular or cable).
Choosing option 1
retains all the remaining operational DS channels active, option 2 retains only
a single DS channel, and option 3 retains all DS channels. Option 1 affects all
cable modems that are receiving service via the affected wideband interface,
while options 2 and 3 only affect the cable modem reporting the impairment.
To control which
option the Cisco CMTS uses when an RF impairment is reported, use the
cable
rf-change-trigger command. This command enables
you to configure thresholds (percent and count) for an event before the event
triggers an action for the cable modem. This command also enables you to
configure a secondary keyword to move all the secondary downstream service
flows of a cable modem to the primary channel interface.
Because the
CM-STATUS messages are received sequentially, the decision to use options 1, 2,
or 3 is made based on whether the trigger threshold is reached or not, and if
the secondary keyword is configured. The table below lists the
cable
rf-change-trigger
command conditions and the corresponding options selected by the
Cisco CMTS.
Table 2 Conditions for
Selecting RF Impairment Handling Options
rf-change-trigger Threshold Reached
|
secondary
Keyword Configured
|
RF
Impairment Handling Option Selected by the Cisco CMTS
|
YES
|
NA
|
Option 1
|
NO
|
NO
|
Option 2
|
NO
|
YES
|
Option 3
|
 Note |
Before the rf-change-trigger count has reached, FrwdIF moves to the
NB primary interface and only after the rf-change-trigger count has reached,
FrwdIF moves to the WB interface. Do not move the previous FrwdIF from NB
primary interface to WB Interface.
|
If the trigger
thresholds for an event are not configured, the state of the non-primary RF
channels always remains up, and the cable modems that report RF failures are
reset after the dampening time specified in the
cable
rf-change-dampen-time command expires. If both
thresholds are configured, then both the thresholds must be reached before
changing the RF channel state to down.
In addition to not
meeting the configured rf-change-trigger, a cable modem that reports
impairments has its downstream service flows modified in option 2 or option 3,
to provide reliable service in the following conditions:
- If the count exceeds the
specified number of cable modems but the percent threshold is not reached.
- If the percent threshold
is reached but the count does not reach the specified number of cable modems.
- If all non-primary
channels of the cable modem are reported down.
Additionally with
option 3, only those unicast secondary service flows (static or dynamic) which
share the same wideband interface as the primary service flow, are moved to the
primary channel interface (modular or cable). Any new dynamic service flows are
created on the primary channel interface.
A suspended RF
channel is restored for all affected wideband interfaces when a specified
number of cable modems report (via CM-STATUS) that the channel connectivity is
restored. The Wideband Modem Resiliency feature defines the specified number of
cable modems as half of the configured count or percentage of
rf-change-trigger, or both. For example, if the count is 20 and the percent is
10, then the number of cable modems reporting recovery should reduce the count
to 10 and the percent to 5 for the suspended RF channel to be restored.
When either option
2 or option 3 is chosen by the Cisco CMTS, the service flows are not moved back
to the original wideband interface until all the impaired RF channels are
restored. However, with option 3 the
existing
dynamic secondary service flows, which are transitory in nature, are not moved
back to the wideband interface even when all RF channels are restored.
The table below
lists the various RF channel impairment handling options that the cable modem
chooses and their applicable Cisco IOS releases.
Table 3 Release Specific
Behavior for RF Impairment Handling options
RF
Impairment Handling Options
|
Applicable
Cisco IOS Releases
|
Option 1
|
Cisco IOS
Release 12.2(33)SCB and later releases.
|
Option 2
|
Cisco IOS
Release 12.2(33)SCC and later releases.
|
Option 3
|
Default
behavior in Cisco IOS Release 12.2(33)SCB and SCB-based releases. All
downstream service flows (primary or secondary) are moved to primary channel
interface.
Configurable behavior in Cisco IOS Release 12.2(33)SCE4 and later releases. The
decision to move all secondary service flows can be configured using the
cable
rf-change-trigger command. For more information,
see the
Cisco IOS
CMTS Cable Command Reference guide.
|