Installation guide

3-32
Cisco uBR7100 Series Universal Broadband Router Software Configuration Guide
OL-2238-03
Chapter 3 Configuring the Cisco Cable Interface
Configuring and Activating Frequency Agility
Tip If you are having difficulty with verification, make sure you entered a valid spectrum group number,
time, frequency, and input power level.
When defining your spectrum, avoid frequencies with known ingress problems such as amateur radio
bands or short-wave bands and spectrum below 20 MHz; allow extra bands for frequency hopping; place
upstream ports in the same combiner group in a shared spectrum group; use the receive power level
setting to perform slight equalization adjustments.
Configuring Spectrum Group Characteristics
After you have created one or more spectrum groups, add characteristics to them. As stated in the
DOCSIS RFI specification, RF channel migration occurs by broadcasting a change in the upstream
channel descriptor (UCD) message to all cable interfaces. The UCD message contains the upstream
frequency and transmission parameters associated with an upstream channel.
The speed of channel migration via the UCD message is typically less than 20 milliseconds (ms). During
this time, upstream transmission is interrupted until the cable interface transmitter adjusts to its new
frequency. Data is stored in the cable interface’s buffers during this time and is sent when the frequency
hop is complete.
Also, per the DOCSIS RFI, station maintenance intervals are used to perform per-modem keepalive
polling. The Cisco uBR7100 series poll each cable interface. When ingress noise causes loss of
keepalive messages from a configurable percentage of all cable interfaces, resulting in those cable
modems going offline, a new frequency is selected from the allocation table and a UCD update is
performed.
Note Also see the “Polling Cable Modems” section on page 5-15.
The migration time is 10 seconds (maximum) for the decision and 20 ms for the frequency hop. The
percentage threshold method prevents a single failing cable interface from affecting service to other
good cable interfaces. The system will not hop endlessly because one cable interface is generating 90%
of the errors and 90% of the traffic.
The minimum period between frequency hops is also configurable, with a default setting of 300 seconds.
If the destination channel is expected to be impaired, the minimum period between frequency hops can
be reduced 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, the minimum period between hops
can be increased.
To adjust the frequency hop threshold percentage or the minimum period between frequency hops, use
the following commands in global configuration mode.
To specify that a particular spectrum group is a shared RF spectrum group, use the following command
in global configuration mode.
Command Purpose
CMTS01(config)# cable spectrum-group
groupnum hop threshold percent
Set the percentage of all cable modems losing keepalive
messages (going offline) that will cause a frequency hop.
CMTS01(config)# cable spectrum-group
groupnum
hop period seconds
Set the minimum time between frequency hops in seconds.
Valid values are from 1 to 3600 seconds.