Specifications

Components Used
The information in this document is based on Cisco hardware uBR7246 VXR (NPE300) processor (revision
C) and Cisco IOS Software (UBR7200−K1P−M), Version 12.1(9)EC and a CVA122 Cisco IOS Software
12.2(2)XA.
RF Plant Troubleshooting Rules
The RF plant can be thought of as a MAC layer two equivalent. Usually, if there is a problem with the
RF plant, then layer two connectivity will not be established. If the show cable modem command
indicates that the online state has progressed past init(rc) status, then this is an indication that layer
two connectivity has been established and usually does not indicate an RF problem. However it is
possible for the Cable Modem to go past init(rc) and even as far as init(i) but still have RF issues. In
this case using a narrower upstream channel may prove the problem is RF−related. See the cable
upstream 0 channel−width xxx command.
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Before installing a live network always verify the cable router configurations in a controlled
environment such as a laboratory where the RF plant characteristics are known. Thus when installing
in a live network the characteristics of the router configurations are known and can be eliminated as a
problem source. A good RF design is imperative to make this work. Documents that should be read
before putting the cable network into production use are Connecting and Configuring the Cable
Headend and RF Specifications.
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The downstream direction is a broadcast domain. a problem that is affecting a large number of cable
modems (or all cable modems), it is likely to be in the downstream plant.
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The upstream direction is based on individual circuits per cable modem. Most cable network
problems are in the upstream direction. A problem affecting individual or small groups of cable
modems may be in the upstream direction. However, loose connections, downstream ingress, and
drop problems could affect the downstream signal to an individual cable modem. Likewise, a problem
with an individual downstream laser, optical link, node, or coax plant beyond the node could affect
just a small number of modems.
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Many upstream cable modem problems are caused by low signal to noise ratio (SNR). However, note
that the Broadcom chip SNR estimate could indicate apparent normal operation, yet impulse noise (or
a similar impairment not indicated by the SNR) may be the real culprit. The show controller cable
x/x and show cable modem detail commands interrogate the Broadcom 3137 chip on the uBR72xx
line cards that computes the upstream SNR value . Note that carrier−to−noise ratio, or CNR, is a more
appropriate term, since SNR is really a post−detection baseband measurement. Below is an
explanation of how the upstream is calculated.
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The settings on an external upconverter used when having uBR7200 and uBR10000 need to be
properly set. Remember that GI Upconverters are configured 1.75 MHz less than the Center
Frequency according to the NTSC table. For an explanation of why this is so, please read Cable Radio
Frequency (RF) FAQs.
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Different MC cards have different output power on the downstream port. For this reason, for some
cards it is necessary to add padding (external attenuation). Make sure that you follow the
specifications on how much padding to add with regards to the specific linecard used. MC11 and
MC16B cards give an output power of 32 dBmV and they do not need padding. However, all the
other MCxx cards give an output power of 42 dBmV and therefore need 10dB padding. For more
information about the cards specifications read Features, Differences, and IOS Support for Cable
Modem Cards for Cisco uBR7200 Series.
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The SNR estimating process uses only packets that are free from uncorrectable forward error corrections
(FEC) errors and is averaged over 10,000 received symbols. If the packet is damaged it is not counted, and the
upstream SNR estimate can read artificially high. The upstream SNR estimate does not take into account the
real world of burst noise, impulse or intermittent noise that is common in CATV upstream networks.
Cisco − Determining RF or Configuration Issues On the CMTS