Cisco − Determining RF or Configuration Issues On the CMT
Cisco − Determining RF or Configuration Issues On the CMTS Table of Contents Determining RF or Configuration Issues On the CMTS................................................................................1 Introduction.............................................................................................................................................1 Before You Begin...................................................................................................................................
Determining RF or Configuration Issues On the CMTS Introduction Before You Begin Conventions Prerequisites Components Used RF Plant Troubleshooting Rules Cable Show Commands for RF Problems DOCSIS Cable Upstream RF Specifications DOCSIS Cable Downstream RF Specifications Notes for Tables Checking the Downstream Checking the Upstream Using the Flap List for Diagnosing RF Problems Related Information Introduction This document describes the troubleshooting steps to determine whether a cable network problem i
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.
Comparing the Broadcom's upstream SNR estimate to what one would measure with a spectrum analyzer will often yield quite different results. The Broadcom chips upstream SNR estimating process is most reliable in the 25 to 32 dB range. If the upstream SNR estimate reaches 35 dB or greater consider the result to be unreliable and gain a true upstream CNR measurement using a spectrum analyzer. The optimal period to collect the 10,000 symbols is 10−20 msec of 100% utilization upstream for a 3.2 or 1.
• The downstream and upstream frequencies used • The noise measurements in dB. Make certain that they are correct and within the allowed limits. A table of the noise limits is included below: DOCSIS Cable Upstream RF Specifications Note: *n indicates additional information below. Specifications UPSTREAM DOCSIS Specifications *1 Minimum Settings *2 System/Channel Frequency range Transit delay from the most distant CM to the nearest CM or CMTS.
+8 to +55 dBmV (16 QAM) Input amplitude to modem card (upstream) Signal as relative to adjacent video signal −16 to +26 dBmV, depending on symbol rate. −16 to +26 dBmV, depending on symbol rate. −6 to −10 dBc −6 to −10 dBc DOCSIS Cable Downstream RF Specifications Specification DOWNSTREAM System/Channel RF channel spacing (bandwidth) Transit delay *3 Carrier to noise ratio Carrier−to−interference ratio for total power (discrete and broadband ingress signals).
Seasonal/diurnal signal level variation Signal level slope (50 to 750 MHz) Maximum analog video carrier level at the CM input, inclusive of above signal level variation. Minimum analog video carrier level at the CM input, inclusive of above signal level variation.
interface Cable6/1 ip address 192.168.161.1 255.255.255.0 secondary ip address 10.1.61.1 255.255.255.
Router# Note: This is showing a downstream receive SNR of 33.6 dB at the cable modem. Acceptance levels are >30 dB for 64 QAM and > 35 dB for 256 QAM. Annex B is the DOCSIS MPEG framing format standard for North America. Annex A is the European standard supported only when using the Cisco MC16E cable modem card and Cisco CMTS images that support EuroDOCSIS Annex A operation. Annex A or B framing format is automatically set when configuring Cisco cable modem cards.
Ranging Backoff automatic (Start 0, End 3) Ranging Insertion Interval 100 ms TX Backoff Start 0, TX Backoff End 4 Modulation Profile Group 1 Concatenation is enabled part_id=0x3137, rev_id=0x03, rev2_id=0xFF nb_agc_thr=0x0000, NB_agc_nom=0x0000 Range Load Reg Size=0x58 Request Load Reg Size=0x0E Minislot Size in number of Timebase Ticks is = 8 Minislot Size in Symbols = 128 Bandwidth Requests = 0x335 Piggyback Requests = 0xA Invalid BW Requests= 0x0 Minislots Requested= 0xA52 Minislots Granted = 0xA52 Minis
Waterfall curves. More complex modulation formats require a higher CNR (or C/N ratio) in order to maintain the same BER. The upstream input power level at the CMTS is normally expected to be 0 dBmV. This power level can be increased to overcome noise in the RF plant. If the upstream input power level is increased then cable modems on your HFC network will increase their upstream transmit power level. This increases the carrier−to−noise ratio (CNR) overcoming the noise on the RF plant.
Range Load Reg Size=0x58 Request Load Reg Size=0x0E Minislot Size in number of Timebase Ticks is = 8 Minislot Size in Symbols = 128 Bandwidth Requests = 0x335 Piggyback Requests = 0xA Invalid BW Requests= 0x0 Minislots Requested= 0xA52 Minislots Granted = 0xA52 Minislot Size in Bytes = 32 Map Advance (Dynamic) : 2447 usecs UCD Count = 46476 DES Ctrl Reg#0 = C000C043, Reg#1 = 0 VXR# You can use the show cable modem detail command to view the SNR estimate for individual cable modems.
In practice, a margin of 6dB or more may be required for reliable operation. Use the show interface cable x/x upstream y command as shown below to check for noise within the RF plant. If the uncorrectable errors, noise, and microreflection counters are high and increasing quickly, then this typically indicates there is noise present within the RF plant. (Please see the table below for a further information on this output.
Uncorrectable Error packets received through upstream interface that could not be corrected Noise and Upstream packets corrupted by line noise Microreflections Upstream packets corrupted by microreflections Total Modems On This Upstream Channel Number of cable modems currently sharing this upstream channel. This field also shows how many of these modems are active. Rng Polls The MAC scheduler queue showing number of ranging polls.
Avg percent minislots lost on late Maps Total channel bw reserved Average percent of slots lost because a MAP interrupt was too late Total amount of bandwidth reserved by all modems sharing this upstream channel that require bandwidth reservation. The Class of Service for these modems specifies some non−zero value for the guaranteed upstream rate. When one of these modems is admitted on the upstream, this field value is incremented by this guaranteed−upstream rate value.
The maximum rate that frequency hopping will occur (in seconds) Corr FEC Errors The number of correctable forward error corrections (FEC) errors on this upstream port. FECs measure noise. Uncorr FEC Errors The number of uncorrectable FEC errors on this upstream port You can use the show cable hop command to check for correctable and uncorrectable FEC errors on a particular interface. Counters should have a low value.
The flap list is an event detector, and there are three situations that causes an event to be counted. Below is a description of these events that cause a cable modem to be entered into the flap list. 1. REINSERTIONS First, we might see flaps along with insertions if a modem has a registration problem and keeps trying to quickly reregister over and over. The P−Adj column may be low.
• Features, Differences, and IOS Support for Cable Modem Cards for Cisco uBR7200 Series • Technical Support − Cisco Systems All contents are Copyright © 1992−2003 Cisco Systems, Inc. All rights reserved. Important Notices and Privacy Statement.
