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Troubleshooting and Testing DOCSIS Cable Modem Installations Presented by: Sunrise Telecom Broadband …

a step ahead


Overview of DOCSIS System Forward Path

100 BaseT Connection to/from Internet and Servers 6 MHz Channels from 88 to 860 MHz 64 QAM 5.056941 Msym/s (30.3 MB/s) or 256 QAM 5.360537 Msym/s (42.9 MB/s)


Cable Modem Termination System (CMTS) at Headend or Hubsite


Cable Modem at Subscriber 2-14-02

Streamed Data on Forward Path

 Data on the forward path is streamed using 64 or 256 QAM.

 Streamed data is broadcasted to all modems connected to the CMTS port.


All Modems Receive the Same 64 or 256 QAM Signal



Overview of DOCSIS System Return Path

100 BaseT Connection to/from Internet and Servers QPSK or 16 QAM TDMA 160 Ksym/sec to 2560 Ksym/s


Cable Modem Termination System (CMTS) at Headend or Hubsite 320 Kb/s to 10.24 Mb/s


Cable Modem at Subscriber 2-14-02

Burst TDMA on Return path

 Modems time share return path under control of CMTS upstream time slot map.

 Bursts are QPSK or 16 QAM depending on CMTS setup.




Vector Diagram of I and Q

Location On Constellation Amplitude of Carrier Phase of Carrier Q Channel I Channel 2-14-02

64 and 256 Constellation

64 QAM Constellation Q Amplitude

7 5 3 -7 -5 -3 -1 1 -1 1 -3 3 5 -5 -7

64 Possible Combinations of I and Q


I Amplitude 256 QAM Constellation Q Amplitude

15 13 11 9 -15 -13 -11 -9 -7 -5 -3 7 5 3 1 -1 -1 1 -3 -5 -7 -9 3 5 7 9 11 13 15 -11 -13 -15

256 Possible Combinations of I and Q I Amplitude 2-14-02

QPSK Constellation

 QPSK has 4 phases with a constant amplitude.

 QPSK is more resistant to noise but has a lower data rate than 16 QAM. Q 01 11 I 00 10 2-14-02

16 QAM

 16 QAM has 2 levels and 2 phases of I and Q making for 16 four bit symbols.

 Data rate is higher than QPSK, but requires a better carrier to noise ratio because the symbols are closer together.

Q I 2-14-02

Decision Boundaries

7   determine the received symbol 3 Noise and interference can cause the received symbol to cross the decision boundaries.

1 -7 -5 -3 -1 -1 1 3 5 7 -3 Correct Locations Fall Within Decision Boundaries -5 -7 Locations in Error Fall Outside Decision Boundaries 2-14-02

Self Provisioning Downstream Scan

    When the modem is turned on for the first time it checks the entire 88 to 860 MHz range looking for a 64 or 256 QAM signal.

When one is found the modem checks to see if the Program ID is for a cable modem. If it is not (ie video) the modem moves on and looks for another channel.

Once a cable modem QAM signal has been found the modem looks for the upstream channel descriptor which includes information on the upstream frequency, symbol rates and modulation type.

Then the modem acquires a map of the upstream time slots to know when to transmit.


Modem finds QAM Channel and receives info on upstream



Power Ranging

 Based on the upstream time slot map received from the CMTS the modem begins to transmit at a low level and increments the level until the CMTS hears it.

 Once it is heard the CMTS checks the receive level and tells the modem on the forward path what level it should transmit to be received at the proper level.

 The modem then goes directly to this level and 2 way communications is established.


Modem transmits at proper freq and modulation at increasing levels




 Once two way communications is established, connections are made with the DHCP, TOD and TFTP servers to establish a full IP connection.

 Once the registration process is completed the modem is ready to surf the internet.


Importance of Testing For Throughput

 Cable modem packets that are received with errors are resent again to ensure all data is correct.

 Data resending applies to the forward and return path.

 When a modem must continually resend, the overall data throughput for all modems on the port drops.

 This may not be noticeable with low traffic or a low number of modems.

 Once loading increases at some point the number of resends gets out of control and the system grinds to a halt.

 Testing the system for throughput can significantly improve the heavily loaded performance.


Forward Path Testing

  Forward path testing consists of testing Modulation Error Ratio (MER), Bit Error Rate (BER), power level and checking the constellation.

MER’s of better than 27 dB on 64 QAM and 31 dB on 256 QAM are recommended for good error free performance.

 There should be no post FEC errors at all and low or ideally no pre FEC errors.

 The constellation is used to troubleshoot the source of poor MER or BER.


Forward Path Tests.

Constellation Automatic Constellation Diagnosis MER PRE and POST FEC BER Power Level Error Sec Severely Errored Sec 2-14-02

Automatic Constellation Diagnosis

   Automatic Constellation Diagnosis automatically interprets the constellation for you and tells you based on the distribution of the cluster the type of distortion present.

Distortions that aren’t readily visible on the constellation will be picked up by the ACD.

Technicians that aren’t familiar with the constellation display can still diagnosis problems.


Return Path Testing

 Tests on the return path include ingress testing, modem level, attenuation, return path BER and lost packets.

 A spectrum analyzer display with zero span is a useful tool for testing fast transient ingress and return path cable modem signals.

 Return path BER and lost packet testing will tell you how well the return path is functioning.


Using a Spectrum Display to Track Ingress and Noise

 Use a spectrum analyzer display to track the source of noise and ingress in the system.

Return Modem Signal tap tap To Headend Node tap tap Return Modem Signal tap tap Check at various points in the system to locate source of ingress or noise Noise or Ingress 2-14-02

Limitations of Spectrum Displays for Catching Fast Transients.

 Scanning Spectrum Analyzers measure only one band of frequencies at any given instant.

Frequency Range Where Measurement is Being Made at That Instant Frequencies Stored From Last Pass of Filter 2-14-02

Limitations of Spectrum Displays for Catching Fast Transients.

 If the spectrum analyzer is at another frequency when the transient appears it will not be displayed.

A transient happening at this time will be missed by the filter unless it is still there when the filter comes by again 2-14-02

Max Hold Function

    Max Hold allows the spectrum display to catch transient signals such as ingress and modems.

Max hold displays the highest level measured and holds it until the trace is cleared by the user or a setting changed.

Max hold will only catch a transient if it is present at the time the sweep passes the frequency of the transient.

Allowing the trace to build up over time using max hold increases the chance of catching fast transients.

Max Hold Trace Current Sweep 2-14-02

Zero Span

 Zero Span mode on a spectrum analyzer is a very useful tool for viewing and measuring fast transient signals or ingress.

 Zero Span mode stops the sweep of the spectrum analyzer and parks it at the center frequency changing the display from the frequency domain to the time domain.

Frequency Frequency Domain Time Time Domain 2-14-02

Zero Span

 By parking on a frequency the analyzer will now display any amplitude variations over time.

 Zero span displays the instantaneous amplitude of the carrier over the time of the display.

Time Modem Bursts Display In Zero Span 2-14-02

Zero Span

  Amplitude variations displayed will include cable modem bursts and intermittent ingress.

Because the center frequency is always tuned to the carrier and doesn’t sweep, extremely fast transients can be viewed.

Cable Modem Signals Noise Floor viewable between modem bursts 2-14-02

Effects of Resolution Bandwidth on Zero Span

 Since the analyzer is parked at the center frequency and doesn’t scan, only signals within the bandwidth of the resolution bandwidth filter will be displayed.

 To accurately display wide band signals such as cable modem return path signals a wide resolution bandwidth filter such as 2 MHz is required to accurately display the signal.

 When viewing narrower signals such as noise and ingress it may be better to use a narrower resolution bandwidth such as 300 kHz.

2 MHz Resolution Bandwidth 300kHz Resolution Bandwidth 2 MHz RBW required to capture entire bandwidth 2-14-02

Level and Path Attenuation

 DOCSIS cable modems have a maximum output level of +58 dBmV (+55 dBmV @ 16 QAM)  Typically the CMTS receive level is set to 0 dBmV.

 These levels require a maximum of 58 dB path attenuation when using QPSK, 55 dB when using 16 QAM.

 Ideally the modem should not be running at full output to allow for system degradations.


Typical Receive Level 0 dBmV Path Attenuation Max 58 dB


Max 58 dBmV Output 2-14-02

Return BER

 Hukk Engineering has developed a patent pending method for measuring the BER performance of the return path without the need for a headend measurement device.

 First the IP of address of the CMTS port is determined by executing a trace route to the DHCP server. The first IP address in the route is the CMTS port.

  Next the instrument sends a ping with known data to the CMTS and returns them to the field instrument on the forward path.

Since the forward path is protected, so long as there isn’t any post FEC errors on the forward path, any errors detected in the returned packet are a result of the return path.

 The instrument counts the errors on the received packets and displays the BER.


IP Pinging to Measure BER

The packets are returned to the instrument. Errors caused by the return path are protected by the FEC.


CMTS Known data packets are pinged at the CMTS port IP address

Proprietary Hukk Technology

Field instrument receives ping packets and calculates BER and lost packets 2-14-02

Cable Modem Diagnostic Displays

Red Indicates Failed Auto Detected Frequencies Modem Performance Screen Green Indicates Passed Limits 2-14-02

Trouble Shooting Installations

 By testing at the various locations in the installation technicians can quickly determine the source of problems and replace only the components that need to be replaced.



Cable Modem TV#1 4 way Ground CMTS TV#2 TV#3 Test Locations 2-14-02

Optimizing the link using step attenuation.

      Many installations have significantly less attenuation between the modem and the CMTS than the maximum 58 dB. In these situations its possible to improve the ingress performance by using step attenuators.

Step attenuators attenuate only the return path and pass the forward path without loss.

The attenuation reduces the amount of ingress from the home.

The attenuation is compensated by the modem’s increased output level.

My measuring the upstream attenuation the technician can optimize installation and install the largest step attenuator possible to attenuate the maximum amount of ingress.

Step Attenuator Response Courtesy of Arcom 2-14-02

Subscriber Owned Modems

 Without fully exercising the system there is no way to know for sure that a subscriber’s modem will work reliably.

 By fully testing the network in both directions from the subscribers home, any problems the subscriber is having is a result of their equipment.

  By fully testing the line subscriber modems become more practical because finger pointing can be eliminated.

The subscriber’s computer can be connected to the cable modem system analyzer to determine if the modem or the computer is at fault.



 Testing and optimizing both the forward and return path can significantly improve system throughput because it eliminates the need for resending  Forward path testing consists of testing Modulation Error Ratio (MER), Bit Error Rate (BER) and checking the constellation.

 Return path testing consists of path attenuation and bit error rate and lost packets.

 Optimizing the return path attenuation can improve overall ingress performance.

 Proper testing of the cable modem system can improve technician efficiency and allow more installations with the same number of people.

CM1000 Cable Modem System Analyzer 2-14-02