This DOCSIS 3.0 presentation was given to the SCTE Piedmont Chapter in Morrisville, NC on January 18th. It covers DOCSIS 3.0 basics, terminology, cable modem registration and troubleshooting.
30. Modulation Error Ratio (MER)
– The quality of a QAM signal
can be defined by the
Q dispersion of the
constellation’s points
considering the target value
– The error or dispersion
I power is calculated by the
value mean square of the
error vectors (real value VS
target value)
• MER is the ratio in dB between the
256 QAM average power of the signal and the
power of the error vectors
E av
MER symb dB 10 log10 N
1 2
ej 30
N j 1
30
For the downstream in DOCSIS 3.0, most legacy testing techniques can be appliedPrimary capable downstream channels can be fully tested with DOCSIS 2.0 or DOCSIS 3.0 meters, identifying and troubleshooting all of the usual impairmentsNon-primary capable downstream channels are currently not widely used due to the relatively low population of D3.0 modems, however this will eventually change. Non-primary channels will ultimately require D3.0 test equipment, especially to identify and resolve impaired or partial service situationsThe upstream still remains the Achilles heel of the DOCSIS network, with the highest density of RF impairmentsDOCSIS 3.0 capable cable modems transmit data across up to four upstream channelsLegacy cable modems (1.x and 2.0) often have multiple upstreams to which they can register and connect with, but do so only singularly. This makes troubleshooting a legacy modem sometimes more challenging because the technician must determine what upstream channel the modem is registered on.Troubleshooting a DOCSIS 3.0 modem effectively does require D3.0 test equipment in order to simultaneously exercise all bonded upstream channel at the same time.Testing a DOCSIS 3.0 network with DOCSIS 2.0 test equipment will make it very difficult, if not impossible to identify advanced impairments such as partial and impaired service outages as will be discussed further in this presentation
Once a partial service is detected, you should work your way back up the plant to determine at what location the partial service started.When the other channel is operational again, it should be thoroughly tested as it could be marginal and pushing you into an impaired service troubleshooting scenario.GB-Tap-Amp-Node
Level of impairment is important here. Many impairments can be masked by EQ technology. You MUST find out how close you are to the “cliff”Seems more difficult that D2. It seems like it would be much easier if there were a single upstream.BUT, you really need to do this on all the upstream channels, not just the one that a D2 modem would be using.Impaired Service can cause packetlossOr Not (Time Dependent Impairments (temperature,…), Proactive Maintenance)Ideally, you want to get the phy layer parameters for each upstream frequency Could do this by: moving the DOCSIS upstream channel to another frequency and inserting a constant carrier from a field meter Some of the data is available from the CMTS on a per-modem basis, you can use a laptop and SNMP to get it Pre-Equalization provides many clues to what is happening on the plant, it is available as described in the PNM Use of burst demodulation in the headend (This gets tricky to get the exact MAC address)The illustrations in this preso will be taken from equipment that retrieves and demodulates the packets from the specific instrument and returns the results to that instrument for display.
When in a bonded environment:Start with one of the channels in the upstream bonding group.Perform Phy analysis Compare against DOCSIS limits where possible Micro-Reflections Group Delay …
Step through the channels 1 by 1.
Step through the channels 1 by 1.
Step through the channels 1 by 1.
This channel is showing CWE, BUT Impaired upstream channels will show up in this manner, even when the no CWE are present.When the technician determines the channel source of the impaired service, the goal is to determine what is causing the impaired service. To accomplish this, the technician should begin traversing the network toward the CMTS, testing at each test point (Ground Block, Tap, Amps, Node) until the point where the affected channel becomes clean. When the channel becomes clean, the technician has passed the impacting impairment. Return to the original failure point and confirm the fix.Must is what I was referring to when we discussed partial service, Just because the channel came back, does not mean it is clean