MacLeech
The one and only
Premium
join:2001-07-14
SoCal
4 edits | reply to funchords
Re: [TWC] Throttling in Milwaukee?
said by funchords  :What is CPD? CPD = common path distortion. It's caused when forward signals are "reflected" back onto the return path. It's more common on the coax distribution system where the forward signals are high (20 dBmV or higher) and the return attenuation is low.
It causes the return noise floor to jump up quite a bit (some times more than 20 dB). It kills the upstream SNR and causes "codeword" errors on the upstream. It usually affects every modem on the upstream CMTS port, but can be VERY intermittent and difficult to track down because it often disappears for hours or days due to being disturbed during the troubleshooting process.
If CPD is strong enough, it'll knock modems offline, if it's not it could just slow things down and cause "hiccups" occasionally because of the upstream errors.
It has a lot of possible causes: corrosion of distribution equipment, loose connectors, bad termination, faulty amplifiers, kinked cable, customer wiring issues, etc.... I've seen alot of it first hand.
This may interest you:
»cable.doit.wisc.edu/cpd/cpd2.v2.html
[edit]Posted better link. |
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funchords
Hello
Premium,MVM
join:2001-03-11
Washington, DC | reply to MacLeech
What is CPD? |
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MacLeech
The one and only
Premium
join:2001-07-14
SoCal
3 edits | reply to mooosenix
said by mooosenix :The modem "config" page is still accessable, and all the lights are still on with the activity light happily blinking away. However, any outbound traffic will fail to reach its destination. How are the modem signal levels during all of this?
Are they stable or does the SNR or Tx level fluctuate more than 1-2 dB?
Is there packet loss during this time?
Can you ping your gateway IP during these times?
Can you ping 192.168.100.1 during these times?
Can you ping 127.0.0.1 during these times?
.
It sounds like a return signal issue from the symptoms listed so far, possibly caused by noise, CPD, transient hum modulation, or return signal reflections... none of which phone support or the techs field techs assigned to service calls can see very well, if at all. Maintenance/line techs can see some or all of it depending on the training and/or tools they have access to. The return issues can cause intermittent upstream interruptions without causing the modem to reset. |
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