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What causes low upstream power?

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What causes low upstream power?

Why would modems have low TX power if the return path has correct levels?

One of our nodes is experiencing a lot of issues with upstream performance. Modems on that node report good downstream RX power +/- 3dB, but upstream TX power is around 35dB. Majority of modems are Cisco DPC3825 and CMTS is a BSR 64k. SNR for the upstream channels on that node is very poor - hovering around 25dB.

My initial reaction is that the return path is running too hot and the modems are dropping their output to compensate. My line techs did a sweep of that leg and report the levels are spot on for that HFC plant, and match the other nodes which are performing great. I just don't have the knowledge about our HFC plant (or RF in general) to speculate what the issue could be. My RF team think it must be an issue with the CMTS or the modems, but there's no difference in the configuration between the cable interfaces and we've already tried swapping ports around. I'm suspicious that there's an issue with the launch amplifier, the fiber transceiver, or even the fiber run itself but my RF team just doesn't agree.

Return Levels are an

Return Levels are an indication of how much attenuation (padding) there is between the CM and the CMTS port. Ideally you want your modems to transmit at 45ish dB (most MSOs have a magic number they target, companies I have worked with have used 42-45 dB) back to the CMTS meaning that there is 45 dB of attenuation between any point on your network and the CMTS. The target level when the upstream signal reaches the CMTS is generally 0 dB. The CMTS will instruct the modem to increase its transmit level so (up to the max 58 dB for Single Channel, 51 dB for 4 Channel US) that the level when it reaches the CMTS is the target level (0 dB), it does this during ranging (D3.0) constantly and can adjust for signal variations because of temperature change etc...

If all the modems in a node (or trunk) are transmitting low this could be an indication that there is not enough attenuation between the node -> CMTS US port. This could be at the node or it could be between Receiver and US Port. Typically you would generate a carrier at the node at your desired Tx level (i.e. 45 dB) outside of your US Carriers. Then as close as you can to the CMTS port (even removing the cable from CMTS port if possible i.e. during maint window) check that carrier level, you want that carrier level to be 0 at the CMTS port, so account for any padding past where your checking.

If your outside plant is designed correctly and setup correctly from any Tap on the network when a CM transmits you should hit the CMTS at 0 dB while transmitting at 45 dB. Now this isn't to say that all of your modems will be transmitting at 45 dB there will be differences for drop length, home splitter arrangement etc.. But the idea is to generally keep those transmit levels below 51 dB so that you can do 4 channel US bonding. This gives you a 6 dB buffer for long drops and splitters at customers homes. If you need a bigger buffer, then you would use a different Tx magic number i.e. using 42 dB as how your setup will give you a 9 dB window for drops/splitters and still be able to do 4 channel US bonding.

If only some of the modems in a node are transmitting low, this could mean that certain legs of the RF plant don't have the right RF padding (attentuation) and need to be checked (swept & balanced).

Low transmit levels can lead to low SNR if the noise floor is a little elevated where your upstream spectrum is located, having a 45ish+ dB TX level at the modem will improve your US SNR.


Thanks for that. We adjusted the padding on our RX port for that node during the maintenance window and SNR improved overnight. Most modems are transmitting at around 45dB now but, still logging T3 timeouts and resets but fewer already. We'll revisit after some soak time. I'm guessing there's still some noise ingress we'll have to trace out.

Download CMTS Simple monitor.

Download CMTS Simple monitor. It doesn't work very well with doc 3 modems but the snr monitor is SUPER helpful as you can use the stats from your cmts to see live snr, have a guy unplug and watch it change... http://www.msarmento.com.br/cmts_monitor.php

It's a little quirky but if you want to look at cnr, just double click on the upstream channel on the first screen and it will open up correctly.

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