What is DOCSIS PNM and How Does it Work?
As seen on the Volpefirm
9.9.13 – Brady Volpe
Recently I wrote about DOCSIS PNM technology and a company working on it called Nimble This. In this article I want to highlight a mechanism behind how DOCSIS PNM works.
DOCSIS PNM begins with a relatively well known DOCSIS feature called DOCSIS pre-equalization. I’ve previously covered adaptive pre-equalization, but here is a refresher. Starting with DOCSIS 1.1, pre-equaliztion was added to DOCSIS cable modems. It was later improved in the DOCSIS 2.0 standard adding more equalizer taps in the cable modem. An equalizer tap is very different from the type of tap used on mainline cable for distributing signal to a subscribers house, so we don’t want to confuse the two. Equalizer taps consists of small circuits in silicon that delay RF frequencies. Each unit of delay is called a tap. Bare with me for now on this very simple explanation. I’ll provide a more detailed explanation of taps and equalizers in a later post.
Once the DOCSIS standard put pre-equalizers in cable modems it created a pretty unique feature set. The CMTS has the ability to look at the quality of the signals coming from the cable modem to the input of the CMTS. When upstream impairments are present the cable modem signal with not look ideal at the CMTS. For example it might have some tilt to it or frequency ripple over its pass-band. The CMTS can make a quick calculation and determine what the cable modem equalizer would need to do in order to predistort its signal prior to transmitting in order to over-come the upstream impairments. The CMTS would then instruct the cable modem to make corrections in its pre-equalizer.
So let’s review how this done in a little more detail. The CMTS looks at RNG-REQ messeges coming from the cable model. The CMTS analyzes the quality of the RNG-REQ messages and determines the ammount of correction needed to be applied to each signal before the cable modem transmits. Then in the RNG-RSP the CMTS gives the cable modem equalizer values to update its [the cable modem’s] internal pre-equalizer. The next time the cable modem transmits a RNG-REQ (or any other message type) it will be pre-distorted as shown in the figure below:
DOCSIS cable modem 64-QAM no adaptive pre-equalization with lots of tilt
Pre-distored Cable Modem Signal
This predistorted signal will travers the HFC network and all of the impairments in the network. In this case there is obviously some significant tilt the signal needs to overcome. When the signal reaches the CMTS it will look like a near ideal signal as shown in the image below:
DOCSIS cable modem 64-QAM with adaptive pre-equalization and no tilt
Near Ideal Cable Modem Signal after Impairments
The CMTS now sees a near ideal signal at its input rather than a signal with significant tilt. This will result in a significant improvement of the upstream MER (SNR) for this cable modem. While the CMTS is making all of its pre-equalization decisions based soley upon RNG-REQ messages from the cable modem, the pre-equalizatioin will equally apply for all upstream messages, this is especially true for subscriber data. This means less uncorrectable codeword errors and better quality of experience for all subscribers.
What does this mean for DOCSIS PNM?
Now we understand what pre-equalization is doing in the cable modem, but how does this apply to DOCSIS PNM? The pre-equalizer in each cable modem has information that tells us how that equalizer has been configured. If we were to get that information, say with an SNMP query we could likely learn a lot about the impairments that the cable modem is overcoming. Once we start monitoring all cable modems in our DOCSIS network on a 24×7 basis, they would likely serve as an early warning indicator for any changes in our plants. Some cable operators are concerned about enabling per-equalizaton because it masks upstream impairments. With DOCSIS PNM we could take advantage of pre-equalization and see the impairments that DOCSIS pre-equalization is over-coming, so there would be no more masking of impairments. It is now a monitoring systems and an impairment identification system. The best of both worlds!
I will explain more on how this works in future articles. Stay tuned.