
Modifying a DOCSIS 3.0 cable modem for high performance - whalesalad
http://www.usbjtag.com/vbforum/showthread.php?t=8259
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tomstokes
A fun endeavor for sure and I like that he measured before & after results for
comparison, but I'm not convinced that he made a real improvement. He notes
that he saw "a 1db increase in downstream levels occurred along with a full
1db increase in S/N" but 1dB is well within the margin of error and day-to-day
variations, especially for the imprecise internal power measurement circuits
of the modem. In fact, simply adjusting and re-tightening your coaxial cable
can easily result in more than a 1dB change. Also, I'm not quite sure where he
came up with the 1000:1 reduction in power rail noise, because his waveforms
show significantly less (although still significant) noise reduction. Finally,
having a 10-ping average of 7ms vs 6ms is very much within the margin of error
one would expect to see.

If he really wanted to improve the RF performance of his modem, he should have
used small, low-value, surface-mount capacitors instead of the large tantalum
capacitors with relatively long leads and large electrolytic capacitors. Cable
modems and other RF equipment operate at very high frequencies by definition,
and tantalum & electrolytic capacitors with long leads are ineffective at
these high frequencies. The long leads of the capacitors act as inductors,
which will prevent the high frequency fluctuations from ever even reaching the
capacitor.

If you look closely, you can see that he added his tantalum capacitors to the
already-present small ceramic decoupling capacitors. If he really wanted to
improve the RF performance of the circuit, he would have been better off
adding more low-value, surface-mount ceramic capacitors in parallel with the
pre-existing ones to help with the high-frequency decoupling, where it counts
in an RF circuit. If he was really adventurous, he could splice some ferrite
beads in series with the supply lines to form an LC filter for even better
noise rejection.

Furthermore, he could have taken measures to improve the S/N ratio of the
circuit by improving the shielding around the sensitive RF circuits in the
middle of the board. You can see where the designer originally made room for a
shield "can" to be soldered over the sensitive components, indicated by the
exposed copper rectangle around the heatsink area. However, you may get
unlucky if your shield can volume has resonant frequencies in the operating
range of the circuit which will trigger feedback and render the circuit
inoperable. A safer option would be to add some RF-absorbing foam over the
sensitive area to absorb the noise.

~~~
jonknee
> Finally, having a 10-ping average of 7ms vs 6ms is very much within the
> margin of error one would expect to see.

It's not like he did a google.com ping on each and called it a day... From the
post:

> I used Multiping set to do 10 pings a second and chart it. My target was the
> CMTS IP. I did weeks of charting and then did mods and did more charting.
> The differences were quite interesting.

Weeks of charting should more than correct for other factors.

~~~
alanctgardner2
But we don't get to see the 'interesting' charts. You're just kind of taking
it on faith that he noticed monotonically increasing performance as he
soldered more and more expensive caps to his power supply.

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thrownaway2424
I'm skeptical that the shown modifications lead to the claimed performance
differences. Those leaded capacitors have no effect above 1MHz because their
performance is dominated by the inductance of their package. Cable modems
operate between 5MHz and 200MHz, where all power supply transients will be
served by those little ceramic surface-mount caps the author bypassed with
leaded tantalums. Above about 1GHz even those are useless and the power supply
decoupling is achieved by the capacitance between the copper planes of the
circuit board.

So it kinda looks like racing stripes on a Geo to me. The claimed improvements
could be due to almost anything including unplugging the coax and plugging it
back in differently, atmospheric conditions, time of day, etc.

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staunch
I'm an avid PC FPS gamer. I'd pay good money for a cable modem that brought my
ping down a bit or even just reduced jitter. I'd kill for FIOS (which is
apparently never expanding in coverage).

~~~
kodisha
same here, someone should recognize this opportunity.

It was the same with PC monitors, for years LCD models had ENORMOUS input lag
and crappy features, but now we see models with 120HZ and no input
lag/passtrough mode appearing.

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aspensmonster
Looks really fun! Expensive, for sure. But I like that he was able to measure
the effects in terms of noise reduction, signal power, and error correction.
This is the sort of thing that is normally a "I have a buddy who swears
swapping out this part for another makes it faster" kind of endeavor. Sure, in
_theory_ there should be an improvement, but you never get to actually detect
it. It's awesome that he's able to measure the increases, though I'm sure the
design team responsible for the devices felt those increases weren't
substantial enough to warrant going all out on component costs (edit: though
according to his buddy at Cox the increases were "dramatic").

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callesgg
I improved my cable modems signal with 7db by soldering the cable directly to
the circuit-bord. Thats a lot more than 1db and the price was essentially
zero.

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taralx
This. So much this. Cheap connectors are a huge source of loss in these
systems, and eliminating them can be a much bigger win than expensive
capacitors.

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sounds
Just went down due to being hit by so many requests. Here is a cached version:

[http://webcache.googleusercontent.com/search?q=cache:iqkzgJs...](http://webcache.googleusercontent.com/search?q=cache:iqkzgJsTGSMJ:www.usbjtag.com/vbforum/showthread.php%3Ft%3D8259+&cd=1&hl=en&ct=clnk&gl=us)

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whalesalad
It's remarkable how sensitive these things are. I had to do some really gnarly
stuff to deal with some voltage backfeeding into my modem from a PoE on a
wireless AP. It helped a lot but ultimately I had to get a tech out (Charter
in SoCal). The guy installed what he referred to as a spider, which was a
hand-made mess of filters and grounding adapters on the coax.

Google "high upstream power level" and there are bazillions of people
suffering from this problem with all kinds of different solutions.

I'm in VA now with Cox instead of Charter, but dealing with a very similar
modem and now, very similar problems. I'm gonna order another one (fairly
cheap online) and see if I can do some of these modifications to improve
performance and upstream power levels.

That or i'll cave-in and switch to Fios.

~~~
mindslight
You can probably achieve the same effect with more reasonable capacitors. I
doubt those rails started off with one hundredth of the capacitance he added.

Low-inductance decoupling across every chip supply is the extremely important
part, and for that you really want ceramic surface mount caps either replacing
or stacked on top of existing ones. I'd recommend skipping tantalum caps
altogether and filling that size/freq tier with larger ceramic multilayers
since you're not designing for cost.

And of course low-esr electrolytics on the rails to reduce power supply ripple
and all that. And some ceramic decoupling caps on both sides of switching
regulators too. But definitely make sure the output of every voltage regulator
(switching and linear) remains stable and hasn't started oscillating.

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klipt
I don't understand why Comcast doesn't support the DPC3010; it seems to be one
of the cheapest DOCSIS 3 modem available.

On the other hand, maybe it's only cheap because Comcast doesn't support it?

~~~
wmf
Comcast has strict certification requirements, so it may have something to do
with that. [http://www.publicknowledge.org/blog/zoom-shows-how-
comcast-a...](http://www.publicknowledge.org/blog/zoom-shows-how-comcast-
abuses-its-market-powe)

I've noticed that Time Warner rents really cheap modems but for BYO they only
support the most expensive models. I don't know why that is (one might
speculate that they want to keep you renting since it's more profitable for
them).

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newman314
I would pay to have someone do similar mods to my SB6120. I get terrible
jitter, no thanks to Comcast...

~~~
voltagex_
I'm on ADSL2+ and am likely to be for a lot longer, thanks to the change in
the Australian government. The phone line coming into my house seems to act as
a big antenna and I get so much noise that I can't maintain a connection above
10Mbit down, 1Mbit up. I'd pay for mods to my modems too.

~~~
mctx
I'm also on ADSL2+ in Perth, my area isn't going to get NBN for the next 3
years. When I've finished moving house I'll open up my modem and have a see if
there are similar power noise problems - I've got some decent electronics
equipment (scope/SMD soldering station/dmms). If you're in Perth then you're
welcome to join in?

~~~
voltagex_
I'll be in Perth for LCAU 2014, so maybe?

Otherwise add me on Twitter or email me and let's talk :)

~~~
mctx
I'll see you there!

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kabouseng
If the Power supply he added is switch mode, I hope he checked that the
modems' nominal current draw is enough for the switch modes minimum load...
Although if it already ran 3 months then it probably is...

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ksec
Still waiting for DOCSIS 3.1 to bring Cable up to speed.

~~~
sliverstorm
What do you mean, "to bring cable up to speed"? Commonly available DOSCIS 3
modems with 8 downstream channels can perform up to 300Mbps, and the
manufacturer can add as many channels as needed.

That's not record-setting of course, but it more than covers the ISP speed
offerings I'm aware of. Aside from fiber, which is not pre-laid for most last-
mile applications, what would be faster?

~~~
ksec
I am pretty sure that is 300Mbps shared. And you rarely get that even in real
world best case scenario. And much worst if you live in City and High Rise
Buildings with High Density population.

DOCSIS 3.1 brings OFDM as well as up to 10 DL /1 UL Gbps. At that sort of
speed, as well other added reliability features finally makes Cable a decent
Alternative. Speaking from experience in EU and Asia Area. No idea if it is
the same in US.

~~~
alcari
In most parts of the US, it's cable we want an alternative to. We generally
only have 2 options for internet service: cable, or DSL via the phone company
(if you live in a large city and you're lucky, the phone company is Verizon
and offers fiber instead of DSL). DOCSIS 3.0 is significantly faster than the
DSL options generally are, so there's really only one choice if you want
speeds greater than ~50 mbps. Here in northern Utah, my options are
Centurylink, which offers ADSL with download speeds of up to 40 mbps, and
Comcast, which offers DOCSIS 3.0 with speeds of up to 105 mbps.

~~~
pktgen
If you have 40/20 (VDSL2, not ADSL) available from CenturyLink, you should be
able to get 80/40 or 100/12 (don't ask me why) bonded. Pretty competitive with
Comcast and more reliable in my experience. Their problem is that you have to
be like 1500 ft. from their DSLAM for 40/20.

