FDX/Unified DOCSIS

fmd · 2025-02-13T20:05:09+00:00

While we wait for the announcement that N+X deployment has officially begun... Unified is the new version of FDX that has downstream going all the way to 1.8 GHz (currently limited by old taps). The return still stops at 684 MHz. Comcast has presented this spectrum layout to begin with: This is sufficient to offer symmetrical 2 Gbps. When more of the legacy channels go away, higher speeds can be offered. 3.1 modems that get FDX-L (limited) firmware can use the FDX channels.

fmd

hex0101 Yes, we’re still patiently waiting for the first FDX post from N+X. 🙂

What I’m also interested in seeing, if your XB7 will get an OFDM in the allocated full duplex band right away when FDX gets enabled.

Aceq2jot

hex0101 I have an outage on the 13th around the same time, so I will have to look for changes after. Xfinity seems to me moving faster since T-Mobile started saturating my area prepping for piper trenching in miles of conduit.

Zenit_IIfx

crucialcolin
FTTP from one of the legacy telcos (ex-Bell’s like Verizon and at&t, ex-GTE fragments (Verizon/FTR), regional independents i.e Shentel GloFiber) or legacy MSOs (Comcast EPON, Cox XGPON, Charter XGPON) is a totally different animal than the new regional upstarts with no industry roots.

The big telecom providers, for all the hate they get, have the following things the smaller players will always lack to some degree:

Formal engineering discipline, to include quality standards (yes, really, even if it doesn’t feel like it all the time)
Significant peering relationships/backbones crossing state lines
Enough scale to drive down equipment cost, improve consistency
Diverse customer base from residential up to enterprise to maintain revenue even in hard times
Staying power

I’ve watched local upstarts completely fall over - over promise, under deliver. Only ONE has succeeded, if only because they have chosen to build their own FTTP in an extremely wealthy area that rewards local roots and quality hometown service that happens to be adjacent enough to the Ashburn internet nexus that bandwidth is cheap to source. The ones that have depended on either the government for subsidy, or another telco big for distribution infrastructure (cough a VDSL2 CLEC trying to run on decrepit VZ copper) have failed, either outright or by being vapor.

I know there are great upstarts that work out, like GoNetSpeed, but they are getting gobbled up.

Telecom is naturally fit for vertical and horizontal integration. Vail was right - the chaos of thousands of providers is a waste of human energy. Today I am watching YET ANOTHER provider dig up the main street by my condo for yet another commercial focused fiber loop. The street already has 8+ carriers not including the local ILEC and MSO on it serving the ring of office buildings and datacenters nearby. This is insanity. Digging in the RoW will be near impossible. Verizon itself has 3 redundant plants in the area that I did not count in the count of 8 - Ex-Bell Atlantic ILEC fiber, ex-MCI fiber, and ex-GTE fiber.

I believe Comcast and Charter will eventually consolidate. It’s fate.

GHz-Man

Zenit_IIfx Enough scale to drive down equipment cost

With XGS-PON, yes, but it seems Verizon was the only one on the NG-PON2 bandwagon.

Not sure if they’re regretting that now, or what the plan is with them absorbing Frontier, since they use XGS-PON.

Or they’ll just decide to migrate everything to 50G-PON or whatever is next.

Zenit_IIfx I believe Comcast and Charter will eventually consolidate. It’s fate.

Agreed, but once that happens they should all settle on a unified standard going forward. Having equipment manufacturers do multiple different things isn’t how you get economies of scale.

Some are doing 3.1 Extended, some are doing FDX, some are doing ESD, some are doing a combination of all.

They seemed to realize that developing them all as separate products wasn’t the best idea, so now they’re working on “unified” 4.0 which supports both ESD and FDX, and it sounds like 3GHz will be fully unified:

https://www.broadcom.com/company/news/product-releases/broadcom-charter-communications-and-comcast-to-jointly-develop-unified-docsis-chipsets-capable-of-25gbps-speeds

Having too many different upgrade paths leads to format wars, like GSM vs. CDMA in cell phones.

hex0101

hex0101 no loss of modem sync, same goes for the maintenance that was on 5/15 @ 12:30 am.

Gateway Restarted 2-3 hours after hitting 30 days of uptime on 5/14. So after 720-722 hours of uptime. No change in firmware version. Rebooted though. Loss of sync also doesn’t cause a reboot on the XB gateway’s in my experience, and neither do the codewords reset. A restart will reset the codewords though.

Gateway Restarted on 5/15 at 3:58 am for firmware update, moved from CGM4331COM_8.3p1s1_PROD_sey (If I recall) to CGM4331COM_8.3p12s1_PROD_sey. Minor bump, but still on 8.3p.

Not sure if they have begun FDX deployments on N+X architecture, especially at scale. (probably not) Still some firmware bugs with the FDX amps that are apparently happening. Heck, the SoC (system on a chip, processor) on the FDX amps are more like the RPD itself due to how FDX works. A lot of software magic is needed to make these work no doubt, especially at peak performance. For now, they can run in Mid-Split mode!

Even during normal FDX operation, traditional 3.1 channels will continue to behave as they always have. If the FDX channels mess up, the modem simply goes into partial service mode and uses 3.1 on the downstream and the sub-split and mid-split upstream from 5-85 MHz. Comcast documentation mentions this as “partial service”. May even require a modem power cycle to get the modem to re-bond to the FDX channels currently, but with time that will likely get optimized more if it hasn’t been already.

Tshirt

hex0101 CGM4331COM_8.3p1s1_PROD_sey (If I recall)

was that maybe
CGM4331COM_8.3p10s1_PROD_sey ?
which is what I’ve had for a while on an XB8.
I thought they’ve gone to everyone on a common build but maybe starting to fork to various versions for different hardware or perhaps based on the local plant hardware.
or maybe I have something new to look forward to? did have a forced reboot thursday early am after 20 days of very smooth sailing.

hex0101

Tshirt CGM4331COM_8.3p10s1_PROD_sey

It was CGM4331COM_8.3p8s1_PROD_sey (installed on 3/23/26), and on 5/15/26 it got upgraded to CGM4331COM_8.3p12s1_PROD_sey overnight at the normal time (between 3:00-4:00 AM EST). 3:58 am in my case for the recent update. Easy to just check on it later on and look at the system uptime and do some basic math to find out approximately when the update was installed since I wasn’t awake lol.

jweaver

I think they just tend to update some faster than others. I have been on CGM4981COM_8.5p2s3_PROD_sey for about 3 days now.

kmax

jweaver You’ve consistently been the canary in the coal mine for firmware updates.

Anything noticible?

jweaver

kmax It has information added on the login page for finding the default password and a bug when internet is down that you can not view the signal status page resulting in an error 500 Internal Server Error and an occasional glitch where 1 DOCSIS channel may show frequency 0 and be called OFDM when it’s a standard 256 QAM channel.

hex0101

jweaver an occasional glitch where 1 DOCSIS channel may show frequency 0 and be called OFDM when it’s a standard 256 QAM channel.

Also saw this bug
Captured on April 12th, 2026
Firmware at the time was: CGM4331COM_8.3p8s1_PROD_sey

kmax

Sounds like they let the guy go (who was superstar dev that solved basic json type stuff) and now back in the dark ages with trusting the signal stats.

crucialcolin

kmax leave it to Xfinity to screw up the basics lol

It’s safe to say signal stats on gateway devices have been untrustworthy for some time. There’s alway some related issue.

Aceq2jot

hex0101 I had two maintenance widows last week and another one tonight. All I can notice is the frequencies on 193 and 194 got lowered. Hopefully i will see high splits soon.

hex0101

Aceq2jot 193 went from 957 to 690 MHz? And 194 is at 957 MHz now?

That seems to be a mid-split setup with 2 OFDM’s. Others on mid-split plant have reported it. Like @Moffetts has two downstream OFDM channels. 193 and 194.

I still only have one OFDM at 957 MHz.

Got bumped from 690 MHz to 957 MHz after the node upgrade was complete (OFDMA was also enabled at the same time). The 10 and 39 Mhz QAM upstream channels were also removed.

Here is an FDX config shared by a customer on Reddit (minus the upstream section of the signal levels)
Issue: XB8 kept crashing and losing connection, swapped to XB10, issue resolved, however the XB10 is only provisioned for mid-split and getting 120 Mbps upload speeds.

I see 193 at 774 MHz, and 194 at 1.086 GHz. (with signal roll off, power levels get way worse on 194). RF impairment. Needs further attention at the customer premises.

cypherstream

hex0101 May be the moca filter starting to carve into that 1.086 GHz carrier.

crucialcolin

cypherstream May be the moca filter starting to carve into that 1.086 GHz carrier.

It’s possible. I see pretty significant roll off even at 957 mhz on mid-split. This is from the main line in from the street at the POE box. That is before any Before MoCA filters, grounding blocks, or splitters installed. Even 193 at 774 mhz sees some degradation.

It’s likely a lot of the coaxial cable running in from taps installed within the last 15-30 years is not actually capable of handling the higher frequencies they were fully spec’d to. Maybe even the taps in some neighborhoods themselves.

Could also be that some contractors are misconfiguring amps in the field too. Who knows. I bet Xfinity is going to start experiencing a lot more service calls generated customer signal issues though as they continue to roll out FDX.

hex0101

crucialcolin I bet Xfinity is going to start experiencing a lot more service calls generated customer signal issues though as they continue to roll out FDX.

Professional Installation is supposed to be required. And a new homerun to the gateway location to ensure signal integrity. Brand new coax! Nowadays I wonder if they can use the existing RG6 coax in my home from 10 years ago and just replace the drop. Interior coax cable runs are already behind the drywall.

Still waiting on the XM4 meter. A signal meter that is designed for DOCSIS 4.0 FDX testing, and used by Comcast technicians. Current setup is the high-split enabled XM2 meter. Signal meters are operated via an iOS app. The iOS device is connected to the Wi-Fi network being broadcasted by the meter.

Also ingress becomes more of an issue with FDX, modems are transmitting on a wide amount of Spectrum vs just the standard 5-42 or 5-85 MHz bands. Local field techs will need to make sure that their HFC plants are clean. All the old wiring, crappy fittings, and gold rated splitters from some of the customer homes letting in ingress 💀

Web-Shooter

hex0101 And a new homerun to the gateway location to ensure signal integrity. Brand new coax!

They’ve been doing this for new builds for years now, probably just future-proofing.

My parents’ house was a new build in a greenfield neighborhood in 2021, and they did home run coax drops to wherever you wanted your single cable outlet.

hex0101

Comcast Technician working in the area yesterday in the morning. “Technician in your area”, when modems go down it changes to an outage for “Unanticipated Network Repairs” Didn’t effect me since they were down the road working deeper on the HFC plant segment. Last Wednesday on 5/20 I did have a similar event and was impacted for a few minutes though with loss of modem sync, but I wasn’t around to see what was happening on the plant. Just some logs and the Xfinity outage map to check status.

Got a 2nd downstream OFDM last night during a 5/27/2026 @ 1:00 am maintenance window.

38.1 SNR good for OFDM when the other one has 39.9 SNR?
How much additional downstream capacity did the node gain?

vCMTS is telling the modem to use DOCSIS 3.0 QAM channels between 441 MHz (Channel ID 13) to
627 MHz (Channel ID 44). Channel 483 MHz is the primary channel. (ID 20).

fmd

hex0101 Comcast published a top 25 chart a while back:

Based on that and your last SC-QAM, your 2nd OFDM has a width of maybe ~160 MHz. That can give you ~1.5 Gbps @ 4096-QAM.

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