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.

Aceq2jot

hex0101 What does the 193 mean. Could you clarify this for me as I see it on my mode. All the nodes are being upgraded in my areas and I have seen a truck a few times this week in my area with a fiber splicing trailer behind it.

Moffetts

193 means you are on a Harmonic vCMTS. You just have not been moved over to the 957 mhz OFDM channel yet.

Aceq2jot

Moffetts Thank you

Devious

Moffetts 193 means you are on a Harmonic vCMTS. You just have not been moved over to the 957 mhz OFDM channel yet.

193 does not dictate it will be 957.
On my modem channel 194 has 957.

Moffetts

Devious Technically true, but when Comcast was running a single OFDM for midsplit areas channel 193 and 957 mhz were synonymous. When a second OFDM was added, mine also shifted from 193 to 194 for 957 mhz (722 mhz is now 193). Who knows what’s going on with @Aceq2jot ’s network. Whatever is going on, it has not been fully upgraded yet.

hex0101

either what @Moffetts said above, post node cut in validations just finished and Comcast is currently running the existing channel config for the time being, or you are on a remote phy shelf like me. The RF comes out of a line card, heads into the analog fiber transmitters, and is transmitted out to my NC4000 node (standard analog node) in the field. This gives you rPHY and vCMTS upgrades, but Comcast does not run mid-split on this setup, so upload speeds will still be the same as before. (Max 35 Mbps before over provisioning.) Comcast wants to reduce power consumption at the hub sites and switching to a rShelf allows this to happen, without having to change any of the equipment out in the field. Bonus points, less having to pay ARRIS/Commscope and Cisco for licensing fees, all of this is simplified with the Harmonic vCMTS core.

I typically ask- How long have you seen the OFDM Channel with ID 193 at 690 Mhz for? Do you only see 4 upstream channels, or 5 or 6? Any recent downtime lasting several hours that you have noticied?

Aceq2jot

hex0101 I have seen channel 193 for about the last year I believe and nothing has changed on that front. as for upload channels i normally see six. But the software sometimes shows the channels and some times not. Like this screen shot i just took with the primary channel being 2959364096 so I think there are bugs in the software as you can see below

I
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hex0101

Anyone know how to find the CMTS MAC? I forgot how lol. PC is connected to XB gateway which is in Bridge Mode. WAN IP lease was obtained via DHCP successfully and the PC is online.

Do I need to simply check the arp table or do something different?

The logs on XB gateways do not show the CMTS MAC like other modems do.

fmd

hex0101 Only the event log can show you. The MAC address is part of the events.

kmax

So just how long does a network upgrade take? Given notices, we are on day 5 of “Network enhancement work”…I get texts, a few random outages, etc.

kmax

Just to add to this, they texted another 2 days for mon/tuesday.

fmd

Comcast said that they are investigating a possible change in how the resource blocks / sub-bands will change between upstream and downstream. FDX allows each of the three sub-bands (108-300, 300-492, 492-684) to change direction independently of each other. If Comcast were to change that, so all three always face the same direction, they can avoid having to rely on echo cancellation in the modem.

videomatic3

fmd if they only operate in 1 direction at a time doesn’t that kinda limit bandwidth too, also make those frequencies basically half duplex
These newer technologies get more disappointing by the day
I guess it’s partially a win for us because they don’t charge modem fees anymore and also kind of funny that now they are interested in making modems cheaper.

hex0101

fmd Ah, so each of the 3 bands have to be fully allocated to either downstream or upstream? I thought that all of the sub-bands can be both download and upload at the same time, making this a true game changer!

Or do the three sub-bands get assigned to either downstream/upstream dynamically by the vCMTS based on the load and can be dynamically changed on the fly by the vCMTS, with no need for modem re-syncs or service interruptions to the customer?

Either way works I suppose If I get symmetrical speeds, but this does make the limitations of using coax long-term seem more apparent.

fmd

videomatic3 It’s about the statistical nature of how customers use downstream and upstream. Comcast has found that it doesn’t make much of a difference.

Due to insufficient isolation in low value taps (introduced by N+X), all modems on the same node leg already share the same resource block config at any given time. This would be a minor change.

hex0101

videomatic3 they don’t charge modem fees anymore

Business customers are still billed $22.00 for the gateway. AT&T mailing promos to business location saying no equipment fees with Fiber. Maybe Comcast will eventually cater. Different service and team, but it runs on the same HFC infrastructure so that is why I am posting this. My node even has a few HFC business customers on it.

fmd

hex0101 That has to do with interference/transmission groups. If there is sufficient isolation between two modems (they are in different transmission groups), one modem can transmit in sub-band 1, while the other receives in it. Thus, seen from the node, you have downstream and upstream on top of each other.

However, due to limitations introduced by N+X, each node leg will be one big transmission group (no overlapping). Traffic engineering wise, Comcast has shown that it doesn’t make much of a difference.

The direction of the sub-bands can change direction at any time, depending on demand. They can do that independently of each other, at least for now (minor change).

fmd

FDX presentation at this year’s Expo: https://techexpo.scte.org/2025/10/docsis-deep-dive-mastering-the-mac-layer-and-full-duplex-architecture/

CableLabs starts by giving an overview of DOCSIS MAC/FDX. The engineer from Comcast is the 2nd speaker.

cypherstream

fmd I wish they would’ve put that video on YouTube or a podcast because the website keeps starting over every time I wanna resume each time I take a car ride. I really wanna listen to this video. Thanks for posting it.

fmd

cypherstream I wanna resume each time I take a car ride

Oh, while driving. 🙂 It is easier to understand when you can follow the slides though. :-)

robry

I ripped the mp4 file out and uploaded it to my google drive:

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