VDSL and G.Fast Coexistence

A few days ago, there’s been a great talk about G.Fast at the Local Chaos Computer Club chapter (C4, Cologne) by Stefan Förster of NetCologne. Here’s a link to the video of the talk (in German). I was particularly interested, because NetCologne recently announced that they have finished their G.fast trial (using the existing telephony copper wiring in buildings) and are about to start commercial service. This sounds like great news and I had quite a number of questions in my mind, including how G.fast can coexist with VDSL Vectoring that the the national incumbent offers in Cologne today.

I was particularly anxious to learn how G.fast and VDSL vectoring can coexist in the same cable harness in the building, because vectoring and G.fast are both about cancelling cross-talk to achieve high speeds. Having two systems running side by side not knowing of each other would be kind of counter productive.

It looks like already today, NetCologne uses Fiber to the Building (FTTB) to connect their customers to the Internet. Cologne has many 5 to 6 floor buildings and their fiber cable terminates in the basement. Over the years, they have installed small DSL DSLAMs to provide up to 100 Mbit/s in the downlink direction and 50 Mbit/s in the uplink direction. For their step to G.fast, they are now replacing the current generation of mini-DSLAMs in the basements with new G.fast capable equipment. Coexistence in the building’s cable harness with DSL Vectoring that is provided by the incumbent from street side cabinets is achieved by G.fast NOT using the first 17.7 MHz in case of VDSL Vectoring, or by NOT using the first 30 MHz of bandwidth in the cable to coexist with VDSL Super-Vectoring. G.fast then uses spectrum up to 106 MHz in the current generation of G.fast and to 212 MHz in the next hardware generation. This way, VDSL (Super-) Vectoring from the street side cabinet and G.fast from the basement will not get in each others way, at the expense of G.fast, as the in-house system will not be able to use 17 – 30 MHz of prime spectrum in the cable.


Still, NetCologne says they can provide 500 MBit/s in downlink and 100 Mbit/s in the uplink direction to the large majority of their customers with the current incarnation of G.fast. Today, G.fast goes up to 106 MHz in the cable, which works well, as the average line length is around 30 meters. Compare this to the length of my line to the street cabinet, which is over 300 meters. The next generation of G.fast will go up to 212 MHz in the cable and there are already plans for G.mgfast that will go up to 424 MHz.

TDD Uplink and Downlink Separation

Another thing I found quite interesting is that while VDSL Vectoring uses frequency multiplexing to separate uplink and downlink, G.fast uses time division multiplexing that can be dynamically adapted. I don’t know how quickly and dynamically this works in practice. Also, the ratio has to be the same for all G.fast modems behind a G.fast DSLAM in the basement. It didn’t become quite clear if NetCologne intends to use this feature to dynamically adapt the uplink downlink ratio depending on the changing traffic of all subscribers, so this remains to be seen.

What Kind of Fiber?

So what kind of fiber access technology does NetCologne use to connect their DSLAMs in the basements? According to the talk, GPON is used today and XGS-PON is an option once the 2.5 Gbit/s in the downlink and 1.25 Gbit/s in the uplink of a single GPON link that can serve one or more G.fast DSLAMs in the basement gets saturated. The G.fast DSLAMs that go into basements seem to be quite small. In a first instance, they will use DSLAMs with 8 and 16 G.fast ports and a version with 24 ports will become available next year. These sizes are quite reasonable for Cologne, with most buildings in Cologne not exceeding 5 or 6 floors with 3 or 4 households per floor. Oh, and by the way, did you know that G.fast offers an option to power the DSLAM from the modems on the other side? This seems to have been standardized to allow G.fast micro DSLAMs to be deployed in cable ducts outside buildings where power is not available.

An Alternative To Fiber To The Home (FTTH)?

So these were my takeaways from the talk. So far, G.fast does not seem to have a lot of traction, but for Germany and perhaps other countries, it could be an interesting alternative in the medium-term before finally getting FTTH directly into apartments. And there’s a good reason. In my Paris flat I have FTTH for quite some years now but it seems to be much simpler for a Telco there to show up and to literally just ‘nail fiber’ to the stairway walls. Unthinkable in Germany…

So this is all nice and well except for one problem I would have, should I get the choice between VDSL Vectoring the I have today and G.fast from NetCologne: They don’t give out public IPv4 addresses for private lines! As I host my cloud services at home, I require a public IPv4. Yes, I could use a redirection but that’s a kludge. And yes, they offer business lines with a fixed public IPv4 address but unfortunately, pricing compared to what I have today is not competitive. But being the optimist, who says that this can’t change at some point?

Overall it looks like competition works and it remains to be seen what the national incumbent will do once a competitor can offer data rates far exceeding their own.

One thought on “VDSL and G.Fast Coexistence”

  1. There have been several studies done on the G.fast and VDSL/Vectoring interference in our company already as part of the standardization discussions.
    Drop my a line and I can provide more information.

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