Potential of FTTdp + G.Fast

Of course, those of us who love our speed would like to have full-fat FTTP, but I completely get why most areas won't be seeing it for a long time.

The idea of FTTdp combined with G.Fast in the meantime interests me greatly as the hype around it suggest it's almost as good as the real thing, but I have been wondering something. I live in an area where the telecoms infrastructure is underground, but would the "distribution point" mean the small single BT manholes dotted all the way along my street, or the larger double-cover one at the very end?

If it's the small one, that is pretty much right outside the door. Even the larger one would give us a boost over the FTTC we have now, though, as it would shorten our copper run to about 200m. My line is currently a relatively long 800m-ish (maybe more) to the cab leading to 30Mb speeds.

I understand that even BT don't fully know the real-world impact of FTTdp and G.Fast yet, but I'm certainly interested in knowing what the "dp" can be, exactly.

G.Fast on a short enough loop, 70m - 100m, will do 1.3Gb/s, shared between upstream and downstream.

G.Fast doesn't have a frequency band for upstream and another for downstream, the division between them is time, not frequency, so the entire bandwidth alternates between upstream and downstream depending on requirements.

1Gb/s down, 300Mb/s up services are possible, as would be 650Mb symmetrical to give 2 examples.

I remember reading something about the time vs. frequency thing - would this have an impact on latency while transferring both ways, or is it simply so fast that it becomes unnoticeable? I would imagine the faster the sync speed, the less of an impact the time division has due to more packet throughput in a certain time.

FTTP using GPON is also time division multiplexing, as there is only one frequency each way (narrowband, LOL).

The DP in underground services is indeed a junction box hiding in a manhole.

In reply to a post by yarwell:

FTTP using GPON is also time division multiplexing, as there is only one frequency each way (narrowband, LOL).

Not quite the same thing. The TDM in GPON is used to share the same carrier with multiple endpoints, in G.Fast there's no frequency set aside at all for upstream, upstream carrier isn't shared between multiple endpoints. Latency on a GPON network can increase as requests for upstream bandwidth rise and each node has to wait longer to be granted its next slot, this can't happen in G.Fast as you're only competing with yourself for slots.

At some point I'll dig my head into how G.Fast works in that regard. I'm familiar with DOCSIS and GPON's request - grant - transmit cycles but am not sure how G.Fast's multiplexing of the resources works. Given vectoring is a required part of the standard there's no reason why each line couldn't run on an independent fixed profile regardless of bandwidth demands, lack of vectoring affects G.Fast profoundly.

In reply to a post by Senn:

Of course, those of us who love our speed would like to have full-fat FTTP, but I completely get why most areas won't be seeing it for a long time.

Alongside the excellent technical discussion so far in this thread, you have to consider the economic and legal issues around FTTDP v FTTP.

To a rough approximation, FTTDP and FTTP involve running the fibre network approximately as deep, so to that point the costs are the same. (There will, of course, be some variance depending on the size of the DP compared to the typical 8 or 12 way split of an FTTP beam splitter).

The question is what option is cheaper from that point.


FTTP involves running a fibre into the premises, but does not involve installing expensive active equipment at the DP, there's no need to get power to the DP, and the installation part of FTTP is pretty much a 'once and for all' deal so far as future network upgrades go. Openreach have already run co-existence tests with prototype 10GPON over their FTTP infrastructure in Cornwall - all they had to do was fit 10GPON equipment at the subscriber and head-end, whilst GPON users on the same split remained unaffected.

FTTDP uses the existing metallic pair from the DP, but does involve installing expensive active equipment at the DP which needs power (which might be back-fed from the end-users). There is fibre at the DP which could be converted into 'full' FTTP later, but if there were future upgrades to the FTTDP technology, you'd have to change the equipment at the DP.


It may well be that both options are rolled out as fibre moves ever deeper into the network.

FTTDP is particularly attractive for multiple-occupier buildings such as flats, because the cables from the DP are typically short, and there's no need to get freeholder permission to run a fibre through the building to each served property.

For houses, it might turn out to be more cost-effective (especially over the medium to long term) to skip FTTDP and go straight to FTTP.


Obviously the economics of both FTTDP and FTTP will change over time as installation techniques are refined and the equipment gets cheaper.

At the moment, it's a hypothetical comparison as commercial FTTDP equipment is some way off and the focus for the next 5 years or so is on the BDUK project targets. It is possible that FTTDP will prove useful in the later stages of the BDUK project. I'm less sure we will see near-term widespread use of FTTDP as a speed enhancement product for those whose connections already qualify as superfast.

the thing that bugs me is such a service will still be subject to the openreach pair lottery, different areas with different quality pairs, so 100m performance is not the same in one area as another.

In reply to a post by David_W:

To a rough approximation, FTTDP and FTTP involve running the fibre network approximately as deep, so to that point the costs are the same.

This is why I'm thinking it's more likely they would end up (at least for streets like mine) making the DP the larger manhole at the end of the street, rather than the small ones outside pretty much every other house. Bringing the fibre almost to everyone's door could, as you say, mean that FTTP actually becomes the better option economically - especially when future-proofing is considered.

So many uncertainties at this point. 30Mb FTTC is doing me well today, but unless I move elsewhere, I figure I'm not going to see anything better for years...

IF you're not in a densley populated area, I would expect LTE to provide better performance than fixed line solutions in that timeframe.

Considering 3G is absolutely terrible here, I don't expect the mobile-focused access technologies to overtake fixed-line solutions for my interest in a fair while.