Maximum speed for line

Why don't BT remove the limit on FTTC lines to allow those of us who are close to the cabinet to get faster than the 80 allowed as they did with ADSL when there was a limit on that?
Will it be something they will do in the future?

It might happen, no visibility of any trials on this yet

BT Openreach have chosen a sensible cut-off for FTTC profile 17a at 80/20 in my opinion. Once you allow for sufficient noise margin for stability on top of the 'achievable' figures quoted by a JDSU or unlocked modem, few will achieve a connection significantly faster than this. The non-Gigabit LAN port of the HG612 (and I believe the ECI modem also) and the limited speed of most people's routers are further constraints on achievable speed.

Offering an uncapped speed product would, in my opinion, benefit consumers relatively little. It may worsen crosstalk, lead to complaints about speed drops as more people sign up to FTTC and crosstalk increases, lead to complaints about inequitable speeds and allow those customers already benefiting from the fastest connections to take an even greater share of contended network links.


If BT roll out profile 30a - which is doubtful considering that the SIN498 specification for future wires only VDSL2 doesn't require profile 30a capable equipment, and a non-trivial amount of BT Openreach equipment isn't profile 30a capable - few will get substantially faster connections.


I believe BT will deploy vectoring in time, but future significant speed increases will require FTTP in most cases. We're now getting very close to what is possible with metallic twisted pair local loops. If a faster speed is 'must have' for you, then FTTP On Demand will be the solution.

Edited by David_W (Tue 22-Jan-13 22:14:16)

Thanks for the reply, no i don't need the extra speed, i was just questioning the artificial limit and whether they would remove it as they did with ADSL.

The move from 2 Meg fixed to up to 8Meg was a much more significant improvement for many people than the 10% who might benefit from going faster than 80 Meg on Profile 17a.

The rules about advertising required at least 10% of customers to be able to achieve the quoted speed. This may well mean there's no marketing gain for the ISPs of uncapped Profile 17a, so there's little commercial reason for BT Openreach to offer it.

As MrSaffron rightly says, few will notice any benefit in having, say, 90Mbps rather than 80Mbps.

In reply to a post by David_W:

If BT roll out profile 30a - which is doubtful considering that the SIN498 specification for future wires only VDSL2 doesn't require profile 30a capable equipment ....

But does specify 17a vectoring capability is required.

Further confirming your opinion.

In reply to a post by RobertoS:

But does specify 17a vectoring capability is required.

Indeed.

I suspect there are also "spectrum management" reasons for having the limit in place...

Without an upper limit, the closer lines will attempt to get as much bandwidth as possible, using as much transmit power as possible. This will cause crosstalk problems for other lines.

With an upper limit, the usage can be spread out over the full frequency range available to the close lines, and allow for reduced transmit power. This, in turn, reduces crosstalk problems on the other lines - which are the biggest restrictions on the performance of long lines.

Doing this makes it more likely that the longer lines will get "reasonable" service, at the cost of denying ab-fab service to the shorter lines. An issue of fairness.

When (if) vectoring is introduced, and crosstalk issues fade considerably, it seems likely that speeds will be higher for both the shorter & longer lines.

In reply to a post by WWWombat:

I suspect there are also "spectrum management" reasons for having the limit in place.

I agree. You've given the fuller explanation of my crosstalk comment.


In time, various changes will take place that may permit higher speeds. Deployment of vectoring will help with crosstalk issues.


I suspect, too, that there will be regulatory changes favouring higher speeds.

It seems clear that the old LLU model is dying from an economic point of view. The important part of unbundling an exchange will be bringing in your own backhaul capability, which you can hook up to the FTTx infrastructure at the aggregation node. There's increasingly little point spending money on equipment to provide 'traditional' phone service and ADSL2+ when consumers will increasingly demand FTTx and voice can easily be provided over FTTx infrastructure.

The cost of rolling out FTTx means that it's only worth one provider rolling out in each location. There's little money in providing alternative 'last mile' connectivity, as shown by the virtual cessation of roll out by the original cable companies in the run-up to their commercial failure and coalescence into Virgin Media, also by Digital Region. This means SLU is only really of relevance where BT Openreach has yet to roll out FTTx. (Incidentally, one FTTC provider per location also offers the best possible scenario for VDSL2 vectoring, with all pairs in a bundle served by the same infrastructure).

If Ofcom is changed the regulatory framework to require all SLU operators to offer wholesale interconnection so there were no lines where FTTx was available but wholesale FTTx was not, the way then appears open to set a date to remove ANFP protection for exchange provided ADSL in each FTTC served area, somewhat like the abandonment of 20CN DSL products in 21CN served areas. This will require removal of ADSL from all lines served from FTTC enabled PCPs, with broadband service moving to FTTC, as the main reason behind the current protection is strong VDSL2 signals at the PCP interfering with weak ADSL signals from the exchange.

Removal of exchange provided ADSL would lower the VDSL noise floor somewhat and, more importantly, permit removal of VDSL power cut-backs put in place to protect ADSL. The removal of the power cut-backs will increase available VDSL2 speeds, especially on longer lines which are most affected by the current cut-backs on the lowest frequencies which are shared between ADSL and VDSL. If necessary, ADSL and VDSL could be served from the same point - from the PCP for lines too long to benefit from VDSL (with the corresponding increase in speeds by removing the E-side cabling from the ADSL) or from the exchange for 'exchange only' lines - though with no power cut-back, VDSL may perform at least as well as ADSL, making ADSL redundant.

The pay back in abandoning exchange ADSL in VDSL2 served areas is clear - there's no need to offer duplicate access methods, and all legacy ATM backhaul can be abandoned. This should permit wholesale port costs for an ADSL replacement VDSL2 service to match existing ADSL port costs. I'm sure 'wires only' VDSL2 will have arrived long before this happens, so ISPs will be able to prepare customers for their upgrade by sending out a new router and filters. Indeed, many of these upgrades will likely happen anyway over time - once 'wires only' VDSL2 is finalised, I expect ISPs will increasingly move to providing multi-mode equipment which works equally well on the various flavours of ADSL and VDSL2, as well as having an Ethernet port to connect to an FTTP ONT and at least one voice port for voice over FTTC.


One unknown in all this is how fast FTTP roll-out will happen, and whether Ofcom will give regulatory approval for BT Openreach to discontinue metallic local loops and traditional voice service when all users have been migrated to FTTP (i.e. Fibre Only Exchanges). The obvious benefit of allowing this is that BT seem destined to have fewer FTTx aggregation points than there are existing exchanges at present, offering savings to BT by abandoning disused exchanges and to other operators from having fewer aggregation nodes to serve.


I expect universal FTTP to take several decades, even with the prospect of consumers helping BT with the costs of upgrading areas to FTTP via the FTTP On Demand product and the inevitable development over the next few years of FTTP or FTTB solutions for blocks of flats and apartments. The problem will be the diminishing proportion of connections that are disproportionately expensive to upgrade to fibre, though it will become standard to install fibre at the same time as - or instead of - replacing life-expired copper infrastructure. All this means there will be a case in the medium term for abandoning ADSL in favour of universal VDSL2, offering the highest speeds physically possible on copper twisted pairs. There may be some incremental advancements still to come in VDSL2, but I do not foresee another major generation of DSL. When there is demand for >100Mbits/second, that will become a driver for FTTP deployment - at the moment, there really isn't a killer application for such connections.