How long is too long?

I've got a friend who is on a very long line - 63db attenuation, but 7db snr, so pretty clean. Connection speeds are usually around 1.8Mb down and around 700kb up.
Their cabinet is just about to be upgraded to FTTC, but I wondered at which point VDSL stops being viable. I've got a rough idea of where the cabinet is and it'll still be a long line, just not quite as long.
Also, are there any particular routers that are better on a longer line? The 2wire died the other day (RIP) which was nice and stable, so I'm on the look out for something to replace it.
A plusnet 2704n is being used at the moment, but it keeps dropping the connection.
Any suggestions?

Attenuation of the exchange to home is of little help, what is needed is the distance to the cabinet itself.

Generally though once line length to cabinet is above 1.5km things start to become less desirable but things vary, due to things like thickness of copper, and if really rural there may be no cross talk issues to worry about.

Thanks Andrew.
I know that the line to the property is new, but the engineer said the line is using the 'last pair' when he installed it. So I think the copper to the pole is new, but after that it's whatever was there before.
I've estimated that the line is about 1.5miles to the cabinet, but that's by road and not necessarily where the copper runs. The cab to the exchange is another 1.1miles by road (again, estimating).

In reply to a post by gromit69:

Also, are there any particular routers that are better on a longer line? The 2wire died the other day (RIP) which was nice and stable, so I'm on the look out for something to replace it.
A plusnet 2704n is being used at the moment, but it keeps dropping the connection.
Any suggestions?

I've had a good experience using a Billion 8800NL on a ~7km line with ~57dB attenuation. The wifi performance is not brilliant only adequate but the ability to tweak/set the SNR margin is great and it has proven to be very stable, holding sync for months at a time.

The Graph on this site may help you -

http://www.increasebroadbandspeed.co.uk/2013/chart-b...

I would recommend that you try to improve the line by doing Quiet Line Tests etc whilst the line is still on ADSL of some variety.

You should also try to pin down the length (and route) of the D-side cable from the PCP to the house, to a reasonable degree of accuracy, as can be seen by the ski-slope in that graph.

Keep in mind that effectively, the above critical D-Side length, will actually increase if upgrading to FTTC/VDSL is carried out.

This additional length is the D-Side link from the FTTC to PCP.

This extra length can vary from being very short; say about 5 Metres when the FTTC and the PCP are close to one another, through in my case 50 Metres; and I have seen suggestions of longer.

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As VDSL works in higher frequency bands than ADSL, the losses per unit length of cable/phone line from the FTTC via PCP to house.

I have not tried working it out in detail; but let's say that the losses are 5 times as great, hence the criticality of that length.

Depending on the precise circumstvances, staying on ADSL with a cleaned-line may give better results than "upgrading" to VDSL.

Also within VDSL, it may be better to subscribe to a 40/10 package, rather than an 80/20.

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But it is difficult to accurately forecast the working results.

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Ironically, "my" FTTC is only 10 Metres from my front door; and the PCP about 40 Metres away.

Yet the D-side from the PCP to my house, wanders about 250 Metres on a green-field site, giving a length of 300 Metres for VDSL purposes - and it may be longer!

That Line Attenuation is about 13.5 db for VDSL.

Compared to 24 db for the previous ADSL, coming from the Exchange; about 1,284 Metres.

I know a couple of people who get 2 and 4 Mbps from at FTTC connection that is about 1.8 miles from the cabinet. So, if the line length is similar to the road distance and there is not too much cross talk then there is possibility that BT might offer their "faster Broadband" product. It will be a case of see what happens.

As for replacing the 2wire - get another one, they are cheap and still plentiful. Just make sure it is a 2700 not 2701.

Getting 8 to 9Mbps at 3km from cabinet is highly optimistic

In reply to a post by MHC:

I know a couple of people who get 2 and 4 Mbps from at FTTC connection that is about 1.8 miles from the cabinet. So, if the line length is similar to the road distance and there is not too much cross talk then there is possibility that BT might offer their "faster Broadband" product.

Several other ISPs may also offer FTTC. It's only BT Consumer that cuts out "Infinity" at 15Mbps estimate.

In reply to a post by gromit69:

The 2wire died the other day (RIP) which was nice and stable, so I'm on the look out for something to replace it.

the ones of these that I have seen fail, have actually been power supply problems, rather than the modem itself.

I am about that distance from the cabinet on VDSL2 and get a stable 4.8Mb, I can get it to sync at 5.5Mb but it will get lots of errors at night. According to the JDSU tester, this line is as good as it gets for this distance.