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[keyap]

Hi, as the title says, I’m looking for a bit of advice regarding fibre packages. Basically, my brother is looking to move from his existing ADSL connection to fibre but we’re unsure which package to go for?

At the moment, he has an ADSL connection with, from what he can remember, an agreed max download speed of 1mb although a ‘line test’ shows a max of 2.5mb is possible. Leaving all the router settings at default, he gets an upload speed of 519 and download of 1179, however, last week I decided to change his SN Margin from its norm, which tends to be around 10, down to 5 and, as a result, the download speed went up to 1639 with what seems to be no negatives. We were a little surprised though as this is above his agreed max speed?

He’s now been running this set up with a connection time of over 4 days and has had no problems (not sure it it’s relevant but during this time FEC errors (up/down) show as 10/0, CRC errors 10/15709 and HEC errors 14/4517). We don’t really know what these mean but, given there’s no disconnects or appears to be other issues we’re guessing everything is OK?

For fibre, 'line tests' show a min speed of 32.4 and max of 43.7 and when we’ve looked at a few company’s packages they tend to have generally two options – one capped at around 38mb and the other at around 76mb. On face value I’m guessing the 38mb would probably be the one to go for but, given the impact of my ADSL SN Margin reduction, I’m wondering if something similar could be done when on fibre and whether it would have the same effect?

I was therefore hoping you guys could answer a few questions:-

1. Is it possible to do on fibre what I did on my brother’s existing connection (assuming he gets a new fibre router that can do this) and, if stable, would it result in an increased speed?
2. If ‘yes’ is it possible to project a realistic new speed?
3. If the answer to 1 is again ‘yes’ and his new speed is above 38mb package limit, if he went for a 38mb package would he be capped at this irrespective of his new speed?
4. If the answer to 1 is again ‘yes’ and his new speed is above 43.7mb max line test speed, if he went for a 76mb package would he be capped at 43.7mb, irrespective of his new speed?
4. Are there any other advantages / disadvantages we need to consider between the two?
5. Which package would you guys go for?

Thanks

[kitz]

32.4 and max of 43.7

The results can sometimes be on the conservative side, but not always.

1.) Its not possible to tweak SNRm on FTTC.  Default is 6dB.  Unlike on ADSL, DLM doesnt use SNRm to increase stability but instead uses speed caps.
2.) n/a
3.) The '38 Mbps' package will sync as fast as it can up to 40Mbps.   The products are either provisioned as 40/10 or 40/2 depending on ISP.
     The exception is BT retail who do 55/10 as standard.
4.) If you go for say a 40/10 product then the fastest it can sync will be 39999 or 40000
     The advertised 76Mbps is provisioned as 80/20.   The line will sync as fast as it can based on a target SNRm of 6dB.
4b) Obviously price.   Upstream is the other.   
5.)  Based on projected speeds.. probably 40/10 or BT's 55/10.   You could always upgrade later if you find you have a lot of surplus.
   

The other thing to bear in mind is that over time FTTC can suffer from crosstalk which may eat away at some speed. 
Also DLM on FTTC is a bit less unforgiving than on ADSL which if you end up getting interleaved could take away some more speed.

[keyap]

Hey kitz, thanks for the detailed reply 

[Weaver]

HEC errors mean that you are receiving corrupted data, which is thrown away. Some applications (e.g. those using TCP) will simply retransmit the lost data and so the only noticeable effect will be a slowdown plus increased data usage. Other applications will have problems because of the data loss.

The line isn't too happy, with those errors. It might be that's why the SNRM was so high (you said it was at 10 dB). Normally it will be initially set at 6 dB on a healthy line, although other options are possible, such as 9dB, depends on the ISP’s choices, so I can't say for sure. It must be a long line anyway.

I have a downstream sync speed of ~2600 on a 4.5 mile long line, set to a downstream target SNRM of 3 dB.

[keyap]

Thanks for the update. I'm not convinced the router is reporting errors correctly! Since my original post, the connection hasn't been dropped but the errors, with virtually another day's worth of stats, now show as FEC 2/0, CRC 2/15832 and HEC 1/4605? Some figures have gone down?

With regard to line length, yep....... it's quite long. Most speed tests quote around 4.5km and line att down is 67, giving a length of 4.9. These feel about right based on my knowledge of where the exchange is.

What I'm not sure about is what distance matters for FTTC? For this, I thought it was distance to cabinet? If it is then I'm hoping the speed will be good as it's around 400 to 500m away if walking, albeit they'll be some line deviation.

Going back to my first point re SNRm and the point kitz made re DLM - I'm assuming the DLM adjustment is automatic, rather than manual, and is based on a number of factors. If this is the case, am I right to assume that, depending on profile, dB will initially be set at 6 or 9 and that the higher the number the lower the speed and vica versa? Is it as simple as, a 'good line' means the value drops whereas a 'bad line' means the value increases?

If it is, is there any 'test' I can do ahead of placing the FTTC order to give me an idea of the SNRm value? We're still debating which option my brother should go for and if SNRm is low then the 76mb sounds appealing whereas if SNRm is high, the 36mb package would seem better.

kitz mentioned the option of 'swopping'  packages - I've not done a lot of digging yet but I'm assuming there'd be a charge which we'd like to try and avoid.

Thanks

[kitz]

HEC's are a type of CRC error check performed on the ATM cell header.  HECs can do a very minimal form of error correction and able correct 1 bit errors, but quite often they trigger a CRC.
http://www.kitz.co.uk/adsl/error_correction.htm#HEC

Its impossible for me to tell if your router is seeing these in the upstream or downstream because different modems may display up or down first.


>> What I'm not sure about is what distance matters for FTTC?

The line between home and the cabinet is still copper so thats the one that matters.   Between the exchange and cab its fibre so no need to worry about that distance.

>> I right to assume that, depending on profile, dB will initially be set at 6 or 9 and that the higher the number the lower the speed and vica versa? Is it as simple as, a 'good line' means the value drops whereas a 'bad line' means the value increases?

Its initially set at 6dB..  and then things differ from traditional adsl which normally would go through the 6,9,12 etc stages.
The NGA (FTTC) DLM doesn't do this.  If it detects a problem, rather than increasing the target SNRm to 9dB, it caps the maximum sync speed.
So say if you were syncing at 40Mbps and it detects the line is noisy, then it will reduce the line rate to say 38Mbps and rate limits it at that speed so you cant sync any higher.  You may well see that your SNRm has increased, but because it uses a hard cap/rate limiting, tweaking SNR doesnt work.

>> is there any 'test' I can do ahead of placing the FTTC order to give me an idea of the SNRm value?

No.  From day one you will sync at 6dB, then any adjustments that DLM thinks needs making will be a reduction of speed cap.
Also, unlike ADSL, you cant turn off interleaving if DLM thinks you need it and why I said NGA DLM is less forgiving than ADSL DLM.

>> I've not done a lot of digging yet but I'm assuming there'd be a charge which we'd like to try and avoid.

Most ISPs are happy to upgrade you for free - 80/20 is more expensive product - so they are getting more £ from you each month. 
They are not so keen if you downgrade to a cheaper 40/10 (less income for them) and some may pass on the BTw product change fee to you. 
Thats why I suggested you start off low as its easier to go up than go down. The only caveat I can think of is watch out for introductory offers.

There is no way anyone can tell you exactly what you will get - even Openreach dont know - until you put it live on the line.   The estimates are based on line length and what people near to you currently get.  VDSL is more sensitive to cross-talk. Also it uses much higher frequencies why by nature are more sensitive to noise.

Crosstalk is far more of a problem on VDSL, most of us here have seen its effects.  A neighbour may get FTTC and suddenly you lose a chunk of speed.
See below for my line..  look what happened at 9am this morning when they turned their router on. That .8 dB drop is a speed loss of  ~ 1.5 Mb if I were to resync right now.   Different neighbouring lines have different effects.  Theres one of my neighbour's cross talk which 'costs' 9Mbps.   

Dont wish to scare you, just make you aware of what happens, and why you cant get ever get an exact quote.    If I went round with a pair of hedge clippers and cut all my neighbouring lines.. my max sync would immediately shoot up to somewhere in excess of 110 Mbps.