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UPDATE2 Speed Boost as BT Openreach Prep 3dB SNR for FTTC Broadband Lines

Tuesday, November 15th, 2016 (7:58 am) by Mark Jackson (Score 7,601)
rural_bt_fttc_street_cabinet_oxfordshire_uk

Customers using one of Openreach’s (BT) ‘up to’ 40-80Mbps capable Fibre-to-the-Cabinet (FTTC / VDSL2) broadband lines might benefit from a free speed boost next year when the operator adopts a new default target downstream noise margin of 3dB.

The SNR (Signal to Noise Ratio) of a standard VDSL2 or ADSL based broadband line (most UK broadband users have one of these) reflects the balance (measured in decibels) between the useful information coming down a line (good signal) and unwanted interference (bad signal / noise).

At present the default target downstream (download speed) noise margin on Openreach’s VDSL2 (FTTC) “fibre broadband” lines is set to 6dB, but by dropping this to 3dB you could deliver a free speed boost to stable copper lines (especially very short ones).

The change has been on the cards since earlier this year when BTOpenreach confirmed that they intended to expand their 3dB trial to 40,000 broadband lines (here), which followed an earlier Proof of Concept test with 150 lines (this indicated that 84% saw some performance improvement, while none experienced a worse service).

How much this performance boost is worth depends on the quality and length of your copper line from the street cabinet, but an extra handful of Megabits per second (Mbps) is not out of the question. If the figures from BT’s earlier tests pan out then an average rate gain of +10-17% seems plausible, although those on less stable lines may not see much of a benefit.

The good news is that BT has just updated their SIN498 document to confirm that the new change will be introduced from March 2017.

1.2.2 VDSL2 noise margins

Currently the default target downstream noise margin is set to 6dB. From March 2017 the target downstream noise margin shall be set to either 3, 4, 5 or 6dB – the actual value shall be determined by the Dynamic Line Management (DLM) algorithm based on line stability.

Expected Outcome

The downstream rates recorded with the 3dB target noise margins should be significantly higher than those recorded with the 6dB target margins. There should not be any significant difference in reported rates between the high and low retransmission profiles.

We should say that the 3dB profile is already supported on Sky Broadband’s unbundled network and other similar platforms have also either implemented or experimented with it. Naturally you’ll need a router that supports the 3dB profile too, but generally most VDSL2 equipped hardware is already capable of this.

UPDATE 17th March 2017

I couldn’t see any point in writing a whole new article on this because not much has changed, but since people keep asking then here’s what we know.

Openreach has confirmed to ISPreview.co.uk that the roll-out is still due to commence this month as planned (phased roll-out) and some updated performance data from their extended trial is due to be shared with ISPs over the next couple of weeks.

We also asked whether ECI based VDSL2 DSLAMS (Street Cabinets) would continue to be excluded from the roll-out (as first reported here) and they confirmed that the enhancement will only be applied to Huawei based DSLAMS. ECI kit has become notorious for causing problems with various VDSL2 enhancements and changes.

Remember, not everybody will benefit from the new SNR profiles and Openreach’s DLM technology won’t apply the lower margin(s) unless your line is deemed likely to benefit.

UPDATE 18th March 2017

A little more detail has come in so I figured we’d add it. Initially Openreach expect to add the new feature to 680,000 Huawei lines between 20th to 31st March 2017 and around 96,000 of those are likely to see a direct benefit as the DLM system will apply a sub-6dB profile (any rate capped and performance managed lines should expect to be excluded).

At this point we understand that Openreach may pause the deployment to assess the initial roll-out and then continue it towards completion by September 2017.

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16 Responses
  1. Max

    This doesn’t raise the 80Mbit cap does it ? The copper run to my house is only about 40 meters, so I sync at 79,999/19,999 – would be nice if they lifted the 80 cap, I could get a good bit more if they did!

  2. Lee

    That’s right, the cap of 80meg remains.

    I can’t see it being lifted on VDSL2 with the imminent arrival of G.Fast. Those close to the cab may see 120 or 130mbps on VDSL2 unrestricted so there wouldn’t be any real incentive for those to “upgrade” to G.Fast at 160mbps whereas the jump from 80 to 160 will seem worth it for some.

  3. Yes no change to the cap, they’re not lifting the rate above 80Mbps (even though we see a lot of FTTC / VDSL2 lines that could theoretically hit 100Mbps), but those who don’t already get the maximum possible capped rate today may well benefit from a 3db related boost.

    Mind you other factors, such as cross-talk (no widespread Vectoring) and the lack of full G.INP support until next year, can still hold back VDSL2 performance.

  4. Marcus

    I guess the question is do people value speed over reliability and stability?

    Would you rather have a 60mbps connection that has over 4+ months up-time (like my line), or have a 70mbps connection that drops frequently or is more susceptible to interference?

    Personally, I value stability above outright speed. I can’t think of much that I could do on a 70mbps line that I could not do on a 60mbps line.

    If I was a decision maker, I’d focus on vectoring for FTTC in the immediate future. I was the first person on my cabinet enabled, and was getting an attainable rate of 120mbps (obviously capped to real 80mbps) – but as the cabinet filled up I dropped to where I am now (~60mbps).

    • The decision makers already looked at that and decided that it would be better to put the investment into a bigger leap via G.fast and FTTP.

      Vectoring VDSL2 had some technical problems and its trials weren’t perfectly smooth, thus Openreach ended up going with a much more limited deployment (i.e. only using it in a few areas of specific need).

    • Simon Farnsworth

      Note that BT will still apply sync rate banding to resolve instability issues; thus, your “80/20” product line, may have a 0.128M to 60M cap applied to the downstream sync speed, causing you to continue to receive 60M speeds, not 70M speeds, when the target SNR margin change takes effect. Thus, you can still have stability even with a 3 dB target SNR margin – it’s just that if the noise on your line varies very little with time, the chances of a resync event decreasing your speed will go down.

  5. Col

    Is there any change for testing/rolling out ‘long reach’ VDSL?

  6. Sledgehammer

    Have BT/OR done any trials on Aluminium lines with G.fast or has it only been with copper lines? Will the lower SNR show any improvement on a Aluminium line? I had a chat with a OR engineer the other day, only to find out that the my DP is supplied with Aluminium cable, probably back to the old cab at the top of the road. Makes me think this may be why BT have delayed the upgrade to FTTC by another 4 months.

  7. Stuart Watson

    I currently can’t get BT Infinity because my Downstream Line Rate(Mbps) is 14.2Mbps will this help that?

  8. GNewton

    All of this won’t make much of a difference in the overall situation this country is in.

    The UK ranks 30th in the world for average download speed and 18th in Europe, coming in just behind Bulgaria or Moldova. The UK’s upload speed comparison is even worse, with the UK coming in at 67th in the world. With an average 39.76 Mbps for download and 8.84 Mbps for upload it is anything a but a leading digital economy.

    The whole broadband policy in this country has gone wrong.

  9. TomTom

    They made the cahnge without warning and stability was gone – connection dropping every few seconds. Took me over a week and multiple calls to convince them that they had changed something on their end and to set it back. Now stable again.

  10. Smallal

    Any news on exactly when in March this is likely to happen.
    There also seems to be some confusion as to whether ECI cabinets will be include in the initial rollout.

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