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

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.

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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.