Having switched my landline telephone and ADSL service I have discovered that my new provider offers a minimum downstream SNR margin of 6dB. Previously, with BT, I had 3dB, and it ran with that for quite a number of years.

Consequently, of course, the data sync speed is somewhat reduced. I am not at all happy because I naturally never gave SNR margin a second thought and the promotional literature from ISPs, including this one, made no mention of the SNR margin.

This raises a number of points:

1. Ofcom does not stipulate that providers must make mention of SNR margin in sales literature. General Condition 23 obligates service providers to advise of the minimum contract term, payment terms and cancellation terms, but not SNR margin.

The response I received from the regulator goes on to say “If this [the SNR margin] is something of particular interest to a consumer, we would expect them to raise this with the service provider concerned before agreeing to the contract.”

So, according to Ofcom, it’s down to the consumer to ask – it’s my fault for not asking! But it’s not something like a new car that I’m looking to buy – where I might have a requirement for a particular style of wheel trim, for example. In essence, I naturally assumed the service would have a 3dB minimum margin – why would I not?

2. An axiom of any free market is that traders must set out information on their products, including properties and specifications. This is so as to differentiate them from those of competitors in the eyes of the consumer.

But the telecommunications regulator sees no reason why consumers would wish to know the minimum SNR margin, which varies by provider. A knock-on effect is that this factor can have no influence on prices because consumers aren’t informed. So a service with a 3dB minimum will, all other things being equal, go for the same price as one with a 6dB. Of course it can’t be equal because the speed will be slower for the latter, and I will come on to this below.

3. As well as modulation scheme, there are several factors that determine the sync speed on a DSL line:

• Line length and quality, including whether any of it is aluminium. Essentially, I assume, this determines attenuation.
• SNR margin.
• Any noise which is present, which may be compensated by increasing the SNR margin.

I am not familiar with the requirements Ofcom sets out to ISPs on how they must indicate broadband speeds. The reason that it is not possible to present a single figure that is reasonably accurate is due to unknown variables.

Where there is a function consisting of one or more unknowns it is imperative to assimilate all those that are known, extending ambiguity in the computed output to the unknowns only. Ofcom does not agree.

4. The response from Ofcom says that for providers using the Openreach network to offer their services, such as my provider, the standard minimum downstream SNR “seems to be 6dB”.

It may be of interest to note that the consumer retail divisions of both BT and TalkTalk offer a minimum noise margin of 3dB. This surely means that not only may their retail customers have this at disposal but that the same exchange hardware which supports other ‘virtual’ operators (like mine) could potentially provide the same margin to those operators.

If the network operators are really supplying other communication providers with 6dB or above then I have to wonder whether this is anticompetitive behaviour. Even if it is not, why has Ofcom not intervened to see that they do offer a 3dB profile?

It must be said, though, that any notion of this being anticompetitive is somewhat mitigated by each individual ISP failing to give open declaration in its literature (as detailed in point 2)! Consumers aren’t generally aware so show only indifference.


I must stress that references herein relate to the minimum possible downstream SNR margin of a service. This might not be the same as the default or standard offered. It is accepted that the margin may need to be increased due to noise. The debate could perhaps be extended to the minimum upstream SNR margin, also.

The variances in speed we are talking about are not insignificant. To give my experience as a guide: with BT I had a SNR margin of 3dB and downstream sync speed of between 12Mbps and 13Mbps. My line, I estimate, is about a mile and a quarter. Initially with my new provider I had a 12dB margin which gave a speed of around the 7Mbps mark. This having been reduced to 6dB now means the line speed is reliably 10Mbps or just above.

While I didn’t expect it to be absolutely the same as with BT – because it is provided by TalkTalk MPF – I really didn’t foresee this fly in the ointment!

This is anything but a new technology still in its infancy. By now I would have expected these sorts of issues to have been ironed out.

Dave wrote on Jul 8^th, 2017 at 8:43am:


There is clearly a case to be made around the need for the SNR margin to be specified to consumers, if it is likely to influence a buying decision, or for it to be somehow factored into declared speeds, if there is a reasonable way of doing so.

This case needs to be presented in objective terms, e.g. with examples of the unexpected detriment caused to specific consumers as a result of its non-disclosure or omission from advertised speeds. One clear way of doing so would be by demonstrating that a high SNR margin is a significant reason for advertised speeds not being achieved in practice.

Ofcom is driven by clearly demonstrable consumer interest, rather than technical precision for its own sake. The overlap has to be demonstrated in order for Ofcom to change its stance.