To BT or not to BT?

We have backhaul via BT (BT plc) and via TT (TalkTalk) now at A&A, and have had for some time.

We have told them both that in the long term we want a pretty even mix of providers between them, and in general we have most customers that can get either TT or BT backhaul. At present it is way more BT than TT.

But what is the difference and why would we, or one of our customers, choose one or the other?

Firstly it is worth explaining that the copper pair, in either case, is BT plc t/a Openreach copper. So either way we have to get a "supplier" to work with BT to install the service, and maintain the service, and fix faults. Some times dealing with the devil you know (i.e. BT) can be easier. For us that is a huge part of the game here - but it is only the few lines with faults that we are looking at day in and day out - for most customers lines work and it is not issue who is fixing lines. If all else fails we can move a line between carriers to get the line fixed.

The technical difference is kit in the exchange and back-haul through the country to interconnect with us. That is where BT and TT are different, but less so than they used to be. They both offer backhaul on a 95th percentile basis now and in spite of surprisingly complex pricing systems by one of them (guess?) they are very similar in price for back-haul. This is why we price both the same to customers. It is, none the less, expensive, and over an order of magnitude more than transit costs for data. Overall the back-haul works, but we have to be on our toes to identify faults and congestion and get them both to sort issues. Both TT and BT can have back-haul issues, and we are good at getting both sorted.

In general, for Home::1 and Office::1 we make a choice of carrier, but you can ask us to change it. For Office::1 we try and make it one of each for redundancy in the back-haul network. For units based the customer chooses.

So what is the difference if not cost?

For a start we can only (currently) do normal FTTC via BT. That will change some time soon, and that is a bit of a game changer as it will allow choice of BT or TT for both ADSL and FTTC services. For FTTC, not only is the copper BT plc, but the DSLAM (the VDSL modem in a cabinet) is also BT plc, so the services really are very much the same in almost all technical aspects.

But for ADSL, there are differences. For a start TT don't have 20CN (ADSL1 only) kit. So where we can do both, if the BT side is 20CN, it is a no-brainer to change to TT. We do this and email customers about the upgrade routinely, just as we do for 20CN to 21CN upgrades using BT kit when they upgrade an exchange. The 21CN (ADSL2+) modems are better and faster, simple as that.

So what else is there on ADSL?

BT have similar kit to TT, but TT allow us to control the profiles directly, and we pass that on to customers. This means you get to choose the margins and settings for the line, and stick with them. For stability, and your choice of "speed vs reliability" you have total control. That is a big plus for TT. On the BT side they have Dynamic Line Management which is impossible to disable in most cases. That will try and get the best from the line, but can make matters worse. This extra control on TT is clearly an advantage for our customers.

Are things changing?

There are several changes in the pipeline, and none look too good for BT...
  • At some point we will get normal FTTC broadband from Talk Talk, and I expect the pricing to be competitive and compatible with BT. This means we can offer an identical service technically from two carriers. This is excellent for dual line services like Office::1 where one can be BT and one can be TT.
  • At some point TT may offer some DLM (Dynamic Line Management) options, which would be an option and not mandatory like BT. If this happens then this can be a sensible default for lines to get more from the line, but with a manual override for the lines that do not work well with DLM. Best of both worlds, and a reason to use TT not BT.
  • At present BT can do a low 3dB margin which TT cannot do on ADSL lines. For a few very clean and stable lines this can mean more speed on BT. We hope TT will offer this soon. That will make the ADSL offerings much more comparible.
  • We may be able to do the "line" via TT instead of BT for the "phone" side when using TT ADSL or FTTC, which has no calls, and that may allow a lower cost for the supporting copper pair.
  • Playing the game! A big thing for us is playing BT and TT off against each other. This means that, to get the best deal, and ultimately the best service and prices we can offer, we may want to move lines between BT and TT one way or the other. Having FTTC on TT will help a lot, as well as 3dB margins on ADSL. Having equivalent services on both makes that easier.
Both back-haul providers have no filtering in place and provide clean 1500 MTU PPP to us.

So we may ask people if they are happy for us to move their line (at no cost to them) between back-haul providers. The impact is a minute or so downtime while re-jumpered. We need to work out the plan - have an opt-out for people, and even, in some cases, a permanent "opt-out" for us tinkering like this. We are happy to respect that some people make a choice and want to stick with it for whatever reasons. Having the flexibility to move most lines ultimately helps all of our customers, but transparency on how we do this, why we do this, and so on is crucial.

We plan to have some BT/TT options on the control pages soon, and notice of changes and opt-out links. If this goes to plan we can get better deals from both carriers, and eventually do more per unit and more base usage on packages for the same money.


  1. That's a very interesting post, thanks.

    Out of interest, which one generally has the lowest latency? I imagine it to be TT, but you guys should know better with all your monitoring kit. :)

    I'm on a BT line at the moment, but I certainly wouldn't object to being switched to a TT line if it's likely to be better latency...

    1. That is a good question. The latency depends on the line sync speed and interleave settings, which are more controllable on TT, but they also depend on the path through the back-haul. I have not done any stats, but generally BT and TT are much the same, but do vary from line to line. As such we could not say that latency would not change when changing carrier.

  2. Out of interest, given your access to the line stats on TT, have you considered writing your own DLM?

    It would look at the line stats every so often (say every loss of PPP, and once a day on top), and change profile if there are obvious issues.

    1. That would end up with my TT line on 15db SNR in about a week. It has days where it works fine, then it rains or is windy and I get a lot of dropouts. But running the line slow enough to prevent them is painful. So I just have to put up with it. 6 engineer visits have tried and failed to improve things.

      The thing that helped in the end is a Zyxel modem, it's a lot more reticent about re-syncing than a D-Link or the TG582n and just waits for the line to come back, which is often quicker than a resync.

    2. I would *hope* that Adrian would be clever enough to have the AADLM only run on lines that haven't opted out - you would obviously opt out, but it might reduce support calls from other users.


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