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UPD3 Latency Woe as BT Deploy Physical Retransmission on FTTC Broadband

Thursday, March 26th, 2015 (10:24 am) - Score 5,468

Earlier this year BTOpenreach confirmed (here) that they were deploying Physical Retransmission ReTX (G.INP – ITU G.998.4) technology to improve the performance of their ‘up to’ 40-80Mbps capable Fibre-to-the-Cabinet (FTTC) lines. But at least one ISP has noticed that this is also increasing latency times by around 15ms (miliseconds) and in other cases a loss of speed is observed on some lines.

The G.INP technology is essentially an error correction solution that can help to tackle spikes / bursts of Electromagnetic Interference (impulse noise), which makes some problematic lines more stable and less prone to errors (Sky Broadband already uses G.INP on their LLU network).

An Openreach Spokeswoman told ISPreview.co.uk (Jan 2015):

Openreach is in the early stages of introducing G.INP correction, also known as Retransmission, for FTTC lines that we think can benefit from it. Retransmission supports our Dynamic Line Management process and will benefit customers by providing a slight improvement in speed on FTTC lines where it has been used. It will also improve the volume of FTTC lines running error free.

FTTC lines will automatically accept Retransmission policies. Those currently with non-Retransmission compliant modem firmware will be identified by management systems and updated, after which Retransmission will become available.

An upgrade process has started which is expected to take several months to complete.”

In keeping with this the ISP Andrews & Arnold (AAISP) has noticed a number of FTTC lines drop and reconnect with G.INP enabled and an accompanying “increase in latency of around 15ms“. Latency is a crucial measure of the time (delay in milliseconds [1000ms = 1 second]) that it takes for a single packet of data to travel from your computer to a remote server and then back again (ping).

In the grander scheme of things 15ms might not sound like much, although it is especially important for fans of fast paced online multiplayer games that like to keep their latency / ping times as low as possible (experiences vary between games, but above 60ms and you can start to notice its impact through synchronisation delays etc.).

A rise of 15ms above the line’s existing latency time might also give some FTTC services a similar or worse latency performance than older ADSL2+ solutions (Ofcom’s comparison).

AAISPs Service Status Update

We’ve been in contact with BT and it does look like BT are rolling out a new profile on to their Huawei DSLAMs in the local green cabinets. It has been expected that BT would be rolling out this new profile, but we didn’t expect such an increase in latency.

The profile adds ‘Physical Retransmission (ReTX) technology (G.INP / ITU G.998.4)’ which helps with spikes of electromagnetic interference and can make lines more stable.

We would hope to have control over the enabling and disabling of this profile, but we don’t. Line profiles with FTTC is managed by BT Openreach and are tricky for us and even BT Wholesale to get adjusted.”

The issue is unexpected since one of the advantages of G.INP is that it should be able to perform its magic without impacting latency, at least that’s what we’ve always been told by firms like Alcatel-Lucent and telecoms operators. AAISP has posted an illustration of the problem (here) and we’ve ask Openreach to clarify.

UPDATE 1:06pm

We still haven’t heard back from Openreach, although feedback from some of our readers appears to be shedding a little extra light on the situation. In short, the latency increase seems to occur when customers have G.INP assigned and yet own a router/modem that doesn’t support G.INP (there will be quite a few of these about).

Some people have been able to solve the problem by upgrading the firmware of their kit, although it is a fact that not all VDSL2 routers support G.INP. Some aspect of this situation appears to then make the line interleaved (or appear to be interleaved), although to the ISP it may look as if no interleaving has been enabled.

The upside is that many of these problems can be fixed with a firmware update and those who own G.INP supporting kit should be fine, we think.

UPDATE 27th March 2015

Sadly we are still awaiting a response from Openreach and have this morning attempted to prod some of their other agents. On a separate note we believe that this issue is probably only connected to Huawei street cabinets (DSLAMS) because we don’t know of any ECIs that have been upgrade with G.INP yet (this will begin around late April 2015).

Meanwhile more ISPs have contacted us to confirm that they’re also seeing the issue, which of course isn’t surprising since most share the same Openreach infrastructure.

UPDATE 27th March @ 11:11am

The reaction from BT is in, although it doesn’t tell us much.

An Openreach Spokesperson told ISPreview.co.uk:

Openreach is rolling out Retransmission technology because it improves speeds on the vast majority of lines – and we consulted with our CP customers before implementing these changes. However, latency may go up for a small minority of people, so we are working with these customers to help resolve the issue.”

UPDATE 8th April 2015

Some consumers with Openreach VDSL ECI modems are also unable to connect to the Internet, although this can be resolved by asking your ISP to action a DLM reset on the line. After doing that the modem should update its firmware automatically and you’ll be able to reconnect.

Take note that the other issues with loss of speed and latency are also known to afflict the BT Homehub 5 (Type A – v4. and TP Link TD W-9980.

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By Mark Jackson
Mark is a professional technology writer, IT consultant and computer engineer from Dorset (England), he also founded ISPreview in 1999 and enjoys analysing the latest telecoms and broadband developments. Find me on Twitter, , Facebook and Linkedin.
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37 Responses
  1. HmmmUK says:

    I don’t suppose anyone has been able to answer my earlier query on G.INP and ECI modem firmware?

    I have an Openreach ECI modem and bought a spare (from the Kelly install engineer!) as a backup. The first thing ISPs often ask if you are troubleshooting a problem is to try an alternate modem/router.

    I’ve been reading that the ECI modem needs a firmware update before the line can use ‘G.INP’. Once the firmware has been automatically updated then the line would be updated. Again I’ve read (on Kitz) that to use a modem with the old firmware you would need a manual DLM reset so that the firmware could be updated before the line uses G.INP.

    I can see this presenting problems for people that keep a spare ECI modem as the firmware won’t be up to date and as far as I can tell won’t allow a successful session once the line upgrades to G.INP!

    If this is correct then it will effect people with spare ECI modems or people that buy them from eBay – and also people that use their own alternative modem/router most of the time but have kept the original as a fallback.

    If you can find out more information on this it would be useful.

  2. Mike says:

    Had G.INP enabled on my line last week. Latency increased by 1-2ms, speeds didn’t change. Estimated 950m of copper between me and cabinet.

    TBB graph shows the jump just after 10am;


  3. adslmax says:

    15ms isn’t acceptable! 🙁

    1. DanielM says:

      not for a “fibre” service it isnt lol

      but since it’s copper who really cares.

    2. TheFacts says:

      What do you do that makes this a problem?

    3. GNewton says:

      @TheFacts: Why do you thing a 15ms increase is acceptable? In fact, why do you thing copper-VDSL is acceptable? Especially in view of your nationwide fibre deployment proposal some months ago!

    4. AndyH says:

      I’d hate to know what you think of the SLA on latency for a leased line between PoP in the UK.

    5. Steve Jones says:

      An increase in 15ms would not be acceptable, but it appears this is happening because the relevant modems aren’t conforming to the SIN which mandates support for g.inp. It looks like for most of those adversely affected it can be fixed with bringing firmware up to date.

  4. Astroturfer says:

    Had this happen to me.

    Openreach systems may have claimed no interleave was present, however it most definitely was.

    D: 1478 632

    I was using an old firmware on my Huawei HG612. Updated to latest and greatest and found myself on a G.inp profile and good to go.

    I suspect that the increase in latency came with a big drop in sync speed; this is due to the FEC overheads that come with interleave.

    Certainly I lost ~12Mb which I recovered via the firmware upgrade immediately.

    1. Matt says:

      Me too… lost lost around 8000 sync and latency went from 10-11ms (Rapidswitch, Maidenhead) to 27ms… Pulsate had no idea why and refused to help…

      Messed with Firmware.. got latest then G.INP was detected and speeds actually went back to normal and then some extra bandwidth too.. gained 3-4000 kbps downstream and now upstream sync at 20000 instead of 19999

      Openreach Broadband Engineer said they have no idea on rollout schedules and also wasn’t aware of the issues…

      It seems the HG612 was not understanding the G.INP profile correctly and started applying high levels of Interleaving and INP to my line…. all been fine since updating to latest firmware..

    2. AndyH says:

      More than likely, your line was excluded from the retransmission update because you had an older firmware. Once you updated, the management systems detected this and updated your line with the retransmission profile which has reduced the error count and increased your sync speed.

  5. adslmax Real says:

    Oh well, G.INP no thanks. Leave mine alone!

    1. Astroturfer says:

      Heh, if DLM feels the need G.inp will arrive.

      You will, however, be fine so long as your own modem supports it.

  6. Astroturfer says:

    I should also mention that the modems aren’t on an interleaved profile according to the DLM. If modems were properly assigned an interleaved profile they wouldn’t be allowed back onto fast path from a single reboot.

    Modems that don’t support G.inp when DLM wants to apply it seem to fall through the cracks and end up on some default interleaved profile even though they’re being offered a fastpath G.inp profile.

    1. Matt says:

      Oh… Just read after replying above…

      Yes.. Exactly what seems to be happening…

  7. Starman says:

    Personally I found the opposite – a decrease from approx 30ms latency down to 15ms and a sync increase from 56Mb to 66Mb. OR modem with current unlocked firmware.

    1. Steve Jones says:

      In your case it is clear that you were running with interleaving on (which normally loses about 10mbps and adds 10-15mbps). I assume DLM did this for stability reasons. Now the g.inp has been enabled, interleaving has been turned off and you’ve got that 10mbps back.

  8. Mark Heath says:

    Absolutely no problems with G.INP with two separate FTTC lines. Both delivered 74 Mbps connection rate and now deliver 80 Mbps with G.INP implemented. No significant chance in latency. Since G.INP has been enabled, there’s hasn’t been a single error on either line.

    More information here: <urlhttp://www.increasebroadbandspeed.co.uk/g.inp

    1. Steve Jones says:

      Clearly it’s a bit more complicated than the headline would indicate. I’ll make a guess that this is probably something to do with an interaction between modems/firmware levels and the implementation in the cabinet level. Some of the symptoms described similar to what might be expected from VDSL interleaving.

      A clearer picture is required. At the moment it seems to be rather anecdotal. Of course and ISP seeing something does speak of something rather more systematic, but then again, possibly there is another factor at play as I don’t know what aspects VDSL settings are under the control of the ISP.

    2. AndyH says:

      The way it should work is that the modem management systems are supposed to detect non-complaint modems (firmware/hardware) for retransmission and not apply the new policies on those lines.

    3. Steve Jones says:

      One thing that might also be relevant is that the BT Openreach SIN for VDSL modems specifies that any modem attached must support g.inp (page 25).

      “The modem shall support downstream PHY layer retransmission as defined in G.998.4[6]. When downstream retransmission is implemented, the “Retransmission used Downstream (RTX_USED_ds)” parameter shall be reported via the DSLAM in accordance with Clause of G.997.1.”


      There is, of course, plenty of potential for issues to arise (as with vectoring, which is also mandated).

  9. Steve Jones says:


    Maybe, but see my post above which states that g.inp support is mandated in the SIN. My suspicion is that some modems may have had interleaving imposed.

    1. Astroturfer says:

      Per my previous post indeed they are interleaved. Openreach probably need to fix their DLM so that it correctly reports that customers have been interleaved. At the moment when G.inp fails they continue to report that this is the profile the modems are on.

      It’s certainly not a ‘standard’ profile as having G.inp compatibility immediately resolves the issue. I guess it’s a fall back for when G.inp is desired but not achievable and the OSS systems aren’t quite there yet.

    2. AndyH says:

      It’s mandated, but some modems may have the older firmware without the g.inp support. If they do, then the retranmission DLM profiles should not be added to those lines.

  10. Gem says:

    not for a “fibre” service it isnt lol

    but since it’s copper who really cares.

    Unless you have native FTTP in your area then all networks run with some copper in them, Virgin and BT both heavily rely on copper so your comment about it being copper and nobody caring is stupid.

    Are you oblivious to the fact that your own “fibre” connection from Virgin media is not that much different from any FTTC service? Or do you think the Virgin connection you have is FTTP and you are far superior to the rest of us?

    1. DanielM says:

      “Unless you have native FTTP in your area then all networks run with some copper in them, Virgin and BT both heavily rely on copper so your comment about it being copper and nobody caring is stupid.”

      Read my comment history. or perhaps you are blind

      “Are you oblivious to the fact that your own “fibre” connection from Virgin media is not that much different from any FTTC service? Or do you think the Virgin connection you have is FTTP and you are far superior to the rest of us?”

      Virgin is copper, i never once called it fibre and never have. you cannot send fibre over copper or a phone line. this is not an opinion but a fact. and for the record Virgin’s service is miles different to FTTC. in most cases the node can be quite a distance.

      and to set the record straight i did have a FTTP connection. that was on a business circuit.

      perhaps in future “Gem” you should read what i have previously said on this matter,

    2. FibreFred says:

      You cannot send Fibre over copper, does that even make sense? 🙂

    3. DanielM says:


      BT appears to think it’s possible. but most of these idiots have never seen a fibre optic cable ever

  11. Carl says:

    The next Ofcom reports on speed and latency times are going to be interesting. I wonder how much worse the latency for FTTC will be.

  12. Steve Jones says:

    On the bright side, by (HH5) is now running at just about its fastest ever rate, albeit only modestly (downstream) at 60mbps (sync) and 56mbps throughput on a 650m line (20.9db) attenuation and, perhaps more significantly, has now been up for 6 days without a resync. Before that once every day or two was normal.

    Latency is roughly the same (16ms to http://www.bbc.com from Maidenhead, Berks).

  13. Eccles says:

    G.inp was enabled on my line last week. Running a Fritzbox 7390. Previously had interleaving on and a few errors. Latency was around 25ms. Now with g.inp I’m seeing no errors and a slight speed increase. Latency has decreased to around 15ms. Line running at 55Mbps and about 600m to cab.

  14. Tim says:

    I’m on Sky Broadband with their black router. How can I tell if G.INP has been enabled?

    1. DanielM says:

      you cannot. you would need a different router/modem

  15. MikeW says:

    Note that a line running with G.INP retransmission (even with the right firmware) can also still include some level of FEC and interleaving configured, and likely does. However, the FEC overhead is much reduced compared to the old setup (5% vs 20%), and the additional latency is much reduced (0.2ms vs 8ms or 16ms).

    If interleaving depth is less than 20 (compared to values around 1,000), then you probably have the right firmware.

  16. SSUK says:

    Another here with reduced speeds and increased latency using a Homehub 5, do i need a openreach modem instead? Once G.INP is applied are you stuck with it?

  17. Kyle says:

    Glad i do not have these issues.

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