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Study Tests the Theoretical Limits of Copper Line Broadband

Thursday, Apr 28th, 2022 (8:28 am) - Score 5,920

New research from the University of Cambridge – supported by BT and Huawei – has attempted to identify the theoretical limits of Openreach’s standard UK copper broadband lines (unshielded twisted copper pairs), while harnessing up to 12GHz of spectrum. The result is faster speeds are possible, but full fibre is still the future.

At present many UK consumers still get their broadband using Digital Subscriber Line technologies like ADSL2+ (up to 20-24Mbps) and VDSL2 / FTTC (up to 80Mbps), which send electrical signals down a tiny bit of twisted pair (TP) copper wire in order to deliver an internet connection into the home. A few people have also adopted the similar G.fast technology (up to 330Mbps).

One important bit of context here is that all of these use different amounts of spectrum in order to carry their data signals. For example, in the UK ADSL2+ utilises up to 2.2MHz, while VDSL2 uses up to 17MHz and G.fast goes up to 106MHz. More modern experimental technologies, such as MGFast, can even harness up to 848MHz for targets data rates up to 10Gbps.


At this point we could also touch on the theoretical Terabit DSL (TDSL) technology, but that adopts the radically different approach of using an existing copper wire as a mere “guide” (waveguide) to help direct a wireless broadband signal in the 100GHz+ millimeter Wave (mmW) band. We’ll leave that out of this one as it would confuse matters.

The big problem with these copper line technologies – particularly the fastest ones – is that the more spectrum frequency you harness, the greater the challenge of mitigating interference over distance (signal degradation / errors etc.). In the real-world, we experience this as a loss of speed or line stability.

For example, an old ADSL2+ line can theoretically deliver speeds of c.20Mbps up to distances of around 1.5km from the exchange, but at 5km+ you’ll be down to 1-2Mbps territory. Similarly, G.fast delivers its best speeds for those within c.200-300 metres of a PCP Street Cabinet, but performance rapidly tails off.

You could, in theory, get 1Gbps+ speeds from G.fast at 212MHz (not deployed in the UK), but you’d need masses of small nodes – all within a few tens of metres of each house – and that’s a very costly and complex build, with issues for long-term maintenance.


The New Copper Limits Study

The new study, which if you’re comfortable with the physics makes for a fascinating read, is designed with a view to improving “achievable data rates over the copper infrastructure so that the bottlenecks over the last mile can be successfully avoided while satisfying the future data demands.”

Study Abstract

This paper explores the behaviour of the ubiquitous twisted pairs at high frequencies and wideband excitation of twisted pairs up to 12 GHz. Higher carrier frequencies on twisted pairs can enable the data rates required by the future communication networks; hence, the existing copper infrastructure can be utilised on the last mile complementing the fibre networks.

In this paper, we show a fundamental limit on the operating frequency of twisted pairs beyond which twisted pairs start to radiate and behave like an antenna. To validate our theoretical derivations through measurements, we designed a microstrip balun to excite the differential mode on the twisted pairs.

At the end, we demonstrate that the standard twisted pairs used in the UK can be used up to 5 GHz carrier frequency without any radiation effect and this upper-bound can be moved to higher frequencies by decreasing the twist lengths.

In conclusion, the paper suggests that its results and design guidelines will help scientists and engineers to better understand the wave propagation on TPs and “enable them to design wideband communication systems operating at higher carrier frequencies.” It’s a useful bit of knowledge to have, albeit not one that will change the current direction of travel, just as G.fast and MGfast failed to do before. Signal degradation at 5GHz would be.. fun.

NOTE: The research suggests that performance could also be improved by “decreasing the twist lengths” on copper lines, which may be true, but that has no benefit to existing cables in the ground.

By now most incumbents in the world, including Openreach, have realised – through a mix of political, technological and competitive pressures – that it makes more sense to spend big on deploying full fibre (FTTP) broadband than it does to continue trying to squeeze extra speed out of ancient copper lines, with all the caveats that invariably entails. Any new copper technology would also take years to develop.

Meanwhile, optical fibres can already handle multi-Terabit speeds and without the hassles of copper, but it’ll be years before such speeds become viable to homes – yet one day, they will. Lest we forget that Openreach also have no desire to run the old copper network side-by-side with Fibre-to-the-Premises (FTTP), thus they’re already working toward its eventual withdrawal.


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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 X (Twitter), Mastodon, Facebook and .
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34 Responses
  1. Avatar photo Ray Woodward says:

    Why (if it’s being scrapped)?

    1. Mark-Jackson Mark Jackson says:

      Curiosity perhaps, but the findings may also have relevance in other fields too. But like you, I’m not 100% sure how much practical worth will be gleamed from this.

    2. Avatar photo Scott says:

      The why is simple – By sponsoring research they may find a recommendation that can be applied to best practice before copper goes fully EOL. It’s going to be around for a while so any reliability improvement is worth it.

  2. Avatar photo Adam says:

    I wonder if BT will rip out all the old copper infrastructure once FFTP is widespread and then scrap this metal to earn income?

    I still don’t fully understand how they are deploying full-fibre; part of me thinks it’s deployed from the FTTC street cabinets to save build costs.

    1. Avatar photo NE555 says:

      > I wonder if BT will rip out all the old copper infrastructure once FFTP is widespread and then scrap this metal to earn income?

      Overhead cables may be reclaimed. FTTC cabinets will be unpowered and hopefully removed rather than being left to rust.

      However, pulling old thick copper cables from ducts risks damaging the fibre cables running alongside them – although the risk is reduced if they are in subducts.

      > I still don’t fully understand how they are deploying full-fibre; part of me thinks it’s deployed from the FTTC street cabinets to save build costs.

      The FTTC network goes:

      headend exchange -> fibre aggregation node -> cabinet -> property

      The FTTP network goes:

      headend exchange -> fibre aggregation node -> splitter -> CBT -> property

      So the answer is no, the FTTC cabinets are not involved in the FTTP network. The splitters and CBTs are normally either underground or on poles, occasionally wall mounted.

    2. Avatar photo Peach says:

      There are some spares that could be used from existing spine fibres but sometimes a new spine cable is required from the parent exchange due to the volume of fibres required

    3. Avatar photo Jason says:

      What are the cabinets that cityfibre uses good for then?

    4. Avatar photo Mel says:

      It might make sense for BT to remove them eventually, to remove the temptation for anyone else to rip them out for their metal value, who wouldn’t care about damaging fibre cables and the ducts.

    5. Avatar photo PedroPony says:

      NE555 – The removal of underground copper cabling is already well underway (and has been done already in the past in regard to the MUCJ core network) with specific teams to do the recovery work. The overhead cabling is, in comparison, worth very little to the large underground cables. Removing these not only generates revenue from the sale but also frees up duct space for future use rather than putting in new ones.

  3. Avatar photo Dassa says:

    Surely the people who develop the copper Ethernet standards have already done this as the size of the data transmission problem can be broadly quantified as speed x distance.

    Whilst they don’t have the same incentive to achieve transmission distance, they’re keen to deliver much higher speeds and Cat 6 cable is only good for 30m at 10Gbits/sec – I’m sure they would have preferred that number to be 100m if it could be.

    Whilst the DSL technologies eventually exceed the capability of Ethernet over a given channel as the amount of processing deemed tolerable in a DSL chipset is several orders of magnitude greater than that tolerable in an Ethernet chipset, the progress (or lack of progress) of Ethernet is a good indication of which is practical.

    Not that I’m saying that 5Gbits/sec over some DSL variant isn’t possible, I just don’t think it will be practical (due to cost / power / etc. constraints).

    1. Avatar photo NE555 says:

      Cat6A can do 10G at 100m.

      However, telephone cable is (a) much lower quality, and (b) only a single pair, not 4 pairs.

    2. Avatar photo Rupert Walker says:

      Only in ideal conditions without interferance from other factors. As normal everything is easy in the lab but once out in the wild things don’t work quite so well.

    3. Avatar photo An Engineer says:

      The whole point of 6A is that it’s more resistant to cross-talk and interference to allow it to carry 10G up to the 100m.

      If in a really EM-noisy environment should be using shielded cabling.

    4. Avatar photo Rupert Walker says:

      I agree “is more resistant” but not immune, I would like to see anybody who has a 100m run in a normal building with lights and 3phase around that can get full speed.

  4. Avatar photo Anthony Goodman says:

    It seems to me, instead of wasting time and money on this Openreach should go full in with Fibre down the water pipes and partner up with the waterboards to deploy it. That seemed perfect allround solution given it means they renew the old leaking water pipes at the same time, have an ability to detect future leaks exactly at point of origin and contaminants/water quality from that same cable.

    1. Avatar photo Alex says:

      This is utter garbage. Really you should think before you write such trash. The old water pipes idea comes up year after year but nobody has ever proven it to be a game changer for large scale broadband delivery. It makes nice PR for whoever’s punting it, but meanwhile Openreach is building FTTP to 1m homes every 3 months using it’s own network so there’s really no need for them to do it another way using a third party network. Also, if you actually read the report you’ll see that Huawei and BT funded the research – not Openreach – so they’re not actually wasting any money or time on that anyway.

    2. Avatar photo Anthony Goodman says:

      Did you miss the part this is about rural houses. Not Urban. I think you did.

    3. Avatar photo GNewton says:

      @Alex: “you’ll see that Huawei and BT funded the research – not Openreach – so they’re not actually wasting any money or time on that anyway.”

      Openreach is a company wholly owned by BT plc, and the network assets would still be owned by BT plc.

    4. Avatar photo Alex says:

      @anthony no, i didn’t miss that. It’s still not proven to be viable, even in rural settings and Openreach is building a huge amount in rural anyway without using water pipes.

      @GNewton this isn’t about ownership or assets, it’s about the time and money spent on research. Openreach didn’t spend any time or money on the research, as Anthony had wrongly assumed.

    5. Avatar photo GNewton says:

      @Alex: Since BT owns Openreach, one can argue that it’s still a waste of money, which would have been better spent with Openreach, to invest in fibre.

    6. Avatar photo Alex says:

      @GNewton – one can certainly argue that! 🙂 Just as one can argue that if you’re a company that owns a massive quantity of a particular asset, it probably makes sense to spend a relatively tiny amount of money on academic research which could prove that asset to be more valuable than the existing consensus.

  5. Avatar photo John H says:

    When you are on the end of a failing copper wire that the OR engineers cannot fix this technology will be even more useless than ADSL. The local network in our area is awash with engineers trying to keep up with the faults. Fibre is the only way to go.

  6. Avatar photo Phil says:

    FTTC isn’t that bad 40/10, 55/10 and 80/20 if there is no FTTP.

    1. Avatar photo Dave says:

      It is when the FTTC connect you have is not even close to what you’ve stated. 10/1 for me due to distance.

    2. Avatar photo Anthony Goodman says:

      How many get those speeds. I live 400m from my cabinet and I only got 26/6. Now I have Cityfibre and get 560/560. I could never go back.

    3. Avatar photo Phil says:

      It’s ok if you got FTTP (lucky for some) as most of us don’t have FTTP.

      If there is FTTP available I will order FTTP 80/20 myself.

  7. Avatar photo Billy says:

    I suspect there’s more to be gained by digging up all the copper and melting it down and selling it. After all, wasn’t that MCI’s idea in the states. I just hope that BT/OR don’t get wind of it and then we end up with GFast 2.0

  8. Avatar photo Chris Jones says:

    A useful study. However, a significant proportion of the “copper” is not actually copper, but aluminium or steel, first installed after the Rhodesia embargoes in the 1970’s. Neither of these alternatives (as I know to my cost) is anywhere near as good as copper. I am only 1.5Km from my FTTC cabinet and in theory should get 15Mbps, yet, because part of my line to the cabinet is actually aluminium, the best I can get is 10Mbps.

    I am pushing for FTTP to be built throughout my community as there are people living further from the FTTC cabinet who cannot get FTTC at all.

  9. Avatar photo Waldo says:

    If copper can deliver Gigabit. Why everybody used turtle speeds for so many years here. Open your eyes.

  10. Avatar photo William Grimsley says:

    Pushing faster and faster speeds down copper is a mistake, the higher the frequencies, the more likely interference occurs, which causes all sorts of problems! FTTP is the future not copper!

  11. Avatar photo Martin says:

    Talk about flogging a dead horse …..

    1. Avatar photo FTTP4ALL says:

      Agreed, exactly..

  12. Avatar photo Mick says:

    Research like this could be valuable in the future if you live in a tower block where the landlord won’t accept/can’t fit fibre into the risers, you can drive the existing copper faster from a fibre connected cabinet just outside the building/in the basement. Openreach is rightly focussed on FTTP rollout, but when the main rush is over, there is always the last 1% that needs clever thinking to resolve.

    1. Avatar photo BenH says:

      Completely agree Mick, I think this is looking for cost effective solutions to patch areas within FTTP rollout, that cannot be easily connected. I live in one of the market towns which is on the Openreach FTTP exchange upgrade list. There are several areas served by the exchange that are a couple of miles away and only consisting of a few properties. Since the exchange will require government funding to be upgraded, I think these outlying areas will not receive the full FTTP upgrade but a patchwork of solutions to boost speed, but not go to the expense of laying new fibre cables.

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