back to article Broadband powered by home gateways? Whose bright idea was THIS?

Fibre-to-the-node can help squeeze the last drop of sweat out of copper telephony networks, but it has a problem: nodes need electrons, and there might not be a copper path upstream to the exchange for 48V power. So the standards body The Broadband Forum thinks powering nodes using household electricity is a good idea. The …

  1. Griffo

    FttDP

    I thought powering the node in FttDP from the users premise actually made sense - as each user in essence had their own NTU in the pit which then connected via GPON back to the network - so each user was only powering their own device, and when they chose or were able to upgrade to FTTP it effectively became GPON to their premise. But maybe I've misunderstood FttDP.

    Using Customer power to reverse feed a cabinet in FttN is just outrageous though - it just highlights one of the fundamentally stupid aspects of an FttN rollout.

    1. Doctor Syntax Silver badge

      Re: FttDP

      I had the same problem with the terminology.

      If distribution point is equivalent to cabinet, as in FTTC, then the amount of ventilation in the existing cabinets indicates a good deal more power is required than could reasonably be sucked down a few hundred yards of telephone copper and on the whole cabinets do seem to be located where power is readily available.

      On that basis I assume they're thinking of something else. There's a BT manhole in the road opposite the end of my garden. My own phone line comes underground from that and the adjacent pole feeds about half a dozen neighbouring houses by overhead wires. Maybe that's what they mean by distribution point.

      If the idea is to pull a single fibre to that they'll be pulling it through a duct which already contains a multi-pair cable which will be being made redundant so they could take as many pairs as necessary from that to feed power. Alternatively the fibre itself can be bundled with power lines.

      And if they plan to run a bundle of fibres to that distribution point, one for each house then powering each house's box individually then power down the existing copper is probably feasible.

      1. Martin-73 Silver badge

        Re: FttDP

        The distribution point is the point the cable from the cabinet terminates and is hooked to the (usually 1-2 pair) wire feeding your building. So in your case it's probably in the joint box you mentioned. If you look at the telephone pole, you'll see a junction box at the top. That's the DP for the overhead feeds (most poles will also be labelled with the DP number, for example "DP 358")

        Seeing as they're running fibre from the exchange, I can't see why they can't also run the 48V feed down the duct from the already existing batteries, to supply power to the DP nodes.

    2. Wayland Sothcott 1 Bronze badge

      Re: FttDP

      If you're powering your own service and you're the only one who loses out if you switch the power off then no big deal. However if you are the only one powering a shared hub then that's a guaranteed nightmare. You could turn your power off when you go on holiday and upset other people in your street.

  2. Ole Juul

    Deal?

    . . . if the nodes only need a few hundred watts, who's going to care?

    That sounds like about a dollar a day to me, so just take $30 per month of my bill. Deal?

  3. DougS Silver badge

    Are they talking about ONE home powering the node?

    Why not have them all power it? With a simple diode inline on the copper to prevent power backfeeding each house will provide a roughly equal share of power, and if some of them lose power the remainder will pick up the load without interruption. If you are worried about the cost of delivering 2 or 3 watts, you probably can't afford broadband anyway.

    1. Vic

      Re: Are they talking about ONE home powering the node?

      With a simple diode inline on the copper to prevent power backfeeding each house will provide a roughly equal share of power

      With simple diodes inline, the node will always be powered by exactly one house. You hope that statistical variation in the output of each will lead to that averaging out over time - but in practice, it probably won't. Tolerances in the PSUs and in the cable resistance will mean that it's always the same house doing the power...

      Vic.

      1. Cynic_999 Silver badge

        Re: Are they talking about ONE home powering the node?

        "

        With simple diodes inline, the node will always be powered by exactly one house.

        "

        No, that is incorrect. Assuming that each supply can be represented by a resistor (the copper wire) in series with the diode, then the current drawn from each supply will automagically adjust such that the source voltage minus the voltage drop across resistance and diode are equal for every supply. The higher the current drawn, the greater the voltage drop across the resistor. Which means that the share of total current from each household will be inversely proportional to the series resistance and directly proportional to the source voltage.

        By deliberately putting in a series resistor that is, say, 10 times greater than the average copper line, the current from each household would be near as dammit equalised.

        1. Vic

          Re: Are they talking about ONE home powering the node?

          By deliberately putting in a series resistor that is, say, 10 times greater than the average copper line, the current from each household would be near as dammit equalised.

          How much power does the unit draw? How much is lost in your resistors?

          Vic.

        2. DougS Silver badge

          Re: Are they talking about ONE home powering the node?

          Assuming that each supply can be represented by a resistor (the copper wire) in series with the diode, then the current drawn from each supply will automagically adjust such that the source voltage minus the voltage drop across resistance and diode are equal for every supply.

          Yep, this is exactly what I do with some satellite and cable gear I have for my business - amplifier, switches, etc. that require about 70 watts of DC power on coax inputs. I had three 24v 2A power supplies that output on coax, so for redundancy I used a 3 way diode steering satellite splitter with the three PSUs feeding into the splitter's "outputs" (the diodes inside a diode steering satellite splitter allow power to flow into the splitter outputs but not the other way) and the splitter's input connected to an 8 way splitter that feeds the power to the various gear.

          Some people were skeptical like the previous poster and told me this "couldn't work" because one PSU would try to power everything and the fuse would blow, but I measured the AC power in watts going into each of the three power supplies using a Kill-a-Watt and they were pretty much identical. If I unplug one, nothing loses power, so I'm protected against one of the power supplies dying.

      2. dotty

        Re: Are they talking about ONE home powering the node?

        if you want simplicity to run a distributed supply for these units its not difficult if you say have 20 subscribers back feed over the copper as a phantom 48V Open circuit at 500ma constant current supply which would give you 24Watts per cabinet feed or 500 watts

        if there is excess it will reduce the current demand on the share depending on the line resistance the maximum demand would be 24 watts could be less

    2. TeeCee Gold badge
      WTF?

      Re: Are they talking about ONE home powering the node?

      Why not have them all power it?

      Hmm, three-phase power with differing phase imbalances depending on site and subscriptions. That'll be fun to build a PSU for.

  4. Voland's right hand Silver badge

    You should see who attends Broadband Forum

    You should see some of the characters attending its junkets. Then, and only then, you will understand why it keeps coming up with such ingenious ideas.

  5. Anonymous Coward
    Anonymous Coward

    The 48V is needed in case of emergency

    and anything between the CPE and the exchange shouldn't be on local utility power.

    1. DougS Silver badge

      Re: The 48V is needed in case of emergency

      That's only true for local loop phone service. For internet it doesn't matter - when you lose power to your house, you can't use internet anyway.

      If everyone's house supplied power to the node, then if a dozen people had their computer along with their CPE on a UPS they would power the node so they could continue to use their computer even when the power went out. But I doubt they care enough to bother implementing this - I've been on VDSL2 for almost a decade, and while I rarely lose power, when I do my internet usually goes even though I've got my DSL modem, switch, wireless router and computer on a UPS. The node is a little over half a km away from me, so most power outages that affect me would affect it as well.

      1. Mike007

        Re: The 48V is needed in case of emergency

        You begin by commenting "when you lose power to your house, you can't use internet anyway", then admit you have your gear on a UPS and therefore you yourself are an example of where this is not the case.

        I have my modem and one of my wireless access points on UPS power and my main computers and home servers are all laptops with internal batteries. I haven't actually had a power cut since getting VDSL so have no idea what would happen to my internet connection in an outage, but in theory if BT are doing their job properly and putting batteries in the cabinets I should stay up. My main systems and hosting infrastructure is tested to remain fully functional for at least 4 hours, and things will degrade to 1 access point with emergency internet access for the next 48 hours - although I doubt they have 48 hours worth of battery in the cabinet.

        1. Martin an gof Silver badge

          Re: The 48V is needed in case of emergency

          Late to this, I know, but I was talking to an OpenReach bloke fiddling inside a VDSL cabinet near my mother's house the other day and he pointed out that the VDSL kit had its own UPS. The battery in this is designed for (something like) 48 hours and is hot-swappable so unless the power outage is very widespread there is a plan in place to keep swapping batteries until the mains returns.

          M.

  6. chris 17 Bronze badge

    Power off modem for 30 seconds

    So when support tell 1 customer to power off their kit for 30 seconds, everyone goes offline too.

    When Eco warier down the road decides he's had enough net, everyone in that node goes dark too

    Not a good way to run a service.

  7. Anonymous Coward
    Headmaster

    Being picky....

    ...but BT is 50v and ramps up when ringing.

    1. Alan Brown Silver badge

      Re: Being picky....

      Being pickier it's 48V (plus a cell on top) and ringing is AC superimposed on the DC.

      48V being the breakpoint between where electrical worker certification is needed or not needed for work on power circuits.

      1. Martin-73 Silver badge

        Re: Being picky....

        It's 50 volts. The tolerances are 49.5 to 50.5 volts

  8. ottsky88
    Megaphone

    RTFM

    If you actually do a little reading, power draw is proportional such that each user only supplies the power required to service their own requirements. So it's scalable - as more users are added, more power is made available. Users don't power other users' services.

    In the event of a power failure, one user with a UPS can provide enough power to continue their own service. So the telco doesn't have to arrange connections to the power grid or back-up batteries, saving money.

    I've looked at mutiple g.fast manufacturers and most seem to offer this feature.

  9. Anonymous Coward
    Anonymous Coward

    Great idea

    We can measure the power each home contributes using its Smart Meter. Genius!

    1. RedCardinal

      Re: Great idea

      That'll be the smart meter I don't have in my house? :P

  10. Bob H

    It seems strange as power is fairly ubiquitous in our streets, where can't you find street power? If you are delivering to homes then those homes aren't usually off-grid and if you are following roads and putting the cabinet or pole next to a road then there is a near 100% chance of finding power. In rural places it might be more of an issue but in those situations the line length might make it problematic again. The only logic I can see to this is the provider wanting to avoid paying for the power to each cabinet rather than the difficulties of getting electricity in the first place.

    1. Anonymous Coward
      Anonymous Coward

      Last mile

      There was a company called Last Mile that was going to put WiFi repeaters in lamp posts - what happened to them?

    2. Tim Jenkins

      I caught two BT-sub-sub-sub-contractors pulling Superfast Cymru-funded multicore fibre up our road one evening in August, and followed them to a new in-pavement hole that recently appeared (complete with BT-branded concrete slab on top). As there are only 20 houses here, I'm assuming we're supposed to be getting FTTRN rather that FTTC, which does leave the question of where they are going to pull power from; it's certainly possible that they'd look at the adjacent streetlight, where there's 'a near 100% chance of finding power' right up until the moment when Powys forgets the node is there and decides to cut off streetlighting to save budget (again)

      1. Martin-73 Silver badge

        They do the cutting off of the streetlights by RF controlled thingies in the socket that USED to be used for a photocell, at least here in Sarf-ampton. You can tell because the entire street comes on at *precisely* the same moment, not 'within a few seconds', but precisely, so the feed to the streetlight itself is permanently live

        1. HieronymusBloggs

          "You can tell because the entire street comes on at *precisely* the same moment, not 'within a few seconds', but precisely, so the feed to the streetlight itself is permanently live"

          Since electrons in a wire travel at a large fraction of the speed of light, and therefore nearly as fast as radio waves through air, I don't think you'd be able to see the difference.

          1. HieronymusBloggs

            Correction

            I'll correct myself there. The electromagnetic wave which carries the power along the wire travels at a large fraction of the speed of light, not the electrons themselves (which drift relatively slowly).

            1. Proton Wrangler

              Re: Correction

              Pedantic very often Mr. Bloggs? :-) Anyway the poster you're responding to was giving the simultaneity of coming on as evidence for RF control and not photocell control.

  11. Anonymous Coward
    Anonymous Coward

    Electricity too cheap to meter

    More power consumed in summer to help it keep cool in Australia.

    Other than that, the power draw is fairly constant. 384 lines per node cabinet (ex last year's blurb). Single-digit watts per line. Likely less than $1/month in electricity cost per line. (Telstra gets $9/month for an ULL.)

    Getting power from the numerous end-points to operate the node is not as silly an idea as grid-connected PV solar.

    Power protection is simple and the power domain easily restricted to individual lines at a primary level, to the boards for the multiple lines at a secondary level and finally to bleed off a little from each of the hundreds for any central functions inside the node cabinet. The bulk of the necessary primary protection circuit is already required to handle surges due to e.g. lightning strikes and accidental connection to the mains grid. The failure of one customer's modem would have no significant effect on operations. The vast majority would have to fail affect the lines of other consumers.

    The technical impediment is that the core design of the circuits inside the cabinet is for power originating from one source being divided to service many lines.

    The loss of utility is the same as with FTTP; no power from the subscriber end and there's not even a basic phone servce; unless the node has a UPS and can drive the lines in POTS-fallback mode. That requires more complex cicuitry.

  12. ChrisLaarman

    The concept reminds me of reality in the Netherlands (where I live).

    My set-top box (cable modem, on copper but not a telephone line) features a double access point. One for my private network, one to be used as a wifi hotspot by fellow customers of Ziggo.

    As long as I leave it enabled, I'm allowed access to their hotspots.

    This public part won't interfere with my bandwidth, but it does add (a tiny amount) to my electricity bill.

    So far, I have enjoyed the benefit and not felt a burden.

  13. Anonymous Coward
    Anonymous Coward

    The Tragedy of the Commons

    A great idea until people figure out how to pirate their electricity from BitTorrent (well, I figured, as long as we're all talking clueless ... )

  14. SImon Hobson Silver badge

    It's not so daft really, but does need "considerable care" in implementation. Taking several things (some of them mentioned above, some not), in no particular order ...

    1) In this case, "DP" means the distribution point close to the subscriber - the joint box up the pole or in the manhole close to your home. Not the Primary Distribution Point where the FTTC cabs are located.

    2) There are already lots of situation where there is something using main power (utility power to our US cousins) between the exchange and the subscriber. Typically these devices run off a nominal 48V and have their own batteries. Maintenance of the batteries may be an issue - I decommissioned a rack (abandoned by it's telecoms company owner) of equipment at work a while ago, lets say the batteries in the bottom were "utterly and completely shagged".

    3) Getting power from the cable in the street is technically very easy, but is also costly. There's a whole section in the FTTC cabinets for handling the power - input termination, isolation, overload protection, and only after all that does it get to the power supplies. Adding this lot to a FTTDP node would add considerably to both it's bulk and cost. There's also the cost of digging up the street and tapping into the power cable - this isn't a case of tipping a guy with a shovel a couple of tenners ! Interestingly, round my way I've observed FTTC cabinets where they've had to trench a lot further than I'd have expected to get to power.

    4) Getting power from downstream isn't itself a daft idea. Most of us are already used to powering ethernet devices through the cabling, and we are pretty well all used to having our phones (a basic one at least) powered from the BT exchange. I see issues (as mentioned above) related to interaction and what if there aren't enough subscribers providing power in order to run the node.

    For part of the node, there's not really much problem powering it off if that subscriber isn't powering it. But there will be a minimum core (upstream interface, management systems) that needs to be powered regardless of how many (or few) subscribers are powering it.

    So there is a minimal power feed requirement, without which you can't run the node supervisory/control functions or communicate upstream.

    5) Doing that powering safely is far from trivial. In theory each supply will have isolation at the supply end, but as this is all stuff under the subscribers' control then it cannot be assumed to be perfect. So the node must accept power from multiple lines, any of which could (through fault, incompetence, or malicious intent) be anything but a "safe" isolated extra-low voltage.

    That aspect aside, a relatively simple switch mode PSU on each subscriber interface could provide isolation from the interface to the internal power supplies - while the internal management system takes care of only powering up the subscriber xDSL interface where there is a power feed present.

    Some reasonable size capacitors could well be needed to handle some transient situations - two such situation that come to mind :

    If there are two (worst case) powering subscribers and one stops providing power, then there will be a delay between loss of that subscriber's power and turning off the circuitry it power.

    If all subscriber stop providing power (street wide power cut, or only one present and that gets disconnected) then ideally the node needs time to communicate upstream that's it's lost power and will be shutting down. Otherwise the management systems cannot differentiate between a node that's gone offline due to loss of power, or one that's lost it's connection (eg broken fibre) - and that's an important difference to know when it comes to fault management.

    6) While it's still not FTTP, I can see why they'd want to do this. That last few yards from the bole/hole to the house is the most difficult and expensive bit. Often it's direct buried cable - so replacement mean digging up the road and subscribers drive or lawn.

    Even where it's overhead, it's not automatically easy to replace - for example, a colleague had been told that his overhead cable doesn't meet current standards for height above the road. As long as it stays as it is then they can leave it, but the moment they do any work on it (such as replacing a failed cable) then they need to do it to current standards which means going to the chimney rather than soffit (it's a bungalow, and long span) with all the "work at height" issues that creates. Or they have to add another pole in the street to make a better span.

    1. DougS Silver badge

      So the node must accept power from multiple lines, any of which could (through fault, incompetence, or malicious intent) be anything but a "safe" isolated extra-low voltage.

      It needs to do this even if the CO was providing nice 48v telco power over copper to the node. If you assume that fault or malicious intent need to be protected against, you have to worry about 240 volts coming up the line from a subscriber's house. Heck, if you want to be extra careful you need to worry about 7200 volts coming up the line (if a tree falls breaks the overhead power and phone lines, and crosses the exposed copper) though it probably would be rare enough you can just eat the cost and replace the whole thing if this happens. Protecting against 240v AC on a line intended to have 48v DC is easier than protecting against both 240v AC and 7200v DC...

  15. Anonymous Coward
    Anonymous Coward

    Must have worked with Comcast?

    Comcast Cable in the U.S. is fond of placing unsecured hotspots in consumer's homes without telling them they are responsible for whomever uses the unsecured hotspot even if it is someone outside of their home, i.e. hackers, neighbors, drive-by's, etc.

    1. Cynic_999 Silver badge

      Re: Must have worked with Comcast?

      "

      Comcast Cable in the U.S. is fond of placing unsecured hotspots in consumer's homes without telling them they are responsible for whomever uses the unsecured hotspot even if it is someone outside of their home, i.e. hackers, neighbors, drive-by's, etc.

      "

      I do not believe you are correct.

      I am not aware that the owner / subscriber of *any* Internet router or hotspot becomes liable for all content transmitted through it. If you cannot reasonably control access or content, then you cannot be held legally responsible.

      1. Will Godfrey Silver badge
        Unhappy

        Re: Must have worked with Comcast?

        Nice theory, but in today's world of guilty unless categorically proved innocent, how do you defend against claims you were dealing in money laundering, terrorist activities and child pornography?

        Oh, and how much will it cost you both financially and in terms of permanent reputation damage?

  16. Terje

    This is the kind of scenario that just makes me wonder who will pay when something goes wrong at one customer and that fries equipment for other users. That scenario will likely be a lot more likely if there is circuitry in place that is actually actively feeding power down the line.

  17. Commswonk Silver badge
    Thumb Down

    Eh?

    Ok I'm retired so such knowledge that I acquired whilst gainfully employed may be rather out of date, but not by _that_ much.

    As things stand my 'phone line has 50 volts DC (nominal) on it when my 'phone is On Hook. The exchange knows when I pick the handset up by the loop current that flows as a result, reducing the PD at my instrument. If I have to feed some voltage _back_ the local BB equipment how is the exchange going to know that my 'phone is On or Off Hook?

    Also as things stand one thing that will _still_ work if the power goes off - rare, yes, but not wholly unknown - is my fixed line telephone; that is why purchasers of DECT 'phones are advised to have an ordinary instrument available "just in case". If instead of my 'phone supplier (OK I'll own up to it being BT) feeding a supply to me I have to feed a supply back to _it_ how is my phone going to work if the power goes off? In fact I cannot see how my 'phone is going to work at all unless the entire system is redesigned.

    And if power is being fed by "everybody" aren't all the lines going to be more or less in parallel, give or take the odd diode, or is there going to be a DC - DC converter on each and every line to provide isolation?

    Grumble grumble... </Meldrew Mode)

  18. raytaylor

    Use the spare copper from the exchange

    12 pairs of 24 AWG copper wire can carry 80 watts over 5kms if each pair carried 96 volts at 100milliamps.

    So if they are running fibre to a roadside node, the now unused copper going back to the local telephone exchange can deliver the power for the remote DSLAM

    That is enough to easily run a mini-dslam and a few repeaters along the way. Using DSL over each of those pairs, and just a single repeater, allows them to bring a shared 120mbits down / 10mbit up. At the remote cabinet, the mini DSLAM can share that amongst 12 or more subscriber lines. If VDSL is used on the remote subscriber lines then they can be short and super fast, or if ADSL2 is used, they can be long and reach further out into rural areas another 3+ kms.

    If two repeaters are used along the path, the shared capacity rises to 180mbits which is more suited to shorter lines at the far end so they can take advantage of that backhaul capacity.

    1. Steven Jones

      Re: Use the spare copper from the exchange

      If you pump 100ma DC down a 5km loop (960 ohms loop resistance), that's a 96v drop. It might draw 100ma at 96V, but the power delivered to the load will be zero (as it will be delivered at 0 voltage drop). I suppose you might consider using earth return, but not by using DC. If you tried that, it would cause enormous problems with electrolytic reaction at the earthing rod. Try it with AC and it will induce horrible signals into adjacent loops.

      I'm afraid the maximum power you can deliver at 96V over a single 5K loop is 2.4 watts at 50ma (48V drop over the feed, 48V at the load). Those twelve pairs will deliver just 28.8W and draw 57.6W.

      Of course there's not a snowball's chance in hell that anybody is going to agree to put 96V DC down a phone line pair mixed up with all those 48V lines. A voltage of 96V DC is plenty enough to give a nasty shock, especially when stuck down a wet hole.

      Those 12 pairs at 5K will therefore deliver just 0.6W each, or 7.2W in total.

  19. Walter Willcox

    Five points on Broadband architecture and power

    1. I recommend reading Neil Fairbrother's article entitled "gfast-a-high-speed-cul-de-sac"

    2. I replicate Dr Peter Cochrane's (Former BT CTO) comment on that article:-

    All this was so obvious way back in 1986....but 'real engineering and economics' has been driven out of the telecoms industry. You can't beat physics (loss and crosstalk) and you can't stop Moore's Law! Mini-DSlams are an insane option! To get network reliability and resilience you have to take out electronics not put more in! To get a 'Green Network' you have to reduce the amount of material used and energy consumed! And Mbit/s are not enough for an obvious future rushing towards us. We have to start talking Gbit/s. But if you want sub-optimal industries and a population who just sit and watch sport on TV....just keep installing copper!

    With a copper network you need over 6000 switch site in the UK. If you install optical fibre this number drops below 70. 20,000 man in van crews goes down to 1,000, and all water ingress related faults just go away. Now redo the economic argument. Go figure!

    Peter Cochrane

    3. I witnessed the commissioning of a FTTC drawing 50 Amps at 48 Volts = 2.4 Kw EVERY time the power is restored, but less when running normally. Let's guess normally 800 W * 50,000 FTTCs = 40 Mw national load. Who do you think eventually pays for this power ?

    4. I think National Grid have stated there is ONE % spare capacity during zero wind winter conditions. Does anybody remember e.g. the Didcot fire or the imminent shut-down of Ferrybridge etc. etc. ?

    5. The nation has to wake up to the fact that a fully future-proofed broadband infrastructure is absolutely vital for the UK. Whereas e.g. Sport content seems a far lesser requirement. The only ones providing secure systems are a few smaller operators such as Gigaclear , Hyperoptic and the stalwart B4RN DIY folk providing 1 Gbps symmetric over REAL Fibre-To-The-Premises. ALL these systems ONLY require power at a far smaller number of distribution cabinets. They can also provide a reliable telephone service without using the worn-out unreliable aluminium alloy and copper twisted pair PSTN, nor do they demand such a connection is paid for.

    1. dotty

      Re: Five points on Broadband architecture and power

      security of supply is a major issue the loss of nuclear is critical at present if you are interested here is the data http://www.gridwatch.templar.co.uk/ the france page is even more interesting as they don't have power shortage issues being self sufficient in nuclear

  20. PassingStrange

    I seem to remember some of the early UK hackers being convicted of "stealing electricity" (because there was no other applicable law at the time - and never mind that the amount of electricity involved was trivial). It seems to me that anyone who attempts to do this without my approval is doing precisely that - just more overtly.

  21. Steven Jones

    Is it possible somebody technically competent could be used to analyse this rather than just write a scare story? Don't you think those looking at the design of what is called reverse power systems have actually thought of some of these issues?

    Any RP system will not be dependent on any single line. It will scavenge power off of several lines and not single line (or even several lines) will cause loss of service. Here's a posting I made from another forum based on calculations of the properties of phone lines.

    *********************************************************************************

    To put all this in perspective, it’s worth doing some calculations to see what level of power can be delivered this way and over what distance. The first thing to note is that the telephone system is (relatively) low voltage and works at a nominal 50V DC. The second thing to note is the resistance of the relatively light gauge of typical phone wire (0.5mm diameter approx). That works out at about 9.8 ohms for 100 metres, although this has to be doubled as there are two wires. So that’s 19.6 ohms per metre “round trip”. The maximum power delivered at the load for a 50m line would be 127W, dropping to 64W at 100m and just 21W at 300m. However, this is highly inefficient as it means drawing double that power at the source as half the power will be wasted in transmission losses.

    A more practical solution is to draw the same current off of each line. If we set that at (say) 100ma, it would provide 4.9W over a 50m line yet still provide 4.4W from a 300m line. The power required at each customer’s site would be 5W. At current UK retail power rates that would be of the order of £5 a year, or about 50p per month. (The node mentioned in the article consumes rather less than 3W per line). Note that the power conversion required at both the customer’s premises and the node will incur some losses, but modern conversion circuitry is now quite efficient at doing this.

    The biggest problem is possibly that the node will only become practical once a certain minimum number of lines are active. Whilst a large amount of the power requires will be proportional to the number of lines actually in use (largely to power the line amps), there will also be a moderately large fixed element to power the optical circuitry, network switches and so on. It’s that minimum power requirement that might be an issue. Clearly my nominal 100ma could be boosted to (say) 300ma which would provide 13.2W at the load over a 100m line (but only 9.7W over a 300m line). If the node’s minimum power requirement was, say, 50W then it might be possible to power it with just 5 active lines. Of course the subscribers would then be faced with another £15 per year on their electricity bills.

    It may well be this “minimum subscriber” issue that’s the biggest issue. Clearly designing nodes with the lowest possible base power requirement will be essential. Ideally you want a node capable of operating at less than 10W with a single line. That’s just about a practical amount of power to deliver over a single line of up to about 250m (that customer would find about £20 on their annual electricity bill).

    So there’s the problem. Design a node that can work off of a 10W power budget when supporting a single line and which scales up power consumption at about 2.5W per line added.

    Nb. battery backup could be provided at the customer premises or the remote node. It would be more efficient at the remote node. Of course if there is a power cut, then most likely the customer nodes would go down as well, so this may only be required for powering phones. In this case I see little need for a very substantial battery.

  22. Will Godfrey Silver badge
    Meh

    Same Old

    So many 'simple' solutions to what is in fact a very complex problem.

    For example, so far absolutely nobody has considered the situation where an entire area goes down power-wise. This is something I've seen several times in the last 4-5 years. On each occasion I was still able to use my land-line and notify the electricity company that there was a fault.

  23. ChrisG13

    Lie back and relax, it's going to happen.

    I think El Reg is behind the curve here. Reverse powering of remote equipment already has a European standard, TS 101 548, published in Sep 2014 by ETSI the European Telecom standards setting organisation. http://www.etsi.org/deliver/etsi_ts/101500_101599/101548/01.01.01_60/ts_101548v010101p.pdf

    It's not some group of nutters proposing this. All the European Telcos and manufacturers have got together and ratified this spec. Anyone can make equipment that meets this spec. and legally install it on the public telecom network. Safety is covered by the usual EN60950 spec. used for all telecom equipment.

    Texas Instruments have published a design note for a 12V 1.2A power supply where up to 8 users can share the powering of one DPU but it must function with power from just one user.

    http://www.ti.com/tool/PMP10099?keyMatch=pmp10099

  24. ChrisG13

    Sit back and relax, it's going to happen

    I think El Reg is behind the curve here. Reverse powering of remote equipment already has a European standard, TS 101 548, published in Sep 2014 by ETSI the European Telecom standards setting organisation. http://www.etsi.org/deliver/etsi_ts/101500_101599/101548/01.01.01_60/ts_101548v010101p.pdf

    It's not some group of nutters proposing this. All the European Telcos and manufacturers have got together and ratified this spec. Anyone can make equipment that meets this spec. and legally install it on the public telecom network. Safety is covered by the usual EN60950 spec. used for all telecom equipment.

    Texas Instruments have published a design note for a 12V 1.2A power supply where up to 8 users can share the powering of one DPU but it must function with power from just one user.

    http://www.ti.com/tool/PMP10099?keyMatch=pmp10099

    1. dotty

      Re: Sit back and relax, it's going to happen

      looking at the schematic it looks ideal DC DC conversion isolation nice to see IT at the front for a change ( used to be an TI Medium range systems engineer )

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