FTTC query

Discussion in 'Broadband' started by Peter Crosland, Aug 24, 2011.

  1. Can any Openreach engineer tell me if the voice part of the signal to the
    FTTC cabinet is over copper or fibre. This may sound a silly question but I
    have had conflicting answers to the question and would like a definitive
    answer TIA

    Peter Crosland
    Peter Crosland, Aug 24, 2011
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  2. Peter Crosland

    Graham J Guest

    From house to cabinet = copper
    from cabinet to exchange = copper

    The DSLAM is installed in the cabinet, and it feeds the VDSL signal
    along the copper wires to the house.
    Graham J, Aug 24, 2011
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  3. Do you mean from the exchange, or from the customer premises?

    It would seem a bit silly to maintain pairs from the exchange to the
    cabinet rather than muxing them over the fibre...mind you, silliness is

    I assume the cabinet to premises is good old copper pairs though with
    phones on the baseband.

    Oh I found something. Yes, its silliness all right


    "The Street DSLAM is served with a fibre back to the exchange to carry
    the Broadband signals. (The telephone exchange at which the Broadband
    service is handed over to BTW is not necessarily the same exchange
    providing telephony services to the End User.). The Street DSLAM is
    connected to the Street Cabinet using tie pair cables. VDSL 2 is used to
    carry the Broadband over the copper pair from the Street Cabinet to the
    End Users’ premises. See the diagram below. "

    (actually there is no diagram below. There is one above: ed)

    "The Public Switched Telephony Service (PSTN) is unaffected by this new
    technology and continues to be supplied over the copper pair between the
    Exchange and End Users’s premises."

    The key thing seems to be that they may decide to string fibre from a
    completely different exchange to the one that feeds the voice.

    Yet another instance of BT failing to bite the bullet and go all VOIP..

    which seems relatively insane really.
    The Natural Philosopher, Aug 24, 2011

  4. Thanks.

    Peter Crosland
    Peter Crosland, Aug 24, 2011
  5. Peter Crosland

    Andy Burns Guest

    I can't claim to be definitive, but i'll give you 500:1 that the voice
    is copper all the way from the exchange, ...
    Andy Burns, Aug 24, 2011
  6. Peter Crosland

    John Guest

    Voice telephony continues to work the same way it always has done, ie,
    copper pair from exchange to cab and copper pair from cab to premises.
    Broadband is provided as fibre from exchange to new fibre cab containing
    DSLAM, copper pair from DSLAM jumpered to existing copper pair in
    existing cab.
    John, Aug 24, 2011
  7. Peter Crosland

    Invalid Guest

    The relative degree of insanity depends on your other

    If you want (have to?) to maintain voice services (for emergency use) in
    the event of power outages, then under a fibre scheme for voice the FTTC
    cabs would need biggish batteries and even the capability to be
    generator supplied.

    If you presume that most people do not run their router off a UPS, then
    you can (reasonably safely) assume that allowing the fibre cab to go
    down in a power outage will not be a big deal - and hence make the
    roll-out a lot cheaper.

    Interestingly OFCOM has a current consultation document at


    requesting comments on the idea that FTTP installations should have the
    requirements for battery backup reduced from four hours to one hour in
    the interest of roll out cost.

    I wonder who is going to be responsible for the long term maintenance
    and end of life exchange of all those batteries?

    In the US Verizon's FiOS product provides voice (but not Voip) for 8
    hours in the event of power outages, but it makes the battery the
    subscribers property/problem (one year warranty on the lead acid battery
    - then if you need to replace it its US$46.00 to buy and you do it

    Invalid, Aug 24, 2011
  8. But you'd need so much more equipment in the cabinet.

    And the work needed when people switched their broadband between FTTC and
    normal would be much much more.

    It seems like a brilliantly simple way of slowly rolling out faster
    broadband to me.
    Brian Gregory [UK], Aug 25, 2011
  9. Peter Crosland

    Steve Hayes Guest

    I've been wondering about this for some years. As I see it:

    The ability of landline phones to work without customer premises power is
    an incidental feature of the common-battery system of phone lines that
    must now be a century or so old. When this system was devised, I doubt
    that the designers even considered this as a requirement. Rather, there
    was the high cost/unavailability of devices that could have used customer
    premises power and the fact that quite a lot of potential customers
    didn't have an electric supply of any kind.

    If we were starting from scratch now, I doubt that the designers would
    introduce significant additional cost, complexity and possible other
    safety issues merely so the phones would work without customer power.

    After all, many people only have cordless phones that stop working when
    the lights go out. They also have mobile phones that, most of the time,
    are a better bet in an emergency. Signal permitting, you can use your
    mobile from anywhere in the building, e.g. from a room you're trapped in
    by a fire. There are no wires to burn through. Intruders can't stop you
    dialling 999 by cutting a wire or putting an extension phone off-hook.

    A pure fibre system is certainly safer when it comes to lightning strikes
    while shorted batteries are a potential cause of fires. These two factors
    alone balance out some cases where someone needed to and could have made
    an emergency call over a landline during a power failure and couldn't
    have used a mobile instead. Then there's the whole area of cost-benefit

    The issue is probably only that the ambulance chasers will be out looking
    for the few people who can claim that they suffered a loss because they
    couldn't make a landline call.
    Steve Hayes, Aug 25, 2011
  10. Peter Crosland

    Graham. Guest

    OK, a good argument has been made for not MUXing the voice lines over the fibre
    but that doesn't mean that they need the extravagance of an exchange pair per sub
    as several FTTC customers could have their voice MUXed and powered over a single copper exchange side pair.
    providing room is available for the DACS hardware in the cabinet.
    Graham., Aug 25, 2011
  11. Peter Crosland

    mr simon Guest

    Couldn't what you're suggesting could be done regardless of whether or not
    they're FTTC subscribers?

    But things will get messy as people change between ADSL/FTTC during house
    moves etc?
    mr simon, Aug 25, 2011
  12. one assumes the exchanges are so equipped. Its only a pair of power
    wires to the cab...

    I dont see there any more equipment being powered up than in the current
    scenario actually.
    The Natural Philosopher, Aug 25, 2011
  13. Peter Crosland

    Invalid Guest

    This discussion started on the basis that it was "...another instance of
    BT failing to bite the bullet and go all VOIP, which seems relatively
    insane really"

    I was merely pointing out that the "insanity" - a phraseology which
    imputes poor/illogical technical & strategic decision making to BT was
    actually a valid and logical consequence of some constraints (mostly -
    I suspect - regulatory).

    3.1c of the general conditions by which BT and other (fixed line) phone
    operators are regulated by Ofcom states

    "The Communications Provider shall take all reasonably practicable
    steps to maintain, to the greatest extent possible uninterrupted access
    to Emergency Organisations as part of any Publicly Available Telephone
    Services offered at fixed locations."

    I don't think requiring the customer to have mains power would quite fit
    with "...all reasonably practicable steps...to the greatest extent

    I would wholly agree that if you were starting with a blank sheet of
    paper (and FTTP may be that blank sheet), then you wouldn't today make
    providing phone service in the event of a premises power cut a core
    requirement - but today it is.

    I don't think its just an "...ambulance chasing" issue I can just
    imagine the political/regulatory/media/consumer lobby furore if BT
    Openreach even suggested that they were not going to provide customers
    with emergency service access when there was a local power cut.

    If I were making strategic design decisions in BT Openreach I would not
    want to even think about going there.

    I think this is also the reason by Virgin provide phone service via
    copper rather than multiplexing it down the co-ax.
    Invalid, Aug 25, 2011
  14. Peter Crosland

    Scott Guest

    I am not sure the 'ambulance chasers' (whoever they might be) would be
    the only people concerned about reducing the resilience of the
    emergency calls system.

    You mention cordless phones but I think you will find that the
    instructions say quite clearly that these should not be relied on for
    emergency use and at least one phone in the property should be a fixed

    Also, I'm not too keen on the idea that mobile phones are a better bet
    'most of the time'.

    Any emergency system should have maximum resilience.
    Scott, Aug 25, 2011
  15. That's 50v DC power. Do you have any idea how thick those cables would
    need to be?
    No, but you're moving a lot of equipment from the exchange to the street
    cab. This means you need much bigger street cabs.


    Denis McMahon
    Denis McMahon, Aug 26, 2011
  16. no reason to feed 50V to the cab:you could down convert it there.

    its only 40mA per phone anyway AFAICR. so a cabinet with a couple of
    hundred phones.. 8A or so?

    If I can get a voip circuit in the same box as a standard router, it
    ain't that big, Basically a DSLAM so equipped would need powering, yes,
    but then I'd guess the cab is emergency powered anyway.

    If BT supplied a customer premises router with VOIP its even better.

    But then they would need some kind of battery backup in that,.
    The Natural Philosopher, Aug 26, 2011
  17. Peter Crosland

    John Guest

    No, because you can't have ADSL if the line is DACS'd.
    John, Aug 26, 2011
  18. Peter Crosland

    Paul Cummins Guest

    We were about to embark at Dover, when (John) came up to
    me and whispered:
    You miss the point. The DACS will be E-side of the vDSL equipment in the
    Paul Cummins, Aug 26, 2011
  19. So you want to convert the 50v exchange battery up to a higher (ac?)
    voltage to transfer it to the cabs, and then convert it down again?

    That would involve 2x conversion losses.

    8A for the phones maybe, but a lot more amps for the electronics inside
    the cab.
    Yeah, but at the moment I think you'll find that VSLAM FTTC cabs are only
    emergency powered for very short periods (< 1 hour) if at all.

    Most exchanges are designed so that the 999 circuits are protected
    indefinitely, firstly by the exchange batteries, and secondly by back-up

    Providing the same level of resilience in street cabs is problematical.


    Denis McMahon
    Denis McMahon, Aug 26, 2011

  20. 1/. The cab needs powering anyway if you want to have broadband thriugh
    a power cut.

    2/. If you think you cant do VDSL to fibre for more than 2W per
    subscriber, then you haven't seen modern electronics.

    http://www.allieddata.com/eur/assets/documents/VDSL2 DSLAM 10024 - 10048 Datasheet.pdf
    says 87W for 48 port DSLAM. About 2W/subscriber.

    If you read the above, there is no reason they couldn't be done at
    minimal power - about 2-3W/subscriber - from the central exchange.
    Its not. Its a piece of piss. Run an armoured cable alongside the fibre
    backhaul to the fibre exchange.

    I dunno how many subscribers it takes to the cab - I'd guess at 100 or
    so max. That's only 400W or so. Not exactly a huge armoured cable is it?
    Even over 10 miles...Probably a couple of amps.

    What IS problematical is probably open reach having to write off a paper
    value in exchange equipment (which would become totally redundant) and
    therefore putting the bean counters in a flat spin.

    And having to implement a SIP style service, which they would doubtless
    make a pigs breakfast of.
    The Natural Philosopher, Aug 26, 2011
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