Tuesday, 7 May 2013

NBN: Can vectoring work on Turnbull's FTTN?

Turnbull made an important point about his FTTN deployment yesterday:
Your existing ADSL modem is still going to work. [However, to get higher speeds, you'll need to buy a VDSL2 modem.]
 This tells us a whole lot about what the Turnbull-FTTN can and cannot do:

  • The Coalition NBN policy claims "no disruption" in cut-over. This is how it's to be done.
    • "Nothing done in the house" is also a listed advantage of the Turnbull FTTN, confirming phone services remain over "baseband" copper shared with DSL.
  • Your NBN connection won't directly use the "Layer 2 bitstream", but the older, more limited & costly and slower current technology, PPPoE.
    • The speed of PPPoE is limited by the CPU speed of the DSL access device you use.
    • Multicast from the FSAM/PoI isn't possible are each client individually tunnels back to the ISP and their PPPoE server. [That's big and chews power, for no good reason.]
    • Services will be limited to one RSP, not the four initially available on FTTP Network Termination Devices.
  • It implies that your fixed phone will still share the same phone line.
    • Which means the line will still have to be balanced & loaded for voice, not DSL.
    • Plus you'll still need performance affecting line-filters. [Now is a really good time to get yourself a high-quality central-splitter.]
    • If the line back to the exchange is cut as is required for vectoring, you'll need an "ATA" (Analogue Telephone Adaptor) either in your home or in the node. This is part of what an NTD does.
  • If you're one of the ~1M lines connected via the 8750 Telstra RIM's (aka mini-muxes), then there's a problem. They are nodes and have to be disconnected and removed. 
    • That conversion won't be simple or transparent.
    • Or free, going on Telstra's track record.
Copper lines can be optimised for either voice or data (DSL), not both.
For DSL to work at all, a high-pass/low-pass filter is needed at the customer premises to separate the high-frequency DSL signal from the voice. You get the choice of a central-splitter, that pulls the DSL signal off when it enters the house onto a separate cable and leaves the house cabling only with voice signals, or a line-filter on every device.

Why you need to do this is simple & unavoidable physics: to DSL, a normal phone looks like a short-circuit and kills the signal. If a filter is not connected or fails, then your DSL service stops, or is severely compromised.

Vectoring relies on treating a whole group as an entity and dealing with near-end/far-end interference & cross-talk, group delay and echos all together.

A major part of the Telstra FTTN plans in 2005 was removing all the special conditioning of copper lines for voice. DSL services work much better, wasting less power and with less line noise, over 'bare' lines. It's a trade off: have your phone work or have DSL work well.

Vectoring will work much better over 'bare' lines as well. You can't have someone with a faulty line-filter or plug-in a new device without a filter.

But as of yesterday, we know that the Turnbull FTTN won't be 'bare' lines with an in-house NTD. It will be a much poorer version of that, with much reduced speeds and missing the essential technical underpinning of the NBN: end-to-end "layer 2 bitstream service" allowing multiple RSP's per premise.


  1. Hi Steve,

    Excellent points.

    Turnbull has often said they will offer a Layer 2 product. Maybe that's only for fibre/wireless and not for vdsl.



  2. David,

    Thanks, esp. for the link.

    I've wondered for a while just what the detail of their design is...

    Having PPPoE in-premises is not entirely inconsistent with offering RSP's a L2 VLAN/bitstream. They get to keep their current PPPoE servers & authenticated and allocate 2 IP numbers to clients.


  3. Hi Steve,

    Could you elaborate on that comment? and how it relates to what you have written in your blog post.

    Specifically, with regards to whether an NTD/NTU will be installed, and whether yours POTS phone will work?

  4. 1. POTS:
    Coalition is *silent* on where they'll do the ATA, because they need to cut the copper AND they claim "no disruption", implying to both ADSL and POTS services.

    2. NTD/NTU:
    Coalition do not mention NTD/NTU for the FTTN.
    No FTTN proposals I've seen have mentioned Customer Premises Equipment.

    The implication is: Customers get to organise for themselves and pay *full* Retail for any in-premises changes required. Like VDSL modem and installing a NEW Central Splitter (ADSL2 ones are different spec, I'm told).

    And yes, you really do want a Central Splitter.
    No, they are not DIY. Need to be a Certified Cabler.

    3. these are the most costly & economically inefficient ways to perform these functions.

    But as it doesn't get billed to the Government/NBN, it is deemed to be "Zero Cost" by the Coalition.

    Write me an email with specific Q's if I haven't been clear.

  5. Apart from the clearly-unworkable claim of "no disruption" on the line, I'm not sure what leads you to think the LBN wouldn't do bare copper from the home to MSAN at the node - supplying both VDSL/ADSL + PSTN from the cabinet.

    MSAN means no ATA required, no voice conditioning required and decent DSL sync rates too. Sure, you need a splitter .. just like today for ADSL. It's not like that first 4kHz for voice steals much bandwidth from data.

    Of course, the copper will have to be fairly short, meaning a lot of (expensive) nodes if they want to be able to guarantee 50mbps.

    Why do you say that LBN can't be a layer 2 network like the NBN? Sure, at switchover time they could be configured for PPPoE as per their old ADSL. I imagine that subscribers could be configured for whatever access their RSP decides: IPoA, IPoE, whatever floats the ISP's boat when they sync up with VDSL2.

  6. Rich,

    On "no disruption": I've seen Telstra write out this in multiple technical documents since Nov 2005. I believe cutting over to nodes like RIMs, with ATA's + DSLAM's can be transparent to user, modulo swapping networks and dropping ADSL sync for a time.

    This promise, "no work in the house needed" is central to the LNP's plan, so I'm saying they have to build Nodes that look like RIMs: phone+DSL.

    Which means they cannot do 'naked' ADSL or VDSL in the first instance - because NBN Co are obligated to keep the phones running without disruption.

    I agree with you, if they ran a pure-digital ISAN/MSAN then no splitter, no voice conditioning.

    But they cannot meet "no disruption" and run

    On the Layer 2 connectivity:
    "No disruption" means ADSL still works, and everyone runs PPPoE, not direct IPoE.

    Turnbull hasn't addressed this issue except indirectly: "You can buy a $50 VDSL modem".

    We just don't know what they plan, but it sure seems like they aren't going to install standard NBN Co NTD's nor run a pure-digital network.

    If, as implied by Turnbull, they are going to allow the same BYOD arrangement for VDSL2 as we have with ADSL1/2/2+, I'm inferring they'll run least-common-demoninator, which is PPPoE.

    Glad you took the trouble to post and put your views.

    If I haven't answered your questions, or you disagree with my logic or assumptions, I'm happy to continue this in the comments, via e-mail or to create another post dedicated to this topic.

    My contacts all in the blog.



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