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ADTRAN and Deutsche Telekom Test cDTA Upgrade for G.fast Broadband

Thursday, July 20th, 2017 (9:43 am) - Score 1,161
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German telecoms giant Deutsche Telekom and ADTRAN, which supplies the kit being used in various broadband roll-outs around the world, have conducted lab trials of the new hybrid-fibre G.fast technology using the 212MHz and coordinated Dynamic Time Allocation (cDTA) upgrades.

At present most of the early G.fast deployments are only using up to 106MHz of spectrum, although the standard has long supported about double that and we’ve already seen several operators playing around with it. For example, Openreach’s (BT) recent Fibre-to-the-distribution-point (FTTdp) based test of pole mounted G.fast distribution points from NetComm Wireless made use of 212MHz to achieve Gigabit class speeds (here).

The downside of 212MHz is that in order to fully benefit you need to use a much shorter run of copper cable (i.e. the signal degrades faster over distance), hence why it works best with FTTdp rather than Openreach’s favoured deployment via extension pods on the side of existing PCP street cabinets. In short, 212MHz isn’t all that new.

The more interesting development here is that of coordinated Dynamic Time Allocation (cDTA). At present G.fast uses a form of half / full duplexing (i.e. the transmission of data in two directions, either asymmetrically or symmetrically) called Time Division Duplex (TDD).

cDTA is an enhancement to TDD that can allegedly improve the upstream performance of G.fast by up to “four to five times by dynamically balancing upstream and downstream capacity to match residential traffic patterns in real-time” (i.e. squeezing a bit more capacity out of the copper line for faster speeds). This sounds similar to the independent DTA (iDTA) method that already exists in the G.fast standard.

Jay Wilson, ADTRAN’s Senior VP, said:

“Operators in highly competitive, dense urban or urban environments are challenged to extend gigabit services due to the time and cost that can be associated with pure play FTTH techniques. With G.fast innovation, operators, such as DT, can significantly accelerate Gigabit Society goals by launching gigabit services over their existing infrastructure dramatically reducing subscriber disruption.”

Wilson believes that the advent of 212MHz and cDTA will “extract even greater overall performance from an operator’s existing assets” and he claims that in many scenarios it may also “eliminate the need for full FTTH for years.” At present Openreach aims to roll-out G.fast to 10 million UK premises by 2020 and the first non-pilot commercial deployments should begin before the end of this year.

However most or all of this initial roll-out is likely to use the street cabinet based approach and as such Openreach has had to cap the top download speed of their G.fast service to 330Mbps (50Mbps upload), although they have previously talked about pushing the top speed to 500Mbps by 2025 (we don’t yet know how much faster uploads could get but cDTA would certainly help).

Meanwhile the Government has been pushing for a lot more 1000Mbps+ FTTP/H and even Openreach are now consulting on a plan that could match the scale of G.fast’s proposed deployment (here), albeit at significantly greater cost and over a much longer period of deployment. Suffice to say that the balance of G.fast and FTTP/H plans in the UK are currently in some flux.

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Mark Jackson
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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4 Responses
  1. Avatar Chris P

    the problem with TDD is that the longer the line the longer you must wait for collision detection etc. Thats why Ethernet had a max distance of 100m, it was because collision detection waited the length of time light took to travel 100m (and a little random bit more) before it knew the line was clear to start transmitting. Full duplex ethernet turns collision detection off and you can safely run further than 100m without issue.
    If i understand this correctly, long G.fast lines would impact shorter lines in the path as to avoid interference they all must be sending or receiving at the same time, hence the enhancement in cDTA being able to send more in 1 direction or other when needed provided the other transmitters are quiet.

    • Avatar CarlT

      It’s not CSMA/CA. If I understand it correctly there are timeslots allocated by the DSLAM depending on upstream and downstream demand, not carrier sensing.

      The modems will be receiving constant messages from the DPU telling them in which direction bursts may be for the next x timeslots, and will transmit their own telemetry on upstream demand to allow the DPU to plan the next slots.

      It can be done both independently (iDTA) per line, and co-ordinating across all lines on the DPU, (cDTA) with vectoring taking up slack to reduce crosstalk.

      I may be completely wrong!

    • Avatar MikeW

      GSM used something known as timing advance, to cause distant mobiles to transmit slightly earlier than nearer mobiles, so as to stay within the timeslot. It worked to 35km.

      I’d imagine that, if G.Fast needed to keep timing tight, it would use a similar design.

  2. Avatar MikeW

    It is notable that neither Nokia nor Huawei seem to be publicising the bleeding edge with either DTA or 212MHz operations. Are they spending their time aiming at BT’s long-range power needs instead?

    It seems NBN is going with Adtran for “fibre to the curb.” They call our FTTC rollout “fibre to the node”, or FTTN, so it looks like -to-the-curb is a shorter-range copper solution.

    212MHz support was part of G.Fast’s amendment 3, and was only standardised in April of this year (the ITU-T spec is now, finally, freely available). It might have been talked about before, but real working equipment is relatively new.

    From recent Adtran graphs, 212MHz gives better speed out to around 350m. If BT chose to use it, they might extend the range of their 300Mbps service a little.

    Note also that the original iDTA seems to be only defined within the annex for a “crosstalk-free environment,” so no vectoring or coordination with other lines is required, so there’ll be no timing clash either. While this annex can be used for twisted pairs, I think it is really being aimed at coax, for FTTB installations and especially for the US. Both the DTA and the “crosstalk-free-environment” annexes were added in amendment 3 too.

    cDTA (c = collective?) looks to be an upgrade to iDTA so that it can be used in crosstalk-ridden environments, such as twisted-pair multi-pair. Even here, CarlT is right – it isn’t a CSMA/CD issue. But timing will be important if vectoring (upstream) is to work correctly.

    cDTA doesn’t seem to be standardised yet.

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