[AusNOG] VDSL / ADSL crosstalk

[Paul Brooks]

On 12/02/2016 7:47 AM, David Hughes wrote:
> Morning all,
>
> Can someone with experience with deployment of VDSL (or VDSL2) based services into an MDU environment (FTTB or "Wireless TTB" for example) comment on the impact it has on existing DSL services from exchanged based DSLAMS.  If memory serves me correctly, VDSL mostly uses frequencies significantly higher than DSL for most of its signalling and drops the volume in the range below 2MHz that DSL utilises with the aim of eliminating cross talk with the legacy services.
VDSL2 *can* have a lower or even no transmission power in the ADSL2+ band, if the
provider programs the DSLAM that way, but by default it uses the same frequencies at
much the same power. The main problem with co-existance with exchange-based gear is
the mis-match of transmission powers caused by the difference in distances from the
DSLAM to the modem. The signal from the basement VDSL DSLAM, being so much closer to
the modem than the exchange DSLAM, tends to swamp the exchange signal which is
attenuated by the time it reaches the building. Also the other way - the VDSL modem
implements "upstream power-backoff" so it doesn't swamp the input of the very close
VDSL DSLAM, meaning the upstream signal becomes swamped by the adjacent ADSL2+ modem
which is transmitting at full power to reach the exchange.

In short, the upstream VDSL2 signal is degraded by the ADSL2+, and the downstream
ADSL2+ is degraded by the VDSL2.


>
> Is that recollection correct?  From practical experience, is it enough to stop local VDSL kit from tromping over services being delivered from the exchange that are running over the same cable plant within the building?
It can help in the downstream direction, but you'd need to also do it in the upstream
direction, and mask out the frequency band used by the exchange ADSL2+ - which will
reduce the VDSL2 performance both ways. Only good pair separation can fix this.
Typically each floor in an MDU is served by a separate cable from the MDF, so you
shouldn't get any impact between VDSL2 and ADSL2+ services on different floors - but
within services on the same floor, there may be impact.

>   
>
> Also, I assume that if at some stage NBN replaced exchanged based ADSL with their own FTTB kit in the building then there would be crosstalk issues between to 2 competing VDSL DSLAMs? 
Yes. Its a real problem that is happening today. Comms Alliance is working on a
co-existance Code to regulate VDSL2 in-building to promote co-existance, but today the
in-building cabling is out-of-scope for the current regulations, so an in-building
VDSL2 deployment can degrade exchange-based services without much of a regulatory
come-back.

This needed the Telco Act changed - see *Telecommunications Amendment (Next-Generation
Broadband Interference Management) Regulation 2015
*(https://www.comlaw.gov.au/Details/F2015L01721).

The action currently is in Comms Alliance WC58 -
http://www.commsalliance.com.au/Activities/committees-and-groups/wc58

Paul.

-- 

*Paul Brooks*

*Executive GM - Networks & Technology*

Email: paul.brooks at tridentsc.com.au

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