his week in Business Spectator the focus is on the long awaited launch of NBN's nascent fibre-to-the node (FTTN) is fast approaching and with the Australian Competition and Consumer Commission potentially set to give the green light later this month, the moment of truth on just what speeds FTTN customers will get is fast approaching for Communications minister Malcolm Turnbull and the NBN.

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The long awaited launch of NBN's nascent fibre-to-the node (FTTN) is fast approaching and with the Australian Competition and Consumer Commission potentially set to give the green light later this month, the moment of truth on just what speeds FTTN customers will get is fast approaching for Communications minister Malcolm Turnbull and the NBN.

In August 2014, Turnbull was on hand to spruik the 98 Mbps download and 33 Mbps upload achieved at the Umina FTTN trial possibly because he was able to imply that the connection speed, which is slightly less than what Labor promised the NBN would deliver, was achievable for every FTTN connection.

Nothing beats a politically motivated media event, except possibly when a politician has his feet taken out from under him and this occurred to Turnbull recently when Telstra, under contract from NBN to carry out the Umina FTTN trial, distanced itself from Turnbull’s hype about the connection speed achieved during the trial.

The search for a better understanding of what FTTN will actually provide over Australia’s aging copper access network continues. A recent exchange between a blogger and a NBN spokesperson highlights how quickly information can be misconstrued especially when draft documentation is involved.

Confusion reigns

In a blog article Kenneth Tsang, identified that FTTN customers would be provided with no more than a connection speed of 12/1 Mbps during the 18 month transition period that occurs after an area is declared by NBN to be FTTN ready.

During the 18 month transition period the existing legacy ISDN and ADSL2+ will co-exist with the new VDSL2 with vectoring, as customers transition to the NBN FTTN.  At the end of the 18 months, the legacy services including ISDN and ADSL2+ will be decommissioned.

Tsang, as it turns out, misinterpreted the ambiguous wording of Section 3.2 (c) in NBN’s draft Wholesale Broadband Agreement 2.2 Ethernet Bitstream Service Product Description that was released as part of the FTTN Business Readiness Testing documentation pack.

A NBN spokesperson subsequently tried to clarify what Section 3.2 (c) meant when hestated in a tweet that “it guarantees speeds of – but does not limit them to – 12/1 during the transition period” and went on to tell ZDNet that Section 3.2 (c) was intended to reflect the potential for interference between the legacy services and FTTN and “hence taking a cautious approach with our customers, the retail telecommunications companies, and guaranteeing the delivery of 12Mbps/1Mbps."

But while trying to clarify what confused Tsang the spokesperson has inadvertently complicated the matter. No guarantees should be given for FTTN connection speeds, and especially not during the transition period, because it's likely that degraded or defective copper sections will be found in abundance.

Section 3.2 (d) states that:

(d)          If the PIR at the UNI used to serve a Premises located in the footprint of the NBN Co FTTB Network or NBN Co FTTN Network is not capable of supporting the provision of an AVC TC-4 bandwidth profile at the UNI of:

(i) where the NEBS is supplied by means of the NBN Co FTTB Network:

(A)   12 Mbps downstream and 1 Mbps upstream for that bandwidth profile; or

(B)   25 Mbps downstream and 5 Mbps upstream for all bandwidth profiles other than 12 Mbps downstream and 1 Mbps upstream; or

(ii) where the NEBS is supplied by means of the NBN Co FTTN Network:

(A) during the Co-existence Period, 12 Mbps downstream and 1 Mbps upstream; and

(B) otherwise:

(1) 12 Mbps downstream and 1 Mbps upstream for that bandwidth profile; or

(2) 25 Mbps downstream and 5 Mbps upstream for all bandwidth profiles other than 12 Mbps downstream and 1 Mbps upstream,

then:

(iii) NBN Co will designate that Remediation is required in respect of the Premises; and

(iv) until Remediation for the Premises is completed, the downstream PIR and upstream PIR at the UNI used to serve the Premises may be significantly less than the downstream PIR and upstream PIR of the bandwidth profile ordered by Customer in respect of the Premises.

What Section 3.2 (d) means is that if degraded or defective copper is found then remediation will be required and until such time as the remediation is carried out the connection speed could be “significantly less” than what is ordered.

It appears that NBN, Telstra or a contractor has been carrying out copper remediation work for the FTTN trials and this in some instances has meant that new copper is being rolled out to replace degraded or defective copper. When the FTTN rollout commences in earnest it would be unthinkable that remediation would include replacement copper as the cost of installing fibre would be potentially cheaper than installing copper. But remediation work that includes replacing degraded or defective copper with new copper would be consistent with the government’s intention to provide Australians with a second rate NBN.

Expect slow connection speeds during the transition period

To understand why NBN's draft documentation included a minimum FTTN 12/1 Mbps connection speed during the transition period we need to understand what the effects are of having legacy access networks and the new FTTN using the copper cabling at the same time.

Communications Alliance (CA), the peak telecommunications industry body, formedWorking Committee 58 on VDSL2 and Vectoring to revise eight industry codes and Australian Standards to “align with international developments in VDSL2 technology” but not to “address competition or commercial aspects of deploying DSL services, including those for Fibre to The Node (FTTN) deployments.”

WC58’s work is important because NBN's FTTN rollout will occur in accordance with the amended Industry Code’s and Australian Standards. In November 2014, WC58 released a statement (PDF) of the current position of the working committee regarding proposed amendments to Industry Code C559:2012 Unconditioned Local Loop Service (ULLS) Network Deployment.

WC58 stated that it was able to draft amendments to C559 that “enable the [government’s] key policy objectives to be achieved, i.e. enable 25/5 Mbps to be available to all, and 50/10 Mbps to be available to 90%, at least cost to taxpayers” with the following assumptions:

• Segregated cables or single vectored VDSL2 provider per area for effective vectoring
• Transition during which vectored VDSL2 will coexist with legacy technologies
• Transition will allow FTTN/B roll out to be engineered for long term, fulfilling least cost policy
• Spectral shaping applied during transition to avoid degrading legacy services
• Following transition, VDSL2 spectral shaping will be removed. Vectored VDSL2 will then fulfil the 25/5 and 50/10 policies where legacy services have been fully retired
• The committee’s submissions to Vertigan and subsequent follow up have assumed that in building cable will need to be subject to the code or a unifying policy that ties FTTN and FTTB deployment rules together

WC58 indicated at the time that the “draft is ready to be issued for public comment, but awaiting confirmation the assumptions are acceptable, alternately, the committee would be pleased to undertake the necessary research to update the code in accordance with the preferred policy clarifications.”

PON vs.VDSL2 vs.ADSL2+

An approximate comparison of the FTTP PON and FTTN VDSL2 with vectoring and the legacy ADSL2+ is provided below. The diagram highlights how technology improvements have significantly improved the DSL connection speeds but at the cost of having shorter distances between customer premises and nodes. The FTTP PON provides a constant connection speed out to the effective range of the legacy ADSL2+ and in Australia the design was for 24 premises to be initially connected to a single fibre providing a download speed of 100 Mbps and that over time this may be increased to 32 premises. However, at the same time GPON is likely to be upgraded to 10GPON providing significantly faster connection speeds.

Source: hwupgrade.it (under laboratory conditions favourable to DSL speed outcomes)

VDSL2 with vectoring achieves much higher connection speeds than VDSL2 by overcoming the effects of far end cross talk (FEXT) which is a source of interference. Vectoring works in a similar manner to noise cancelling headphones and is a cyclic process that includes measuring the noise on copper pairs in a cable bundle, determining sequences of noise cancelling codes to be transmitted on each of the copper pairs and transmitting the codes over the copper pairs.

Source: Telebyte

However, vectoring is significantly less effective when there are legacy access networks utilising the same cable bundle causing cross-talk and other forms of interference between the copper pairs in the cable bundle, or when different vendors are providing VDSL2 with vectoring over copper pairs in the same cable bundle from separate nodes. What this means is the vectoring is unlikely to be very effective during the 18 month transition period if there are legacy access networks utilising copper pairs in a cable bundle.

There is another important cause of interference that will exist during the 18 month transition period and this is what is known as spectrum interference or interference caused by the different allocation of spectrum being used by each of the DSL variants.

Source: Communications Alliance

The various forms of DSL including VDSL2 and ADSL2+ utilise a number of frequency bands for upstream and downstream transmission over copper pairs and there are a number of “Plans” of how the frequencies can be allocated. During a transition phase from a legacy technology to a new technology it is likely that there will be different Plans in use such as Plan 997 and Plan 998.

As CA WC58 identifies it is “not straightforward for CA to mandate a VDSL2 band plan under current regulation” especially during the transition period and this means that there will be interference between copper pairs within a cable bundle caused by the varying frequency use.

WC58 has recommended that VDSL2 spectral shaping be applied during the 18 month transition to reduce the affect of different spectral plans being used by ADSL2+ and VDSL2 with vectoring.

Spectral separation between each of the DSL variants can minimise noise within a cable bundle, and spectral separation is an important techniques used to overcome what is known as mid-point injection noise, which is where different DSL variants use copper pairs within the same cable bundle but only for different lengths of copper.

Source: Communications Alliance

So the effects of having legacy access network technologies utilising copper pairs in the same cable bundle as VDSL2 with vectoring led to the key CA assumptions:

• Transition during which vectored VDSL2 will coexist with legacy technologies
• Spectral shaping applied during transition to avoid degrading legacy services
• Following transition, VDSL2 spectral shaping will be removed. Vectored VDSL2 will then fulfil the 25/5 and 50/10 policies where legacy services have been fully retired

So it will only be when the legacy systems are turned off that NBN will be able to ramp up VDSL2 with vectoring to its full capability, but let’s not forget that as more VDSL2 with vectoring customers come online connection speeds will drop due to cumulative noise effects within cable bundles and for many Australians this will mean a connection speed that is less than stellar and far lower than what FTTP would have provided.

Mark Gregory is a senior lecturer in the School of Electrical and Computer Engineering at RMIT University.