Network providers bringing high-speed communications to multi-dwelling units have a noise issue to resolve. Ian Millner explains.

The talk goes on and on – FTTN, FTTB, FTTxxx – and we wonder whether we should care, what we need to know and what we can do.

This is probably in the minds of many cablers when they hear or see information flying backwards and forwards about the National Broadband Network (NBN).

The NBN is rolling out more slowly than most of us would like, and most of us are keen to get it. We know it will be rolled out using a range of technologies.

With the technologies being deployed you will get much higher speeds than at present, so the technology is not so important.

The problem lies in apartment buildings and townhouse complexes – or as the industry calls them, multi-dwelling units (MDUs). An MDU may have two or more network service providers operating from different locations.

To start, it is important to understand how telecommunications services are currently delivered to an MDU using the existing copper network.

Figure 1 shows the copper cable from the telephone exchange to a pillar on the street, and from the pillar to the main distribution frame (MDF) in the building. This copper is owned, operated and maintained by the carrier.

Copper cable then runs from the MDF to telephone outlets in each apartment. This customer cabling belongs to the owners corporation (body corporate), which operates and manages the common property. Fibre to the node (FTTN) and fibre to the building (FTTB) will connect to this existing copper cabling (Figure 2).

FTTN will most commonly be connected to existing pillars in the street, and FTTB is connected to the MDF in the building. If Provider A wants to connect to the pillar it must have an agreement with the owner of the copper network in the street (Telstra in Australia) in addition to other approvals and authorities.

If Provider B wants to connect to the MDF it will need permission from the owners corporation to install equipment in the building and add a cabling termination module to the MDF.

From a purely mechanical perspective these solutions seem to be able to coexist. Unfortunately, from a technical transmission perspective, there are issues involving the vectored VDSL (very high bit rate digital subscriber line) technology used for FTTN and in some cases FTTB.

The biggest challenge in transmitting high-speed data on the existing copper cable is the noise generated by the signal. The twisted copper cable used for telephony behaves more like an antenna as the signal frequency increases.

When all that was transmitted on a telephone cable was the plain old telephone service, then the highest frequency was 4,000Hz. With Vectored VDSL the highest frequencies being pushed along a twisted pair can be as high as 30MHz. At such high frequencies the twisted pair cable is a very good antenna and radiates these signals.

Figure 3 is a closer view of the twisted pair that runs from the node to the modem in the customer’s dwelling. It shows the noise from one pair, but all pairs will be emanating noise. Because this twisted pair is such a good antenna, it radiates the signal it is transmitting and receives the signal from adjacent pairs as noise.

So here’s the clever thing with vectored VDSL. Engineers know how much noise is being broadcast by adjacent pairs, so they can cancel it at the receiving end. This works because most of the noise when transmitting vectored VDSL comes from adjacent pairs that are also carrying vectored VDSL signals.

The problem is when noise received by the modem comes from an external unknown source. Figure 3 shows noise from an external source being induced into one pair. This type of noise will have an adverse effect on the speed of the broadband service.

This is the challenge in a building with two sources of broadband – FTTN and FTTB. Figure 5 shows two digital subscriber line access multiplexers (DSLAMs). One is installed as a node connected to a pillar, and one is in the building connected directly to the MDF.

The building has a DSLAM, which is shown as supplying three broadband services to different apartments. The noise generated by Provider A’s services cannot be cancelled by Provider B’s equipment, as it is ‘external noise’, and vice versa.

Consequently the services can be substantially degraded, leading to loss of the vectored technology’s benefits.

Communications Alliance is working on this to minimise the degradation, but it is generally accepted that two providers sharing a cable binder will result in lower data rates.

Should the owners corporation give a provider permission to install FTTB, as this may affect overall performance? It’s a tough one. Getting high-speed FTTB now rather than FTTN at some future stage could seem very attractive.

The questions to the FTTB service provider are:

• What range of retail service providers (RSPs) will be available to occupants via your network?
• What guarantee can they give the owners corporation as to service levels when FTTN is rolled out?

Note that FTTN and FTTB network providers are limited to providing the infrastructure. Occupants then need to contact the RSP to get broadband or other services installed.