Electrical Connection Spring 2015

5 8

E L E CT R I C AL CONNE CT I ON

S P R I NG 2 01 5

NETWORKING

Fibre – with a twist

he 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

MDUmay have two or more network service

providers operating fromdifferent 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

Network providers bringing

high-speed communications

to multi-dwelling units have a

noise issue to resolve.

Ian Millner

explains.

Figure 1.

Figure 2.

Copper

distribution

cable

MDF

MDU

Copper cable

Telephone exchange

Pillar

Copper cable

Fibre to the node (FTTN)

Fibre to the building (FTTB)

PROVIDER B

PROVIDER A