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74 E L EC TR I C AL CONNEC T I ON
SUMME R 20 1 6
BEAMME UP/DOWN
I
nternet access in the home has become
an essential part of daily life.
In all that follows, link speeds are
given in bits per second (bps). Some
websites mention transfer rates in
bytes per second. The two should not be
confused: 1 byte per second is roughly
equivalent to 10bps.
ADSL
ADSL stands for asymmetric digital
subscriber line. It is called asymmetric
because the download speed is greater
than the upload speed. It operates at much
higher frequencies than voice. Filters in the
home and at the exchange separate the
voice and data traffic.
ADSL operates over what was
essentially designed to be a voice-grade
cable network. Because it tries to squeeze
the last ounce of bandwidth out of this
type of cable, it does have some limitations.
For any given signal passing along a
cable, the signal loss is directly proportional
to the length of the cable and is a rapidly
increasing function of frequency.
Crosstalk is another factor that
can degrade the signal. Again, it is an
increasing function of frequency. As the
bit rate for any given type of data signal
increases, so must the bandwidth (higher
frequencies) increase to transmit it.
For the above reasons, the greater
the distance between the end user and
the exchange, the slower the maximum
achievable ADSL line speed.
Cable quality, number of joints, cable
conductor size and the amount of traffic
in adjacent cable pairs will also affect the
maximum achievable download speed.
For these reasons, most carriers offer
a standard service, or ADSL1, and a faster
ADSL2+. These variants have different
maximum link speeds and reach (maximum
distance from residence to exchange).
With ADSL1, the maximum achievable
download speed is 8Mbps for locations
up to about 2km from the exchange,
and 1.5Mbps for locations at about 5km
out. Beyond that, the error rate rapidly
increases to the point where the link
becomes unworkable.
With ADSL2+, maximum theoretical
download speeds of about 20Mbps are
achievable out to 0.7km, dropping to
12Mbps at 2.5km, 8Mbps at 3km and
1.5Mbps at 5km.
Practical experience has shown that
actual speeds achieved are usually much
slower than those quoted above. According
to one service provider, only about half of
ADSL2+ users achieve maximum download
speeds in excess of 10Mbps.
HYBRIDFIBRECOAXIAL
Hybrid Fibre Coaxial (HFC) is a
high-frequency cable network technology
that employs both fibre and coaxial cables.
It was originally installed by cable TV
operators in metropolitan areas to deliver
pay TV to the home (eg: Foxtel).
The network topology employs fibre
trunk cables from the head end to optical
nodes, where the signal is transferred to
coaxial cable.
Coaxial cables are reticulated down each
street (with amplification as necessary) in
a tree-like topology. Coaxial taps are used
to provide service to individual households.
With the spread of the internet in
the 1990s, HFC networks were adapted
to pass data traffic using a technology
called Data Over Cable Service Interface
Specifications (DOCSIS).
The DOCSIS Standard has since
undergone several revisions, with
commensurate increases in link speed.
The current version used by Telstra is
DOCSIS 3.0. With this version, Telstra
offers two types of internet access using
its HFC network – a standard service with
a maximum download speed of 30Mbps
and a ‘Boost’ service with a maximum
THERE ARE THREE COMMON
FIXED-LINE INTERNET ACCESS
TECHNOLOGIES FOR THE HOME.
GEORGEGEORGEVITS
OUTLINES
THE PROS AND CONS OF EACH.
TECHNICALLY SPEAKING