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Unfortunately, the copper telephone
network was designed many years ago for
voice communications, and line bandwidth
was not the main consideration. In fact,
lines using the old paper-insulated cables
sometimes have a more restricted upper
audio cut-off frequency of about 2.4kHz
but still seem to work.
Notwithstanding the above limitation,
it is possible to create a relatively high-
bandwidth link over a telephone pair if
some basic design rules are followed.
These rules are necessary. As the link
bandwidth is increased, the line starts to
behave as an RF transmission line, and
network topologies that work fine at
audio frequencies can no longer be used.
In particular, bridge tap connections
(line stubs that consist of a short line with
an open circuit at the far end) can’t be
used due to the impedance mismatch
and high-frequency signal reflections
they create.
Similarly, any loading coils on
the line must be removed. For data
communications, what is needed is a
clean balanced pair from end to end.
High bit rate data over balanced pair
telephone cable has been achieved by
developing the xDSL series of digital
subscriber line protocols. Here the ‘x’ is
a generic variable, with ADSL and VDSL
being just two common variants.
For a high-speed data service, two
parameters define its usefulness - reach
(maximum distance between customer
and exchange over which the link will be
reliable); and maximum link speed.
However, these parameters compete,
because cable losses and crosstalk (a
form of noise) increase with cable length.
Thus a low-speed data circuit will have a
longer reach, all other things being equal.
DSLVARIANTS
In order to understand the need for
G.Fast technology, and its capabilities,
some explanation of xDSL is in order.
ADSL, or asymmetric Digital
Subscriber Line, is a broadband internet
access service using standard telephone
cable pairs.
It is called ‘asymmetric’ because the
maximum downstream bit rate is
much higher than the upstream rate.
This is because internet traffic flows
mostly downstream.
The link speeds discussed below
represent the absolute maximum speed
on good copper cables having no bad
joints, faulty insulation or disallowed
connections such as bridge taps and
loading coils.
The actual link speed may be
considerably less than these maximum
values, depending on other factors (line
equipment characteristics, node traffic
volume, other pairs in the same cable
carrying active xDSL services, etc).
ADSL comes in three forms; basic (also
known as ADSL1), ADSL2 and ADSL2+.
These technologies offer progressively
better reach and speed characteristics.
ADSL1 has been around for well over
10 years. It can theoretically support
maximum downstream bit rates of up to
about 8Mbps to a reach of about 2km,
and up to 1.5Mbps to about 5km using
standard telephone cable.
The ADSL1 signal occupies bandwidth
of a little over 1MHz on the cable pair.
ADSL2 is an improved version, with a
maximum theoretical downstream speed
of 12Mbps to about 1.5km, degrading to
8Mbps at 3km and 1.5Mbps at 5km.
ADSL2+ is the fastest version, with a
theoretical maximum of almost 24Mbps
to 0.7km, dropping to 12Mbps at 2.5km,
8Mbps at 3km and 1.5Mbps at 5km.
Achieved speeds are substantially
slower due to less-than-perfect cabling
and equipment characteristics.
VDSL, or very high data rate DSL,
offers downstream speed close to
50Mbps. The reach at this speed is
limited to about 1km and the required line
bandwidth is 12MHz.
VDSL2 offers improved performance
through crosstalk mitigation techniques
(known as vectoring), with a downstream
speed of about 350Mbps close in, rapidly
degrading to 100Mbps at 0.5km, 50Mbps
at 1km and 25Mbps at 1.5km.
VDSL2 technology has been selected
for use on the copper part of fibre to the
node (FTTN) technology being installed
by NBN Co in some locations.
G.FAST
G.Fast technology is similar to VDSL2 in
terms of its application.
It is intended to drive short balanced-
pair copper lines at high speed from a
nearby node fed by optical fibre trunks.
G.Fast technology is specified to a
maximum link of 250m. This limits its
applications to high-rise buildings and
areas of high population density.
The technology is capable of a
maximum downstream speed in excess of
1Gbps on very short links by using more
than 100MHz of cable-pair bandwidth
and advanced crosstalk cancellation.
At a reach of 250m, maximum
speed drops to about 150Mbps. Again,
these figures degrade rapidly if cable
performance is not up to scratch.
Note also that the close-in downstream
link speed for G.Fast is about the same as
that achieved with Gigabit Ethernet, the
latter requiring at least Category 5e cable
and connectors.
The main difference is that G.Fast
achieves similar performance on
short links using telephone-grade
cable, which has much poorer
transmission performance.
Late last year, NBN conducted a field
trial of G.Fast technology in a Melbourne
office block. It achieved link speeds in
excess of 600Mbps on a 100m length of
copper that was more than 20 years old.
This implies that very high data rates
can be achieved in a building using
existing telephone cabling.
G.Fast is likely to be very useful for the
FTTN system being rolled out by NBN,
particularly for the commercial sector, as
the 50Mbps offered by VDSL2 may not
be enough.
BY
GEORGE
GEORGEVITS