Network Communications News (NCN) December 2016 | Page 18

F E AT U R E

PON

PON: The evolution of data delivery to the home
Paolo Novellini, system IO specialist and Antonello Di Fresco, strategic
application engineer, at Xilinx, explore the next steps in home data delivery.

F
Paolo explores the
next step in data
deliver to the home

ifteen years ago, the most
common way of connecting
to the Internet was through an
analogue modem, sending data
over a standard telephone voice
channel. This technology quickly
dominated the communications
market because it was very cheap for
the user and required no change to
‘last mile’ technology because it used
existing twisted-pair wires already
deployed for standard phone calls. No
digging up roads; no changes in the
central office (CO); it was quite attractive
as you can imagine.
Modem speeds peaked at 56Kbps.
Why 56Kbps? Why not higher? The
simple answer is: it was not ‘theoretically’

possible. This theoretical limit set the
stage for ADSL technology.
Analogue modems used the existing
voice channel, strictly standardised by
the ITU-T committee. This channel has a
limited bandwidth (4KHz, including the
guard band), which is hard-filtered at
the CO (central office) before entering
the Muldex (Multiplexer Demultiplexer).
The Muldex is the device your telephone
connects with in the CO.
What is the maximum data rate
you can transfer over a 4KHz analogue
channel? The answer to this question is
the key to understanding ADSL.
The correct answer is: ‘It depends on
the noise level of the channel.’ Famous
American mathematician and electrical

engineer Claude E Shannon gave us this
answer back in 1948. You can transfer
at any bit rate, provided the noise level
is low enough. This result sometimes
surprises us. Shannon was actually more
precise in relating, in a quantitative
way, the maximum bit rate with a given
channel bandwidth and noise level. You
use his famous formula:
Where C is the maximum bit rate in
bits/sec (capacity), B is the bandwidth
in Hz, S/N is the signal-to-noise ratio of
the channel.
The ITU-T specifies the bandwidth
and the noise level for the voice channel,
limiting de facto its maximum bit rate on

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