VoIP, The PSTN Killer, Won't Kill Local Loops
by William Flanagan, Publisher
Telecom engineers continue to amaze.
With only the primitive transistors of 1960 they encoded voice for
digital transmission via channel banks and T-1 circuits. In
short order carrier networks, both transmission and switching,
became all-digital. That left the copper local loops as the
remaining analog portion of the PSTN, providing plain old telephone
service (POTS) to traditional phones.
When ISDN attempted to extend digital transmission to every customer
device the electronics of that era limited the capacity on most
pairs to 2 Mbit/s or less--more often 160 kbit/s for a
maximum-length residential line in a standard serving area around a
central office. That isn't enough for today's World Wide Web
and streaming video (which is why ISDN mostly faded away).
So the folks in the back room have sent out another level of
capability. Building on decades of Moore's Law, R&D
steadily improved the electronics for Digital Subscriber Line (DSL)
technology to squeeze more and more bandwidth from the
copper. Advances came in several forms:
The term vectoring isn't instantly clear, but it's sometimes
compared to noise-canceling headphones that actively counter
unwanted sounds. Each wire pair in a cable, or at least in a
binder group, in effect monitors all the other pairs for potential
interference. When a signal on an one pair affects the signal
on another pair the result is crosstalk, which makes it harder for
the receiver at the customer end to read the data correctly.
- Many carrier frequencies on a pair. Orthogonal
Frequency Division Multiplexing (OFDM) put hundreds of virtual
modems in parallel, each on its own carrier frequency, over one
- More efficient coding. How a bit appears on the
wire changed from simple on/off signals (T-1) to 4-level signals
(ISDN), to adding a phase change (quadrature coding in
modems). The number of bits per baud (how many bits each
digital symbol conveys) went from 1 to 64 and may go higher.
- Improved signal-to-noise (SNR) ratio. Echo
canceling, first applied to voice, does wonders for data
- Interference canceling. The latest is highly adaptive
"vectoring" among all the pairs in a cable.
To implement vectoring, a controller in the central office DSL
Access Multiplexer (DSLAM) monitors the signals on all pairs in the
local loops and calculates how they affect each other. The
controller adjusts the transmitted signal on each pair to counteract
offending interference from all the other pairs. Yes, that is
computationally intense. Cutting crosstalk means that the
signal received by the customer is "cleaner" and more easily
understood. That in turn means the signal can be weaker yet
still work reliably which extends the reach and/or raises throughput
on each pair.
As true for all DSL, the product of 'bit/sec speed' times 'cable
distance' is roughly a constant that is determined by the Signal to
Noise Ratio (SNR). Less noise, including crosstalk, means
higher speeds and/or longer reach at a given speed. Recently
Alcatel-Lucent briefed consultants on some new equipment that
applies vectoring to local loops. The following chart reports
their tests that show more than double the throughput compared to
previous DSL methods.
Throughput of 100 Mbit/s over DSL at 1000 ft and 50 Mbit/s at half a
mile should get existing copper loops serious consideration when
customers demand more capacity. They will: for streaming
video on demand, video conferencing, and other unified
But it gets better, at least for the carriers, because progress in
speed x distance comes with simpler management. Most of the
DSLAM configuration downloads from a central server. The
crosstalk canceling functions (vectoring) are dynamic so need no
Next step will be dealing with random external noise from electric
motors, switches, welding, etc. This "impulse noise" is the
subject of a recommendation, G.imp, in development at the
International Telecommunications Union.
The sum of these improvements will give customers more capacity for
new uses. People will get the speeds and service qualities
they want. Under central control, a Software Defined Network
(SDN) will use the increased bandwidth, or what portion the customer
has arranged to pay for, to set up services requested by software
applications or devices like IP telephones.
Don't hold your breath for this kind of activity on your lines, but
if you breath normally for a few years you may just have it.
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