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POLE POSITION
Posted Date: 20 Mar 2008 Resource Type: Articles/Knowledge Sharing Category: Computer & Technology
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Posted By: arunkumar Member Level: Gold
Rating:  Points: 5
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How does ADSL work? What is VDSL? When, and how, will true broadband
reach your home?
Who needs 5 Mbps? Of course you do,
so you can (illegally!) download a
movie in 20 minutes. But who needs
54 Mbps? You’d need to buy a new hard disk
every day if you kept downloading at that
speed! But streaming video, teleconferencing
and even HDTV become possible at 54 Mbps,
which is the theoretical speed offered by
VDSL—Very-high-bit-rate DSL. VDSL is the
hybrid (fibre-cum-twisted pair copper wire)
evolution of DSL (Digital Subscriber Line). OK,
it’s sort of cheating to call it DSL, but then,
much of the last mile is over telephone lines.
If you’ve lost us, refer Turbocharging The Telephone,
Digit July 2004. There, we talked about
DSL and how it works. Back then, DSL wasn’t
really available in India except for a few select
cities; now, corporations such as BSNL, MTNL,
and Airtel offer ADSL (Asynchronous DSL) in
several cities. The services are very expensive
for those of us who like to download a lot of
stuff (and that’s most of us!), but it’s there.
Plans from the companies mentioned above are
pretty competetive for casual surfers.
In any case, our purpose here is to talk
about what VDSL is and whether it’s possible in
India. But before that, a recap about how DSL—
specifically ADSL—works.
ADSL
ADSL provides a bandwidth that differs from
area to area, and between implementations; a
typical figure would be 1.5 Mbps to 8 Mbps
over a single copper pair, at distances up to
18,000 feet, and at a wire thickness of 0.5 mm.
You probably know that with ADSL, you
can use your phone at the same time that you
surf the Net. You’re able to do this because the
ADSL system divides the frequency spectrum
into three parts—one for phone service, one for
upstream data and one for downstream data.
How does ADSL work? What is VDSL? When, and how, will true broadband
reach your home? Phone service is at the lower end of the spectrum;
data is at the higher end. To implement
this frequency splitting, you need to
install a splitter along with the DSL modem.
Voice can typically be transmitted within the
4 KHz range.
The actual splitting of the frequency range
is done using one of two methods—Frequency
Division Multiplexing (FDM) or Echo Cancellation.
FDM uses separate frequency bands for the
transmitting and receiving channels. In Echo
Cancellation, transmission and reception
happens on the same frequencies—that is, the
bands overlap.
The advantage of FDM is that it is cheaper
to implement in the sense that the hardware
needs to do less work to separate the transmitted
and received signals; the advantage of Echo
Cancellation is that it transmits at a relatively
lower end of the spectrum, meaning less signal
attenuation with distance. See figure Frequency
Splitting In FDM And Echo Cancellation.
Actually, it’s not quite as simple as that.
There’s modulation involved as well (refer
this month’s Fast Track for more on modulation).
Modulation is required because of
several factors, including the noise associated
with transmitting data over the kinds of
frequencies we’re talking about. In ADSL, the
signal from your home is modulated (most
usually) using a technique called discrete multitone
(DMT), which is also what has been
approved by the standards authorities
for VDSL.
DMT splits up the spectrum on which the
downstream signal travels into 247 channels,
each of these 4 KHz wide (and the downstream
spectrum into 32 channels). The biggest advantage
of splitting up the band into so many
channels is the continuous quality check that
is possible.
Each of the hundreds of channels is monitored,
and if and when it is seen that quality is
impaired on a particular channel, the data on
that channel can be shifted onto another one.
This happens continuously—the system is
constantly re-assigning data to channels. What
results is the “best” channels at any particular
moment being chosen.
After all the data is switched onto the right
place, your phone line transmits everything
Frequency Splitting In FDM And Echo Cancellation
In FDM (left), the frequency band is split into ranges for telephone service,
upstream data, and downstream data, at the frequencies indicated
above. In Echo Cancellation (right), the frequency bands for upstream
and downstream data overlap
4 kHz ~100 kHz
180 kHz
Telephone
1 MHz
4 kHz ~100 kHz
Signal Strength
Upstream
Downstream
Telephone
Upstream/Downstream
together to your service provider’s Central
Office (CO). At the CO, a DSL Access Multiplexer
(DSLAM) aggregates all the ADSL lines coming
in to it from all over the neighbourhood.
Because of the frequency separation, the DSLAM
can figure out what’s voice and what’s data.
Voice is redirected to the PSTN (Public Switched
Telephone Network), and Internet data is
switched to the backbone connected to the ISP.
Now, the problem remains of the signal
dropping as distance from the CO increases.
See figure Data Rates And Distances for how
much this drop is, with ADSL compared against
VDSL. It’s clear that ADSL can carry data reliably
over much longer distances.
VDSL
Now here’s the 54 Mbps we were talking about!
VDSL offers such fibre-like speeds over copper
wiring, but like we mentioned, some of the
path from the home to the central office (full
form: central telephone switching office) is
indeed fibre.
VDSL isn’t new. It’s been around for a while,
but is being rolled out mostly in Japan and
Korea. We know it hurts, but all we can do right
now is sit and watch providers in these countries
battle it out to deliver the fastest speeds to
their customers!
As indicators, in Korea, we’re looking at 50
Mbps downstream and 11 Mbps upstream being
offered. In Japan, it’s more like 70 Mbps downstream
and 30 Mbps upstream—courtesy
VDSL 2 (See Jargon Buster). In Japan, NTT charges
a premium of just about $2 (Rs 90) per month
over the cost of ADSL service!
So why doesn’t everyone just move on to
VDSL? The answer lies in what we’ve been talking
about all along: fibre. (For example,
markets in the United States have not been too
responsive to VDSL.) To understand this, we
need to understand what the last-mile problem
is all about.
Fibre For The Last Mile
ISPs (In India, VSNL, Bharti and Reliance
Infocomm) are connected to the Internet backbone.
Telephone service providers (TSPs, such
as MTNL) are connected to the ISP, and your
home is connected to the TSP’s exchange. In
some cases, you might directly be connected
to the ISP’s central office (CO). If you’re not
connected directly to the ISP’s CO, it’s copper
followed by fibre that carries data to the ISP.
It’s the distance between the CO (or telephone
exchange) and your home that is called
the last mile, and here’s where all the issues
arise. “The last mile” typically means a couple
of kilometres, but could be longer.
ADSL2+ (See Jargon Buster), within a mile (1.6
kilometres) of the CO, can deliver about 25
Mbps. That should be enough, one might say,
but faster is better, and there’s no questioning
that. VDSL can reach 100 Mbps within 1,000
feet of the CO, but as the distance from the CO
increases to more than a mile, the speed begins
to compare with ADSL2+’s 25 Mbps.
Here’s where fibre comes in. VDSL deployments
use some length of fibre from the CO.
How much, depends on the particular deployment.
There’s Fibre To The Neighbourhood
(FTTN), Fibre To The Curb (FTTC), and Fibre To
The Basement (FTTB). Of course, there’s also
Fibre To The Home (FTTH), which means fibre
all the way with no telephone lines involved.
All these terms are somewhat self-explanatory,
but to elaborate, FTTB is fibre all the way
to the basement of a tall building, from where
copper takes over to the individual offices in
the building. FTTC means fibre would reach
your street. And FTTN—the most practical possibility
being talked about these days—involves
the setting up of optical network units (ONUs)
in a neighbourhood to which, on one side, fibre
connects to the CO, and on the other side,
copper connects to the home or office. The
ONU’s job is to convert the electronic signals
from the copper to light signals, which are
transmitted over the fibre.
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