ADSL
An ADSL circuit connects An
ADSL modem on each end of a twisted-pair telephone line, creating three
information channels: a high speed downstream channel ranges from 1.5 to 8
Mbps, a medium speed duplex channel ranges from 16 to 640 Kbps and a POTS
channel. The POTS channel is split off from the digital modem by filters,
thus guaranteeing uninterrupted POTS (the old voice calls), even if ADSL
fails.
ADSL modems can be purchased
with various speed range and capabilities. The minimum configuration
provides 1.5 or 2 Mbps downstream and 16 Kbps duplex channel; other provides
rates of 6.1 Mbps and 64 Kbps duplex. Products with downstream rates up to 9
Mbps and duplex rates up to 640 Kbps are available too. Downstream data
rates depend on number of factors, including the length of the copper line;
it’s wire gauge, presence of bridged tapes and cross-coupled interference.
Line attenuation increases with line length and frequency, and decreases as
wire diameter increases.
Ignoring bridged taps, ADSL
will perform as follows:
|
Data Rate |
Wire Gauge |
Distance |
Wire Size |
Distance |
|
1.5 or 2 Mbps |
24 AWG |
18,000 ft |
0.5 mm |
5.5 Km |
|
1.5 or 2 Mbps |
26 AWG |
15,000 ft |
0.4 mm |
4.6 Km |
|
6.1 Mbps |
24 AWG |
12,000 ft |
0.5 mm |
3.7 Km |
|
6.1 Mbps |
26 AWG |
9,000 ft |
0.4 mm |
2.7 Km |
Premises beyond these distances
can be reached with fiber based digital loop carrier systems.
Why ASDL is better than cables:
Many applications envisioned
for ADSL involve digital compressed video. As a real time signal, digital
video can’t use link or network level error control procedures commonly
found in data communication systems. ADSL modems therefore incorporate
forward error correction (FEC) that dramatically reduces errors caused by
impulse noise.
The main technologies used by
the ADSL are:
Discrete Multitone
Code & Error
Correction Framing & Scrambling
Discrete MultiTone
Copper lines have a frequency
spectrum of 1.1Mhz which can be used to data communication under two main
limitations:
1.The lower 4Khz are being used
by the POTS
2.The amplification isn't the
same in all frequencies.
The technology being used is
DMT which divides the frequency range to 256 sub-frequencies from 64Khz to
1.1Mhz Each sub-frequency is an independent channel and has it own stream of
signals. The ADSL protocol defines a basic stream of data which is known to
both endpoints in advanced and enables them to find the specific SNR for
each sub-frequency, and uses this information to split the data over the
sub-frequencies.
Dividing the spectrum to 256
sub-frequencies
It is important that the ADSL
won't effect the POTS in any way, to accomplish that the lower 4Kbps is
separated by a analog circuit called Splitter. In this way you can make
simple phone calls and use ADSL services in the same time.
Adaptive SNR discovery
The DMT technology is also very
useful in the asymmetric mode where the sub-channels are divided into
groups, one for the upstream data (from the endpoint to the PTT) and the
other for the downstream data (from the PTT to the end user).
Code & Error Correction
One of the most important
technology breakthroughs that helped the ADSL is the coding. Using a method
called constellation encoding and decoding the information on the line can
be damaged and yet the decoder rebuilds the information in a very high
reliability. To improve the performance of ADSL system some companies use 16
state 4 dimensional trellis code on top of the constellation encoding.
Framing & Scrambling
As most of the Computer
communication networks, the ADSL uses a specific framing method. The main
frame is called Superframe and it is composed of 68 ADSL data frames , the
ATU-C sends a superframe every 17 msec . Each data frame gets his
information from two data buffers (interleaved buffer and fast buffer),
which are scrambled, at a specific sequence, this scrambling method makes
the error correction and coding more efficient. |
