some technical questions regarding ADSL

As I understand it g.dmt use 247 4khz channels,do these start at 0hz ?

if so do this mean that 246 are available for DSL (I presume that the 1st
channle is for pstn,but I am unsure as to wether the typical filters cut
off is sharp enough)?

How much data is carried in 1 4khz channel (is it even fixed) ?

Which range of channnels are typically used for upstream and downstream ?

I understand that each channel is created using OFDM and QAM
modulation,how big is the QAM(ie 4 state 64 state etc)


Thanks
Martin Warby

"Martin Warby" <(E-Mail Removed)> wrote in message
news(E-Mail Removed)...
> As I understand it g.dmt use 247 4khz channels,do these start at 0hz ?
>
> if so do this mean that 246 are available for DSL (I presume that the 1st
> channle is for pstn,but I am unsure as to wether the typical filters cut
> off is sharp enough)?
>
> How much data is carried in 1 4khz channel (is it even fixed) ?
>
> Which range of channnels are typically used for upstream and downstream ?
>
> I understand that each channel is created using OFDM and QAM
> modulation,how big is the QAM(ie 4 state 64 state etc)
>
>
> Thanks
> Martin Warby

Martin,

There are a total of 256 DMT tones, separated by 4.3125 kHz, each with a
different centre frequency, fc. Since the POTS channel resides at the lower
end of this spectrum, only tones 7 -255 are available. For full-rate ADSL,
the downstream uses DMT tones 7 - 255 (echo cancellation) or 32 - 255 (FDM).
The upstream channel uses tones 7 - 31. Bit-loading is adaptive and varies
from 2 to 15 bits per tone (subchannel) depending on the relative noise of
each carrier. When high noise levels are detected in a given subchannel, the
DMT modem can shut down a particular subchannel altogether.

The upstream channels are divided into 32 subchannels.

For the reduced bandwidth of G.lite, there are a total of 128 DMT tones. The
downstream channel uses 7 - 128 tones and the upstream, 7 - 32. Bit-loading
varies from 2 to 8 bits per symbol.

Each channel in the combined signal is capable of carrying up 64.7 kbits/s.
This yields a maximum capacity in excess of 16 Mbits/s (for 256 channels).
In practice the higher frequency channels are attenuated and noisy giving
significantly less usable bandwidth.


When ADSL uses Frequency Division Multiplexing (FDM) to create the multiple
channels, it splits off the POTS channel into a separate 4KHz channel which
is situated at the low end of the 1.5MHz frequency spectrum of the carrier
(often called the DC end). This is because voice only uses 3.4Khz of
bandwidth.
Then it assigns a channel to the duplex multimedia stream that operates at
between 16-832Kbps, this band sits between 25 and 160KHz. Finally the
high-speed downstream multimedia stream that operates between 1.5Mbps and
6.1Mbps is placed at 240KHz to 1.5MHz. Each of the multimedia channels are
Time Division Multiplexed (TDM) into further lower speed channels. The
upstream channel has 32 x 4KHz sub-channels and the downstream channel has
247 x 4KHz channels. The common method of splitting up the bandwidth into
sub-channels is called Discrete Multitone (DMT) and is designed such that if
problems occur at a particular frequency such as interference or bridge taps
on the wire path, then only one or two sub-channels become affected, data
can still be carried on the unaffected sub-channels.

Hope this is a little of help although it does not answer all your
questions...

Duncan Newell

On Thu, 02 Sep 2004 18:34:28 +0100, Martin Warby <(E-Mail Removed)>
wrote:

>As I understand it g.dmt use 247 4khz channels,do these start at 0hz ?

G.992.1 Annex A (ADSL over POTS) starts at 25 kHz (Tone 7)

>if so do this mean that 246 are available for DSL (I presume that the 1st
>channle is for pstn,but I am unsure as to wether the typical filters cut
>off is sharp enough)?

There's a guard band from 4 to 26 kHz.

>How much data is carried in 1 4khz channel (is it even fixed) ?

Noise dependent. Hence adaptive, 2 to 15 bits per tone.

>Which range of channnels are typically used for upstream and downstream ?

7-31 for upstream (26kHz to 138 kHz)
32-255 for downstream (138 kHz to 1.1 MHz)

>I understand that each channel is created using OFDM and QAM
>modulation,how big is the QAM(ie 4 state 64 state etc)

Up to 15 bits, AFAIK.

Thanks for all your responses,was searching google for ages and couldn't
seem to find many of the hard technical facts,just very simplified
diagrams

Thanks again for your help
Martin Warby