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'[EE] ADSL line detection'
2009\06\15@233017 by David Duffy (AVD)

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I have the need to detect the presence of an ADSL connections where no
modem has been installed. I know I could just plug an ADSL modem into
the line and see what happens, but this is for installers so they can
determine if the line has a DSLAM available.

A bit of researching reveals that the ADSL modem can issue "R-Tones"
down the line and listen for the response from the DSLAM. Can anyone
point me towards a source that explains exactly what these "R-Tones" are
please? All I can find so far is that they "change phase every 16ms".
David...

--
___________________________________________
David Duffy        Audio Visual Devices P/L
Unit 8, 10 Hook St, Capalaba 4157 Australia
Ph: +61 7 38235717      Fax: +61 7 38234717
Our Web Site: http://www.audiovisualdevices.com.au
___________________________________________

2009\06\17@072635 by M. Adam Davis

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The phrase you'r looking for is "ADSL handshake" - this is the process
where the modem makes itself known and attempts to locate the dslam.

http://www.google.com/search?q=adsl+handshake

>From www.commsdesign.com/design_corner/showArticle.jhtml?articleID=16502772
:
"The initialization process consists of four major phases. The first
phase is a handshake using the G.994.1 or G.hs protocol. (G.hs is a
precursor to the G.992.1 specific initialization and is used by other
DSL and telecommunication devices.). The remaining three phases -
transceiver training, channel analysis, and exchange - are covered
directly in the G.992 standard and apply specifically to
standards-based ADSL networks.

The G.994.1 handshake is used to determine the nature and capabilities
of the endpoints (such as an ADSL modem) and to indicate which
protocol will be used for the remainder of the initialization. The
signaling method used for the handshake interchange is designed to be
robust to address channel characteristics that could be atrocious.
Biphase shift keying (BPSK) modulation is used to modulate multiple
single-tone subcarriers, all carrying the same data.

The subcarriers used are selected based on the typical impairments
likely to be present in a given global region. The handshake has
several possible variants, but, fundamentally, the two endpoints
exchange a message which contains information about the endpoint type,
and a number of related subparameters such as the frequency range and
number of DMT subcarriers supported. "

Looking up G.994.1 provides some useful information:

http://www.google.com/search?q=G.994.1

The actual spec is at the ITU, and may not be available without
membership in that organization, but further searches down that path
may yield what you're looking for, a simple tone you can send downt he
line that the dslam will respond to.

Have you listened to and recorded a real conversation between the
modem and the dslam?  I'd think that if you recorded an ADSL modem
while it's trying to connect on a line without a DSLAM and then played
it on a line with a DSLAM, you'd have a very simple method of
determining DSLAM availability.

-Adam

On Mon, Jun 15, 2009 at 11:30 PM, David Duffy
(AVD)<spam_OUTdavidTakeThisOuTspamaudiovisualdevices.com.au> wrote:
{Quote hidden}

>

2009\06\17@073414 by M. Adam Davis

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This paper:

http://www.iol.unh.edu/services/testing/dsl/training/An_ITU-T_G.994.1_Protocol_Analysis_Tool_For_ADSL.pdf

Appears to show how one can use matlab to analyze an ADSL handshake
session, so while I haven't read through it you may find it contains
enough information to get you started.

It leads to knowing that you have to implement DBPSK on 6 different
frequencies on the line.  Since during handshaking all 6 frequencies
transmit the same information, you may be able to get by with one
frequency.

There's lots of information out there about DBPSK.

If you search around enough for G.hs and G.994.1 you may find a freely
available preliminary specification from the ITU...

-Adam


On Wed, Jun 17, 2009 at 7:26 AM, M. Adam Davis<.....stienmanKILLspamspam@spam@gmail.com> wrote:
{Quote hidden}

>> -

2009\06\17@172152 by David Duffy (AVD)

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Hi Adam,
I hooked up an ADSL modem yesterday and found that it's putting out 3
frequencies simultaneously with a cadence of about 1.5 secs on, 1.5 secs
off.

As you say, the three tones seem to be required to cover all ADSL
variants. I've not measured them exactly yet, but in the spectrum mode
on my Bitscope, they appear to be roughly 39KHz, 72KHz and 107KHz.

Some of the documents I've found mention the tones "change phase every
16ms" but I don't really know what that means in this context.

I'll see if the standard is not too expensive. Hopefully it contains the
details I need.
David...

M. Adam Davis wrote:
> The phrase you'r looking for is "ADSL handshake" - this is the process
> where the modem makes itself known and attempts to locate the dslam.
>  
--

___________________________________________
David Duffy        Audio Visual Devices P/L
Unit 8, 10 Hook St, Capalaba 4157 Australia
Ph: +61 7 38235717      Fax: +61 7 38234717
Our Web Site: http://www.audiovisualdevices.com.au
___________________________________________

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