Modem and/or ADSL Daughter card

GNU Radio
1

Hello,

I was just wondering if anyone has used GNURadio for any non-radio
applications, specifically phone line modulation/demodulation.

It shouldn’t be too difficult to design a daughter board for the USRP to
sample phone line voltage (with an appropriate line interface circuit of
course). After that it should be rather easy in GNURadio to generate
DTMF
tones for dialing, and modem tones for data communication.

Any thoughts?

Mike

2

It shouldn’t be too difficult to design a daughter board for the USRP to
sample phone line voltage (with an appropriate line interface circuit of
course). After that it should be rather easy in GNURadio to generate DTMF
tones for dialing, and modem tones for data communication.

DSL is a good example – it would be great for students, technicians,
and researchers to be able to study the performance of DSL modulation
techniques, and try making improvements.

You can probably add a telco block to the low frequency tx and rx
boards without trouble.

Don’t forget to support voice communication as well as DSL and modem!

(Hmm, anyone want to reimplement the Telebit modem for fun? It did
half-duplex communication on hundreds of ~2-bit-per-second carriers,
avoiding the frequencies that didn’t get through cleanly on your
particular phone network, and flipping back and forth from TX to RX
many times a second. There’s probably an acronym for it now.)

Hacking up an interface and code for 10 megabit Ethernet on coaxial
cables would be fun too. That could be done on standard TX and RX
boards, probably by just wiring them to the “transceiver cable” of an
Ethernet transceiver. Ethernet modulation is pretty straightforward.

For true incestuous pleasure, we could code up the signal modulation
and electrical interface for USB 1.1.

John
3

John G. wrote:

(Hmm, anyone want to reimplement the Telebit modem for fun? It did
half-duplex communication on hundreds of ~2-bit-per-second carriers,
avoiding the frequencies that didn’t get through cleanly on your
particular phone network, and flipping back and forth from TX to RX
many times a second. There’s probably an acronym for it now.)

Yes, it’s called OFDM :slight_smile: :slight_smile:

For true incestuous pleasure, we could code up the signal modulation
and electrical interface for USB 1.1.

It just figures that “true incestuous pleasure” would involve a
daughtercard.

Ok, ok, I’ll stop now…

4

On Thursday 04 May 2006 14:01, Michael M. wrote:

It shouldn’t be too difficult to design a daughter board for the USRP to
sample phone line voltage (with an appropriate line interface circuit of
course). After that it should be rather easy in GNURadio to generate DTMF
tones for dialing, and modem tones for data communication.

Any thoughts?

Yeah, just one: 47 CFR Part 68. As in FCC regulations on items that can
be
connected to the PSTN. A part 68 interface card that could be used to
provide an interface to an LFRX/LFTX pair would be ideal.

Marcus can elaborate, being that he is employed in that field.

Lamar Owen
Director of Information Technology
Pisgah Astronomical Research Institute
1 PARI Drive
Rosman, NC 28772
(828)862-5554
www.pari.edu

5

On Thu, May 04, 2006 at 02:01:31PM -0400, Michael M. wrote:

Any thoughts?
Mike

A simple DAA (Data Access Arrangement) plus the LF daughterboard would
probably work. There are two chip DAA’s available. Been a while
since I looked, but I think that Crystal Semiconductor used to make
them. Google “solid state daa” There’s also the classic Midcom
transformer plus a couple of other discretes. These things are tricky
to get right. You definitely want to find somebody’s reference design
and use that. Or use the pre-approved chipset.

The USRP is pretty much thermonuclear overkill for this application.
You could just use a sound card with a DAA. That’s how all the
“soft-modems” work. IIRC some of the the AC97 codecs support a second
channel for telecom (modem) apps.

Eric

6

Lamar Owen wrote:

Yeah, just one: 47 CFR Part 68. As in FCC regulations on items that can be
connected to the PSTN. A part 68 interface card that could be used to
provide an interface to an LFRX/LFTX pair would be ideal.

Marcus can elaborate, being that he is employed in that field.

Just because I work for a “traditional” telecom equipment vendor doesn’t
mean I know anything about “traditional” telecom equipment :slight_smile:

But I strongly suspect that you’d need to get “type approval” of any
device
you plan to connect to the PSTN.

Unlike the airwaves, where ham radio operators are free to put
experimental
devices on the air, within their allocated bands, without type
approval.

[That’s not to say that hobbiests haven’t been connecting “experimental”
devices to the
PSTN for decades. It’s just that if you want to bring an actual
device to “market”, the
feds will land pretty hard if that device isn’t type approved for that
particular application].

–
Marcus L. Mail: Dept 1A12, M/S: 04352P16
Security Standards Advisor Phone: (ESN) 393-9145 +1 613 763 9145
Strategic Standards
Nortel Networks [email protected]

7

Quoting M. Milner [email protected]:

Any thoughts?

Mike

I’d like to mention that there are currently a number of soft modems
out on the
market which occasionally have linux drivers for the audio portion. I
don’t
know the details of getting them to “pick up” the phone and such.
Asterisk
(www.asterisk.org) has for a long time supported a card with the Intel
IA-92
chipset. (I have one of these cards – it worked OK, but not great esp
with odd
phone line conditions, it could get confused as to whether it was on
hook or
off, or whether the other side had hung up.) I believe that libtiff (or
some
other unrelated-sounding library) implements V.34 (? the fax one)
modulation. I
would guess that you can also use [AM]C97 modems to such an end, though
I
haven’t actually done that myself. Keep in mind that these cards are
usually
designed for “FXO” signalling, i.e. they can’t power other devices, and
rely on
a line voltage to be supplied externally.

While you could use these cards for regular use (i.e. under the 64kbit
ISDN
channel assumption), they of course can’t be used for ADSL and the like,
since
that uses a much higher frequency than the cards support.

(Not a USRP solution, but GNURadio could be made to work with these; I
bet the
required interface is very similar to what Asterisk uses.)

-Ilia

8

On Wed, May 10, 2006 at 07:24:07PM -0700, Eric B. wrote:

The USRP is pretty much thermonuclear overkill for this application.
You could just use a sound card with a DAA. That’s how all the
“soft-modems” work. IIRC some of the the AC97 codecs support a second
channel for telecom (modem) apps.

Here is an example that uses a sound card,
http://www.araneus.fi/audsl/.

It would be neat if there were the PC audio-out equivalents of the iGo iTips power adapter plugs, with plugs that adapt audio-out to media such as AC powerline, phone wire, Cat5, and television cable. A suitable software modem would adapt to whatever medium was on hand, adding a new dimension to "ad hoc networking."

Dave

–
David Young OJC Technologies
[email protected] Urbana, IL * (217) 278-3933