Forums

300baud FSK over GSM

Started by Denis Gleeson September 29, 2004
In article <cjeg5d$15q$2@titan.btinternet.com>, 
nospamclayton@btinternet.com says...
> > "Frithiof Andreas Jensen" <frithiof.jensen@die_spammer_die.ericsson.com> > wrote in message news:cje5sc$g9b$1@newstree.wise.edt.ericsson.se... > > > > "Denis Gleeson" <dgleeson-2@utvinternet.com> wrote in message > > news:184c35f9.0409290304.1fed30b4@posting.google.com... > > > >> Although the GSM codecs distort the phase of audio tones they should > >> not effect the frequencies. > > > > I think you are doomed ;-) > > > > The GSM Codecs work by building a model of the human "speech machinery", > > determining the model parameters and the initial stimulus parameter and > > then > > send the parameter set across; not the actual data: The voice at the > > recieving end is merely a simulation of the speaker! The Codecs are > > speech-only!! > > It is a pretty good simulation, since one can normally recognise the voice > of the caller.
The technology is utterly jaw-dropping. Even at 1200 bits per second you can still recognize the speaker with one codec I've played with, but if they hold up a radio playing music to the microphone, you'll hear nothing but garbled mush. If GSM uses codebook-based compression, you can forget about the survivability of Bell 103 or anything like it. -- jm ------------------------------------------------------ http://www.qsl.net/ke5fx Note: My E-mail address has been altered to avoid spam ------------------------------------------------------
Hello everybody

Thanks everybody for your excellent input. Its given me lots of room
for thought.

Slight mistake in my description of the problem. Let me start again.

What I said was;
 
"Essentially the requirement comes from the fact that the equipment
at the far end can only communicate at 300baud FSK over the PSTN
network.
Our end will work with GSM and needs to communicate at the 300 baud
FSK."

What I should have said was;

Essentially the requirement comes from the fact that the equipment
at the far end can only communicate at 300baud FSK over the PSTN
network.
We are connecting a board to this equipment which will have a GSM
module (modem) on it.
Our end will be a standard PC with a PC modem(connected to PSTN)."



I had hoped(and I doubt my wisdom on this now) that we could place the
call with
the GSM modem and simply connect the PSTN output of the equipment to
the GSM
modules audio input. Of course there is some electronics involved
where the audio out of the GSM module is connected to the equipment's
PSTN output.

What I am thinking following the discussion here is that I need to
take
my 300baud FSK and convert it to a digital data stream (using
Microprocessor
and zero crossing detector) and use the data communication
capabilities of my
GSM modem to send the data to my PC modem. Then, of course, I will
also need
to take the returned data from my PC modem and convert it to FSK
(using Microprocessor and tone generator) for the equipment.

There will also be a requirement to do the tonal handshaking required
by the 300 baud FSK modem before communications begins. This can be
achieved with the
Microprocessor being used. 


Now that sounds like a lot of work but I think I could be guaranteed 
that it would function.

In all this I am assuming that when communicating with my PC modem
from
my GSM modem that the network is taking care of the change in
protocols
required. Is my understanding clear with this?

Many thanks again

Denis
_____________________________
http://www.CentronSolutions.com
[Note: F'up2 list cut down --- should have been done by OP]

In comp.arch.embedded Denis Gleeson <dgleeson-2@utvinternet.com> wrote:

> Essentially the requirement comes from the fact that the equipment > at the far end can only communicate at 300baud FSK over the PSTN > network. > We are connecting a board to this equipment which will have a GSM > module (modem) on it. > Our end will be a standard PC with a PC modem(connected to PSTN)."
Sorry, but I think this clarified version actually makes no more, possibly even less sense than the original. In a system of three devices, all of them being a modem, something can't be right --- you'll eventually end up with a loose end that is a telephone jack connected to nothing.
> What I am thinking following the discussion here is that I need to > take my 300baud FSK and convert it to a digital data stream (using > Microprocessor and zero crossing detector) and use the data > communication capabilities of my GSM modem to send the data to my PC > modem.
Yes. In the effect, you need a bona fide 300 Bd modem, and no, a GSM phone's audio in/out can't be used in that function. You may also need a PSTN line simulator --- a simple pair of cables won't do, IIRC. Then you'll need a micro that connects to both this modem and your GSM modem. That will *not* use any PSTN modem standard, though, but rather a digital-only transmission method (SMS, CSD, HSCSD, GPRS, ...). If you have that thing call a PSTN number in data mode, then the GSM provider will indeed have to convert from GSM digital to analog signals. Which thus drives the total number of modems involved in this scenario to a rather crazy 6: * 3 at the location of that remote device-thingy * 2 somewhere in the GSM provider's infrastructure * 1 at your PC's location Are you *sure* getting a PSTN land line to that remote device's location wouldn't be a whole lot less hassle? -- Hans-Bernhard Broeker (broeker@physik.rwth-aachen.de) Even if all the snow were burnt, ashes would remain.
I read in sci.electronics.design that Hans-Bernhard Broeker
<broeker@physik.rwth-aachen.de> wrote (in <2s2aumF1fugmvU1@uni-
berlin.de>) about '300baud FSK over GSM', on Thu, 30 Sep 2004:

>Even if all the snow were burnt, ashes would remain.
... together with oaks and beeches, I hope. -- Regards, John Woodgate, OOO - Own Opinions Only. The good news is that nothing is compulsory. The bad news is that everything is prohibited. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
Hi guys

The equipment in discussion is a piece of security equipment.
The requirement exists such that if the PSTN line goes down or is cut
then
the equipment must be able to continue to communicate. Hence our GSM
modem connected to the equipment at the remote site.

> Then you'll need a micro that connects to both this modem and your GSM > modem. That will *not* use any PSTN modem standard, though, but > rather a digital-only transmission method (SMS, CSD, HSCSD, GPRS,
Ok the modem we have is capable of making a CSD data call(not GPRS).
> ...). If you have that thing call a PSTN number in data mode, then > the GSM provider will indeed have to convert from GSM digital to > analog signals.
OK. This is an area that I am uncertain about. It appears that I would be at the providers mercy, dependant on what services he provides for GSM data communication. Would I be better off attempting GPRS communication?
> Which thus drives the total number of modems involved in this scenario > to a rather crazy 6: > * 3 at the location of that remote device-thingy > * 2 somewhere in the GSM provider's infrastructure > * 1 at your PC's location
Im not sure about these figures. Can you explain.
> * 3 at the location of that remote device-thingy
No? My GSM modem at "that remote device-thingy" can send/receive data and voice.
> * 2 somewhere in the GSM provider's infrastructure
OK. If the provider has this capability thats fine by me.
> * 1 at your PC's location
ok. There has to be one there at any rate. Many thanks for all your help. Denis linus@magrathea-telecom.co.uk (Linus Surguy) wrote in message news:<415af09c.602244343@10.0.0.3>...
> dgleeson-2@utvinternet.com (Denis Gleeson) wrote: > > >Hello All > > > >We are attempting to get 300baud FSK data over a GSM link. > > > >Ive seen different postings on this in the past and the concensus > >is that it may be possible given that its FSK and at a slow rate. > >Although the GSM codecs distort the phase of audio tones they should > >not effect the frequencies. > > > >Essentially the requirement comes from the fact that the equipment > >at the far end can only communicate at 300baud FSK over the PSTN network. > >Our end will work with GSM and needs to communicate at the 300 baud FSK. > > Given the earlier thread, this is not a credit card terminal by any chance!? > > Linus > > > -- > Linus Surguy - Magrathea Telecommunications Ltd. Wholesale and retail telephone > services. www.magrathea-telecom.co.uk www.uknumber.co.uk www.callthrough.co.uk > www.telesave.co.uk: UK 2.5p/1.5/1p South Africa 6p US,France,Germany,Eire 2.5p > Looking for VoIP? We will gateway SIP & IAX to and from the PSTN. > > > -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- > http://www.newsfeeds.com - The #1 Newsgroup Service in the World! > -----== Over 100,000 Newsgroups - 19 Different Servers! =-----
[F'up2 cut down. Again!]

In comp.arch.embedded Denis Gleeson <dgleeson-2@utvinternet.com> wrote:

> The equipment in discussion is a piece of security equipment. The > requirement exists such that if the PSTN line goes down or is cut > then the equipment must be able to continue to communicate. Hence > our GSM modem connected to the equipment at the remote site.
That covers the "why", but not the "how". You said originally that this device of yours can _only_ communicate at 300Bd over PSTN. I took that a face value, i.e. assumed it means that the PSTN line jack is the *only* external data connection this device has. Is it? If so, you will need another modem capable of doing 300 Bd over PSTN to attach to that, with a (simulated) PSTN sitting between the two, because that's the only kind of device your security thingy can talk to at all. Whether you implement that modem yourself, or buy a modem-on-a-chip is beside the point.
> OK. This is an area that I am uncertain about. It appears that I would > be at the providers mercy, dependant on what services he provides for > GSM data communication. Would I be better off attempting GPRS > communication?
Maybe, maybe not. GPRS is less widely available than CSD, and neither of them is unconditionally included in a plain vanilla GSM service contract. You'll have to check that out with the GSM providers in your place.
> > * 3 at the location of that remote device-thingy > No? My GSM modem at "that remote device-thingy" can send/receive data > and voice.
Yes, but as has been stated here by others, it can't serve as an analog modem to partner with the existing analog modem in the security device. And even if it coulde, it quite certainly wouldn't be able to service two independent communication links simultaneously. I.e. the GSM device can't act as a gateway from 300 Bd FSK to CSD, but only as a modem, which translates between RS232 (or alike) and CSD. -- Hans-Bernhard Broeker (broeker@physik.rwth-aachen.de) Even if all the snow were burnt, ashes would remain.
"Denis Gleeson" <dgleeson-2@utvinternet.com> wrote in message 
news:184c35f9.0409300315.20bbcc4@posting.google.com...
> Hello everybody >
snip
> > What I should have said was; > > Essentially the requirement comes from the fact that the equipment > at the far end can only communicate at 300baud FSK over the PSTN > network. > We are connecting a board to this equipment which will have a GSM > module (modem) on it. > Our end will be a standard PC with a PC modem(connected to PSTN)." > >
Please get someone to formalise the requirement.
> > Many thanks again > > Denis > __________________________
As I now understand it the remote equipment is a remote fixed security device (e.g. and alarm) and what you want is a back up service using GSM for an existing 300baud PSTN dialled connection. Presumably if the PSTN connection fails the call will divert to your board. There seems to be some confusion about whether the underlying data stream (at 30cps) is available or not. So your problems are: - 1. Instructing the GSM module to place the call. If the module contains a modem or is essentially a GSM phone that contains Hayes Compatible one (e.g. Motorola Time-Port connected by cable rather than IRDA) then this should be fairly straight forward, if not you will struggle. 2. Can you run 300FSK over GSM. I will presume that the near end will answer incoming calls and the modem will sync to whatever actually arrives. Alternatively you could connect another modem enabled GSM phone to a spare serial port. So you possible sequences are: - "dumb" option - 2 wire interface device fails to establish comms on main [PSTN] path and tries backup path. board notices presence of carrier from device board places voice call to PSTN number and simple connects the [polling?] device to the central station. 300FSK communication then takes place over the established voice channel. when carrier drops from device the board disconnects the call. "intelligent" option - serial interface device fails to establish comms on main [PSTN] path and tries backup path. board receives request from device on serial port (e.g. ATDxxxx...) at 300bps board wakes up GSM phone and passes command to phone phone calls central station and established GSM modem connection. serial communication is established via GSM modems at 9600baud, albeit that the available bandwidth is significantly underused. call is hung up under device control (ATH) If the device is not moving then call quality will be improved, and a proper aerial can be used.
Would a Nokia 22 be any use?
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=34165&item=5723529928

You put a sim card in it, and you can then connect a normal phone to it
It apparently does data too.

Sparks... 


Hi Guys

Thanks again for your responses.

Really Im down to the issue of GSM to PSTN modem communications at
this stage.

If I have a GSM modem using CSD to place a data call at the remote
equipment
and wish to communicate with my standard PSTN modem in my PC what
problems am I likely to encounter? Once both communicate at 9600 will
it work?

If I then want to be able to initiate the call from my PC will this
also work?

Denis




"Sparks" <this.is@invalid.invalid> wrote in message news:<415dd432$0$94916$bed64819@news.gradwell.net>...
> Would a Nokia 22 be any use? > http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=34165&item=5723529928 > > You put a sim card in it, and you can then connect a normal phone to it > It apparently does data too. > > Sparks...
R. Mark Clayton <nospamclayton@btinternet.com> wrote:

> "Frithiof Andreas Jensen" <frithiof.jensen@die_spammer_die.ericsson.com> > wrote in message news:cje5sc$g9b$1@newstree.wise.edt.ericsson.se... > > > > "Denis Gleeson" <dgleeson-2@utvinternet.com> wrote in message > > news:184c35f9.0409290304.1fed30b4@posting.google.com... > > > >> Although the GSM codecs distort the phase of audio tones they should > >> not effect the frequencies. > > > > I think you are doomed ;-) > > > > The GSM Codecs work by building a model of the human "speech machinery", > > determining the model parameters and the initial stimulus parameter and > > then > > send the parameter set across; not the actual data: The voice at the > > recieving end is merely a simulation of the speaker! The Codecs are > > speech-only!!
> It is a pretty good simulation, since one can normally recognise the voice > of the caller.
That's because the human brain is good at speach recognition... -- Mark Evans St. Peter's CofE Aided School Phone: +44 1392 204764 X109 Fax: +44 1392 204763