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 Posted: Feb 28, 2012 - 10:40 PM |
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Joined: Apr 25, 2009
Posts: 30
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Hi,
I need to build a project which will use a GSM modem in order to offer a normal PSTN phone line. Any idea of how I would implement a PSTN line? (I have already built the part of GSM, Calls etc)
Thanks in advance
Theodore |
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Posted: Feb 28, 2012 - 11:17 PM |
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Joined: Aug 04, 2002
Posts: 1705
Location: Brisbane, Australia
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Theodore,
Go and read up on the signalling and 4 wire interface used in the PSTN exchange. The interface ( repeater) will come out of Your understanding of PSTN standard. ( the project is not the building perse, its the understanding of the standard interface.
Theodore talk to Your local PSTN provider company and ask them for the 4 wire , signaling and 4 wire to 2 wire interface standards.
This should get You going. |
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Posted: Feb 28, 2012 - 11:38 PM |
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Joined: Dec 30, 2004
Posts: 8774
Location: Melbourne,Australia
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Posted: Feb 29, 2012 - 06:23 AM |
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Joined: Apr 25, 2009
Posts: 30
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Thank you both.
I am going to make some research based on your comments and I will be back with more specific questions. |
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Posted: Feb 29, 2012 - 06:27 AM |
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Joined: May 02, 2007
Posts: 3016
Location: Nieuwegein, Netherlands
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| note that each country probably has its own telephone standard and thus as such you probably need to do things to make the stuff work on a normal telephone line in different countries. |
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Posted: Feb 29, 2012 - 11:02 AM |
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Joined: Jul 18, 2005
Posts: 62299
Location: (using avr-gcc in) Finchingfield, Essex, England
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| Note that there are a lot of ATA adapters already available. They interface a POTS phone to TCP/IP. I imagine if you study the designs of some of those it would be close to what you want to do as I guess that across the GPRS/EDGE/whatever you want to carry SIP/RTP to do VoIP? |
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Posted: Feb 29, 2012 - 01:06 PM |
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Joined: Apr 25, 2009
Posts: 30
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clawson wrote:
Note that there are a lot of ATA adapters already available. They interface a POTS phone to TCP/IP. I imagine if you study the designs of some of those it would be close to what you want to do as I guess that across the GPRS/EDGE/whatever you want to carry SIP/RTP to do VoIP?
Actually I don't want to use nothing more than GSM functionality of my GSM/GPRS modem.
The final product will be something like this:http://www.alibaba.com/product-gs/523662600/1_sim_card_1_port_GSM.html
It will be used as an emergency call device |
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Posted: Feb 29, 2012 - 01:39 PM |
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Joined: Jul 18, 2005
Posts: 62299
Location: (using avr-gcc in) Finchingfield, Essex, England
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I gotta ask but what possible advantage does that $50 device have over a $50 mobile phone? You still have to stick a SIM card in it and you don't even get a dial-pad or a pretty LCD display  |
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Posted: Feb 29, 2012 - 02:26 PM |
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Joined: Apr 25, 2009
Posts: 30
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clawson wrote:
I gotta ask but what possible advantage does that $50 device have over a $50 mobile phone? You still have to stick a SIM card in it and you don't even get a dial-pad or a pretty LCD display
The project is intended to be used in places with weak signal (or without signal at all). This is the reason why we need to offer the phone line. The GSM is going to be mounted in a full signal environment and from that point we will send a cable (phone line) 10-30 m away. |
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Posted: Feb 29, 2012 - 02:39 PM |
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Joined: Jul 18, 2005
Posts: 62299
Location: (using avr-gcc in) Finchingfield, Essex, England
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| Oh I see. If it's just a question of 10-30m would an external cellphone antenna not suffice? |
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Posted: Feb 29, 2012 - 05:06 PM |
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Joined: Dec 29, 2007
Posts: 499
Location: Ontario, Canada
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Posted: Feb 29, 2012 - 05:28 PM |
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Joined: Jul 18, 2005
Posts: 62299
Location: (using avr-gcc in) Finchingfield, Essex, England
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Posted: Feb 29, 2012 - 07:06 PM |
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Joined: Apr 25, 2009
Posts: 30
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Sorry I had to clear that 'm' comes from 'metre' (indeed we (Greece) use SI).
Quote:
Oh I see. If it's just a question of 10-30m would an external cellphone antenna not suffice?
If you mean an antenna with 10-30 metres cable then it is an expensive solution (too expensive cable and too much lose of signal). If you mean just to use a stong external antenna then no this is not a working solution (the device should be able to work well even in 3rd basement of a building) |
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Posted: Feb 29, 2012 - 08:50 PM |
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Joined: Dec 30, 2004
Posts: 8774
Location: Melbourne,Australia
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| You do realise these devices are commercially available? I don't know who makes them, but inMalaysia i used to see a lot of them used in new installations. Just a box on the wall with an antenna and a wall wart. They wired up to a normal telephone. |
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Posted: Feb 29, 2012 - 09:08 PM |
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Joined: May 02, 2007
Posts: 3016
Location: Nieuwegein, Netherlands
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Quote:
Sorry I had to clear that 'm' comes from 'metre' (indeed we (Greece) use SI).
not to be rude, but where could we have seen that before?
I'm also in the Si lane and had guessed meters.
If you are only going to deploy in the UK then you only need to make sure you comply with the greek telephone standard. If you intend to use it in more countries then you need to make sure that you support all possible connected telephones.
I have made such product a long time ago and then we decided to use a standard slic interface IC as that could cope with a lot of different telephones. From the head a later version even had some sort of auto detection that was able to adapt to the phone connected.
have had a quick look in my archives, but could not find a part number. from that IC.
also a possible problem will be the number of phones connected to your line. (answering machine/automatic fax/ multiple phones in parallel.)
it will be a challenge. |
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Posted: Mar 01, 2012 - 09:20 AM |
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Joined: Jul 18, 2005
Posts: 62299
Location: (using avr-gcc in) Finchingfield, Essex, England
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Quote:
If you are only going to deploy in the UK then you only need to make sure you comply with the greek telephone standard. If you intend to use it in more countries then you need to make sure that you support all possible connected telephones.
No you don't - there's no intention that this device be connected to PSTN landlines. All the conformity regulations required for phone/modem equipment on POTS is simply to prevent you device doing something stupid like injecting 240V into the telephone circuit and killing one of their engineers at the exchange.
This proposed device is going to simulate POTS on the side that connects to existing analog phones then use GSM/GPRS over the air on the outbound side. I suppose the only homolgation that might be required will concern the mobile network operator (though they don't seem too fussed what kind of device injects RF into the airwaves as long as it doe not "jam" their existing signals). |
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Posted: Mar 01, 2012 - 12:33 PM |
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Joined: Apr 25, 2009
Posts: 30
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meslomp wrote:
...also a possible problem will be the number of phones connected to your line. (answering machine/automatic fax/ multiple phones in parallel.)
First version should be able to work with one phone. Later on I am sure we have to expand it for more devices.
clawson wrote:
...This proposed device is going to simulate POTS on the side that connects to existing analog phones then use GSM/GPRS over the air on the outbound side.
This is exactly what I have to implement!
clawson wrote:
... I suppose the only homolgation that might be required will concern the mobile network operator (though they don't seem too fussed what kind of device injects RF into the airwaves as long as it doe not "jam" their existing signals).
In the side of outside communication I am going to use the Telit GE865 modem. Any transmitted airwave will be created by the modem so I don't think rest devices will inject any RF signals into the GSM's airwaves. |
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Posted: Mar 01, 2012 - 01:04 PM |
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Joined: Jul 18, 2005
Posts: 62299
Location: (using avr-gcc in) Finchingfield, Essex, England
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| But what kind of data are you planning to send? I assume you are going to digitise the voice input from the phone and send by RTP using VoIP or are you really intending to make a voice rather than a data connection. Also the old phone will presumably be outputting DTMF when it dials so you'll need a DTMF decoder to work out what the destination number is. If you are doing VoIP you'll also need a DAC to convert the digitised audio back to analog audio to play out into the analog handset's speaker. |
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Posted: Mar 01, 2012 - 02:11 PM |
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Joined: Apr 25, 2009
Posts: 30
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clawson wrote:
But what kind of data are you planning to send? I assume you are going to digitise the voice input from the phone and send by RTP using VoIP or are you really intending to make a voice rather than a data connection...
I can't send data (we don't have to demand SIM cards with enabled GPRS functionality) so I have to use only GSM services.
clawson wrote:
Also the old phone will presumably be outputting DTMF when it dials so you'll need a DTMF decoder to work out what the destination number is...
Yes I will (at least I think so!!!). I found some DTMF decoders so far but I didn't make up my decision so far. |
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Posted: Mar 01, 2012 - 02:29 PM |
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Joined: Jul 18, 2005
Posts: 62299
Location: (using avr-gcc in) Finchingfield, Essex, England
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| Ah you learn something new every day I see that if you send ATD1234 to a GSM modem it dials 1234 and sets up a data connection to it (presumably assuming the other end is a modem) but if you send ATD1234; it dials the number as a voice call. |
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