Re: 300baud FSK over GSM
From: R. Mark Clayton (nospamclayton_at_btinternet.com)
Date: 10/01/04
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Date: Fri, 1 Oct 2004 14:18:43 +0000 (UTC)
"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.
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