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Avatar of andrewjb
andrewjb🇬🇧

GSM modem on PABX
I'm trying to connect to a GSM modem, from a standard in-laptop (Windows) modem behind a PABX
i.e:

Laptop contains modem
Modem connected to PABX in the office
PABX linked to the outside world

GSM modem sitting on my desk
Connected to a COM port.

I am simply using Hyperterminal to connect to both modems. I ATDxxxx on the laptop, on it's modem, and dial the GSM modem.
I'm also connected to the GSM modem via Hyperterminal.

I get a 'RING' 'RING'.
I 'ATA' to answer

Both modems squark for a while... then I get NO CARRIER.


If I try it from outside, i.e. without going through the PABX, then it works.

If I try it with two GSM modems, it works.

i.e the PABS seems to be either the problem, or the straw that breaks the camels' back.


Anyone got any suggestions?

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Avatar of TheLANLordTheLANLord

Are you on a digital or analog line card?
Avatar of andrewjbandrewjb🇬🇧

ASKER

Hi - sorry - been ill!

I've tried both, though now assume that the digital should never even have got close.

Plus, it isn't necessarily GSM that causes the problems, that's just what I happen to have been playing with. If we have two ordinary modems going through certain exchanges, they often don't connect. Is there a more generic problem?
Avatar of TheLANLordTheLANLord

Could be too much noise on the line for the modem to establish its "handshake"
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Avatar of andrewjbandrewjb🇬🇧

ASKER

It 'sounds' OK - there's no audible noise if I simply dial the number and listen.. Are you aware of any way to determine what's going on when the negotiation is in progress? It'd be _really_ nice to have a debug log from the modem.... "Trying 56kbaud.. fail... trying 33kbaud..." etc. :-)
Avatar of TheLANLordTheLANLord

I haven't used AT commands in a while but I recall something where you can turn messages on. Also you might try disabling compression (v.90 or whatever you're using). Also try setting it to a slower speed. I believe there are AT commands for this as well. I wish I could tell you which ones but its been years since the dialup BBS days.
Avatar of andrewjbandrewjb🇬🇧

ASKER

I have been playing... \N0 and %C0 (or something like that) get rid of compression and stuff, and I've tried forcing down the baud rate.... The only AT commands that I've found relating to debug info. only seem to apply once you've got the CONNECT sorted - i.e. aren't much use here....

Looks like this is going to fail... !!
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Avatar of TheLANLordTheLANLord

there is a command to "use default settings" try that.
Avatar of andrewjbandrewjb🇬🇧

ASKER

&f or &f1 - yup!
Avatar of ☠ MASQ ☠☠ MASQ ☠

Try forcing the modem down to 33.6 with V.34
  Use "+ms=v34,1,1200,33600"

PABX's are notorious for this though as you lose high frequency and line quality throughout the system.  Some of the newer exchanges can be configured for modem support on selected extensions but even so you'll be lucky to get upwards of 40k

Have you tried AT &V1 to get the last connection log?

Also try AT I6 I7 I11

But fundamentally this is a transmission quality issue

M :o(

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Avatar of andrewjbandrewjb🇬🇧

ASKER

I'll give those a go next time I'm on site.
The lines we're using are analogue cards in the PABX i.e. are intended specifically to support modems etc. (as opposed to the digital cards which are used for most of the 'normal' phones in the office)
Avatar of andrewjbandrewjb🇬🇧

ASKER

OK - we think we've found the problem.
(Thanks goes to Melissa at Orange for her help!)

The company uses a call routing service to find the cheapest call. So a call to a mobile get routed - God knows where. If you use the magic prefix to prevent routing (i.e. just go through BT) it works. So that seems to be the problem. Yippee!
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Networking Hardware-Other

Networking Hardware-Other

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Networking hardware includes the physical devices facilitating the use of a computer network. Typically, networking hardware includes gateways, routers, network bridges, modems, wireless access points, networking cables, line drivers, switches, hubs, and repeaters. But it also includes hybrid network devices such as multilayer switches, protocol converters, bridge routers, proxy servers, firewalls, network address translators, multiplexers, network interface controllers, wireless network interface controllers, ISDN terminal adapters and other related hardware.