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I have a cisco router, and I need an emergency backdoor to it from a remote site in case the internet connection is down (for any reason). I've connected a good old analog modem to cisco console port by serial cable and I have the similar modem on the client side, connected to a PC and local PSTN line. The problem is: there's no PSTN line on server side. So I've setup a small asterisk box with GSM and FXS modules and a simple route to forward a call from one port to another. The call is passing just fine and server-side modem picks it up. But after a minute or so of trying to handshake it hangs up. Any ideas?

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  • There are a number of possible reasons - and you have not told enough detail to comment on any one of them. You need to have e.g. "PPP" (Point to point protocol, or similar) drivers active on both ends - then do what is necessary to route IP-packets through there. Whether that is close to possible on a router without open source software is questionable.
    – Hannu
    Commented Jan 17, 2017 at 17:40
  • Turn logging all the way up and find out what's going on during that minute or so.
    – David Schwartz
    Commented Jan 17, 2017 at 18:39
  • superuser.com/a/748163/1686
    – grawity_u1686
    Commented Jan 18, 2017 at 5:07

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You will have 9600 bits connection only in case if ALL following is true

1) you have internet(voip not work without it,right?)

2) ALL path is ulaw or alaw codecs. Any compressed codec will loose data(all codecs are lossly), which will result loss of carrier on modem

3) Fax detect have be turned off(in most cases modem connection will be detected as fax).

There is almost no chance get 14000, any other bitrate is theoreticaly impossible, codecs allow do only voice,not all frequencies and codecs do only 8khz.

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  • No internet, no VoIP. Just GSM channel and FXS channel. The modem on client side makes a call via local PSTN line, dialing a GSM SIM card number. This call comes to GSM module with a SIM card inside PBX box, which forwards it to modem, connected to FXS port. The goal is to get the cisco shell at the remote PC as it was done quite frequently in good old dial-up times. All would be well if I had a conventional PSTN line on cisco side, but there's none, such stuff becomes a rarity.
    – user686946
    Commented Jan 18, 2017 at 19:27
  • I am sorry, via gsm codec can work only very old modem with 400bod support. GSM operator use gsm codec.
    – arheops
    Commented Jan 19, 2017 at 0:01
  • As other option you can create software modem(yourself), with similar max bandwidth. For example send every 0.1 sec different tones, based on tone decode information, will be~200bod modem.
    – arheops
    Commented Jan 19, 2017 at 0:02

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