[cisco-voip] FXO port to Viking C-1000B door entry controller

[Wes Sisk (wsisk)]

IMHO this is where it gets interesting. We have to leave the ‘digital’ world and enter the ‘analog’ world. The “FXO” emits a signal, it travels across the wire, and might possibly be echoed back at the electrical level. Get the o-scopes ladies and gents. When the firmware of the FXO card reports this:

30278353: Jun 14 14:09:32.746: htsp_process_event: [0/1/1, FXOLS_CONNECT, E_DSP_SIG_0100]fxols_normal_battery
30278354: Jun 14 14:09:32.747: htsp_timer_stop2 fxols_disc_confirm

30278358: Jun 14 14:09:32.747: //3699561/56508068916F/VTSP:(0/1/1):-1:1:1/vtsp_process_event:
   [state:S_CONNECT, event:E_TSP_DISCONNECT_IND]

I suspect there is something *interesting* going on at the analog level. We start getting into pseudo-manchester-encoding territory here. Just how much “signal” is required to assume a 1 vs a 0?

Think of it as someone with ‘super hearing’ vs. someone that might have handled one too many chain saws or attended one too many metal concerts.

Given the disconnect occurs when you press any (assumption on my part) DTMF, then I wonder what analog signal is reflected back down the wire at the analog level. Reflection is a common byproduct of impedance mismatch. This is part of the reason high impedance interfaces are so valuable almost universally. This is where I buy my friend with an electrical degree a few beverages of his choice. http://www.cisco.com/c/en/us/support/docs/voice/ip-telephony-voice-over-ip-voip/64282-impedance-choice.html

How much knowledge or control do you have over all aspects of the physical implementation?  24km of 26AWG copper pair can have ‘interesting’ electrical properties, let alone what ‘actually’ happens over that 24km.

Impedance (AC resistance at various frequencies) may not be the same as Resistance (DC resistance). https://en.wikipedia.org/wiki/Electrical_impedance . This gets into the physical details ,and sometimes anomalies, of the environment.

Hat tip Ryan for brining up the i(impedance) vs r(resistance) angle.

-w

On Jul 1, 2016, at 11:52 AM, Ryan Huff <ryanhuff at outlook.com<mailto:ryanhuff at outlook.com>> wrote:

Only after the local-end generates an oscillated sine wave (in this case, a DTMF event), the call is somehow terminated.

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