Loop-start And Ground-start

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Asterisk Users 2 Comments

Hi list

First of all could you please explain loop-start and ground-start for me? What are they used for?

Next, I have the following configurations:

dahdi-channels.conf :

context=pstn-channels signalling=fxs_ks channel=>130
context=phone-channels signalling=fxo_ks channel=>127

chan_dahdi.conf :

[channels]
cidsignalling=dtmf cidstart=dtmf

signalling=fxo_ls pulsedial=no usecallerid=yes context=pstn-channels channel=>130

signalling=fxs_ls context=phone-channels channel=>127

extensions.conf :

[pstn-channels]
exten=>_.,1,Dial(DAHDI/127/${EXTEN})

[phone-channels]
exten=>_.,1,Dial(DAHDI/130/${EXTEN})

/dahdi/system.conf fxoks5-128
fxsks9-132

I can hear the dial tone from the phone with these configurations. As soon as I comment “signalling” type in chan_dahdi.conf, dial tone comes up. What’s the problem with these configurations?

2 thoughts on - Loop-start And Ground-start

  • Loop start is the simplest type of analog phone signalling. It’s used to connect an FXS (like a central office) with a very simple FXO device (like a handset). In this signalling mode, the FXS device basically sends a DC
    current down the line to the FXO. When the FXO device (handset) goes offhook, it shorts both wires (tip and ring). The FXS device will then detect this “offhook” state from the DC current that returns on “the loop”.

    Kewl start is Asterisk’s name for Loop Start with the “disconnect supervision” feature. This is most likely what you want to select over regular loop start. http://en.wikipedia.org/wiki/Disconnect_supervision

    Ground start is typically used to connect an FXS (like a central office) to something more complicated like a PBX. In this signalling mode, both devices are able to provide a DC current on the line. This affords some extra line supervision options that are handy to have with a PBX. From what I understand, this is a pretty rare mode these days …

    Signalling is determined by the device you want to signal TO. So if channel
    130 is connected to the pstn, you likely want fxs signalling. This would mesh with how you have dahdi-channels.conf and /etc/dahdi/system.conf.

    Same here as above. If channel 127 is connected to phones (fxo) you probably want fxo signalling here. Also, dahdi-channels.conf is generated by dahdi’s best guess and is typically intended to be #included at the bottom of chan_dahdi.conf. Not sure if you left that out on purpose.

  • Thanks for your help. It’s very useful. I have change my chan_dahdi.conf file as you said:
    >