Network Connectivity - E1 CAS signalling

Table of Contents

dahdi_cfg displays errors for channel 16 when configuring an E1 with CAS signalling

For an E1 with CAS framing, as opposed to a T1 using a RBS signalling type, the robbed bit always comes from channel 16 instead of that particular voice channel.

This means that channel 16 must not have any kind of signalling assigned in order for the E1 span to function properly.

Starting with DAHDI 2.4.0, dahdi_cfg will display an error if channel 16 on an E1 CAS span is assigned a signalling type. Prior to 2.4.0, no error would be displayed but Asterisk would have issues with hangups and other call problems whenever any calls were passed on the span.

For example, say you are getting just 18 voice channels from your provider using CAS signalling. You try to configure it like so:

This would result in an error message that will look like this:

Selected signaling not supported

• FXS signaling is being used on a FXS interface (use a FXO signaling variant).

• RBS signaling is being used on a E1 CCS span.

• Signaling is being assigned to channel 16 of an E1 CAS span.
  
  

The important message in this particular case is the final one, "Signaling is being assigned to channel 16 of an E1 CAS span".

This message is telling you that some kind of signalling (the specific type doesn't matter) is being applied to the 16th channel of an E1 CAS span. Channel 16 needs to be left unconfigured in this case in order to function properly.

Here is an example of a correct system.conf for this on a dual-span card where the full range of channels is used.