[asterisk-ss7] Link inactivity detection with dahdi based MTP2

[Matthew Fredrickson]

Marcelo Pacheco wrote:
> Hi Matt and asterisk-ss7 members,
> 
> Looking at the dahdi-linux code, there seems to be an important feature
> lacking on the dahdi mtp2 feature:
> 
> Should the mtp channel go down, without the E1/T1 span go down (no
> alarms), there's no notification sent to the application (usually asterisk).
> 
> 
> I suggest an inactivity timer be implemented on dahdi, so if no new mtp2
> messages get received over a 100ms period, a HDLC Abort message (or any
> other specific message) gets send to userland. This way should libss7
> receive say, 2 messages in close proximity it will consider the link
> dead and initiate realignment.
> 
> Additionally should this error condition be raised to userland, upon
> receiving a valid repeated message, it would be sent to userland to
> indicate things are back to normal.
> 
> Without this, technically libss7 isn't compliant with a few of Q.7xx
> specs. I've seen real world situations where lack of this feature would
> cause serious problems.
> 
> Specially this would be a serious issue with E1 links between Asterisk
> and a TDM switch, where the signalling link is switched to a STP using a
> semi permanent call. Should the STP or the link between the TDM switch
> and the STP die, the only available indication would be that the
> signalling link would go dead (with perhaps one HDLC Abort or CRC error
> being detected, should the link go down preciselly between FISUs, then
> no error at all might be detected). This is very common in Brazil, the
> largest carriers use this layout exclusively on interconnects.
> 
> 
> Matt, what do you think ? Do you have a better suggestion than an Abort
> message ?

Actually, this sounds like a very good idea.  I think that a better 
indication might be a specific event related to this though, rather than 
trying to overload abort events (since you can legitimately have abort 
events on a link).  I will think about this and see if I can implement 
something like this for the next release of DAHDI.

Matthew Fredrickson
Digium, Inc.