[Freeswitch-dev] TDM questions: OZ, signaling on digital spans

[Michael Collins]

Guys,

 

You better run for cover because Mikey has been reading technical books
again and now he has questions! :-)

 

First, I was reading in Matthew Gast's T1 - A Survival Guide about
channelized T1's and channel associated signaling.  Just to make sure I
am understanding it correctly - CAS and robbed bit essentially refer to
the same thing on a channelized T1 don't they?  The bits that are robbed
include the signaling bits - AB or ABCD depending on whether you have SF
or ESF.  Please confirm if I am understanding this correctly.

 

Second, aren't FXO and FXS signaling on a digital channel really just a
matter of interpreting the meanings of the A and B bits being high or
low?  Obviously there isn't an actual loop start or ground start on a
digital channel, but rather just a nomenclature for deciding that a
particular combination of A and B bits being high and/or low has a
meaning, like A high + B low means on-hook in one signaling type but
could mean ringing in another signaling type. Again, correct me if I'm
wrong...

 

Third, if FXO/FXS signaling is just a matter of manipulating the AB bits
in CAS, then shouldn't it be relatively easy to add E&M signaling since
that is essentially just another set of meanings for each AB bit
pattern?  Or am I oversimplifying it?

 

Last question - how does OZ, in zap_zt.c, determine the signal start
type?  (E.g. see code starting at line 87.)  AFAIK it is not set in
openzap.conf.xml nor in openzap.conf.  Does it get that info from the
zaptel driver?  I know the start type is set in zaptel.conf, I just
didn't know if the OZ was smart enough to figure out the start type
without requiring the user to specify it in a conf file.

 

Thanks for letting me ask all these questions!  I'll do my best to
update the wiki as appropriate as I get the answers...

 

-MC

 

 

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