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

[Anthony Minessale]

It's a trickle-up methodology.

The low level driver interprets the voltage change
or the bits flipping or whatever and turns it into
driver specific events which in turn are changed
into openzap specific events by the zap_<whatver>.c
zap_zt.c, zap_pika.c etc.  when you apply a signaling
to a span in the mod_openzap config the signal specific
module will monitor the span for these events and decide
what they mean in reference to whatever signaling type
it represents.

so

voltage changes on zaptel card

zaptel specific ring event is caught by zap_zt.c

openzap ring event is queued on the channel and
event data flag is set on the span.

zap_analog.c sees the event data flag and gets the 
ring event on the channel

ring event is sent up to mod_openzap who starts a channel
in freeswitch.


so implementing another sig type like e&m just requires
zap_em.c and proper interpretation of the bit shifting 
events.  I belive zaptel drivers and pika 
can do it but I don't know if the native sangoma can yet.



 
Anthony Minessale II

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----- Original Message ----
From: Michael Collins <mcollins at fcnetwork.com>
To: freeswitch-dev at lists.freeswitch.org
Sent: Tuesday, February 5, 2008 1:39:13 AM
Subject: [Freeswitch-dev] TDM questions: OZ, signaling on digital spans





 
 

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Guys,
 

  
 

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

  
 

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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