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

Tue Feb 5 20:16:39 EST 2008

see inline comments.

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 6:34:50 PM
Subject: Re: [Freeswitch-dev] TDM questions: OZ, signaling on digital spans

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From: freeswitch-dev-bounces at lists.freeswitch.org
[mailto:freeswitch-dev-bounces at lists.freeswitch.org] On Behalf Of Anthony Minessale

Sent: Tuesday, February 05, 2008
6:19 AM

To:
freeswitch-dev at lists.freeswitch.org

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

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.

 Okay,
so the driver-specific events get captured and translated… Makes sense.  Curious:
does this all happen at OSI layer 2?

I don't know exactly how it works.

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.

I think I get it.  Next question: on a
digital span that is set up in the <analog spans> section of
openzap.conf.xml – does zap_analog.c do the signal translation even
though the span is technically digital?  It almost sounds like
zap_analog.c is agnostic to the analog/digital nature of the span simply
because it’s receiving events from the driver via openzap… Last
question: if zap_analog.c is handling the translation of FXO/FXS signaling then
couldn’t we almost just copy zap_analog.c to zap_em.c and then redefine
the bit patterns that represent on-hook, ringing, hangup, etc.?  Or am I oversimplifying
again? ;)
Yes that is more or less correct.  In fact once you took out all the analog state machine etc from zap_analog.c
zap_em.c will be rather tiny just starting calls when it gets off hook and ending them when it gets onhook.

Thanks
for the explanation – I will distill this thread into it’s
components and add it to the wiki when we’re done.

-MC

P.S. –
the zaptel drivers do support E&M, E&M wink, etc.  I’ve used
E&M with the orange splat and connected to my NEC PBX with the exact same
tor2 and trunk-side T1 card. 

Anthony Minessale II

FreeSWITCH http://www.freeswitch.org/

ClueCon http://www.cluecon.com/

AIM: anthm

MSN:anthony_minessale at hotmail.com

GTALK/JABBER/PAYPAL:anthony.minessale at gmail.com

IRC: irc.freenode.net #freeswitch

FreeSWITCH Developer Conference

sip:888 at conference.freeswitch.org

iax:guest at conference.freeswitch.org/888

googletalk:conf+888 at conference.freeswitch.org

pstn:213-799-1400

----- 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

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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