[asterisk-dev] Doubt Pseudo channel

[john at spectross.com]

Hi,

I was trying to understand the chan_zap/chan_dahdi code.

I have a few doubts, listed below:

What is the concept of using sub-channels (SUB_REAL/SUB_CALLWAIT/SUB_THREEWAY): Well I understand each of these sub-channels when allocated would have a corresponding owner ast_channel (generic to rest of Asterisk).
I also understand 
SUB_REAL -> ast_channel currently interacting with the phy channel.
SUB_CALLWAIT -> ast_channel waiting on the phy. channel to be available.
SUB_THREEWAY -> ast_channel held on MOH (during transfers etc...).

What I do-not understand is why do these sub-channels need a "fd" to "/dev/zap/pseudo", well one reason could be to aid conferencing but when I see instances like the one below (sending RBT to a pseudo-channel) it doesn't appear that pseudo channels are merely for conferencing:

/* Make ring-back */

if (tone_zone_play_tone(p->subs[SUB_CALLWAIT].zfd, ZT_TONE_RINGTONE))

ast_log(LOG_WARNING, "Unable to generate call-wait ring-back on channel %s\n", ast->name);



above are lines from zt_call - chan_zap (Asterisk 1.4.18).

Could someone plz. help me understand the concept of pseudo (sub-)channel.


Regds.
John
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