+/* HOW TO audio?
+
+Audio flow has two ways:
+
+* from channel to the upper layer
+ -> sound from mISDN channel
+ -> announcement from vbox channel
+
+* from the upper layer to the channel
+ -> sound from remote channel
+ -> sound from asterisk
+
+Audio is required:
+
+ -> if local or remote channel is not mISDN
+ -> if endpoint is linked to asterisk
+ -> if call is recorded (vbox)
+
+
+Functions:
+
+* PmISDN::txfromup
+ -> audio from upper layer is buffered for later transmission to channel
+* PmISDN::handler
+ -> buffered audio from upper layer or tones are transmitted via system clock
+* mISDN_handler
+ -> rx-data from port to record() and upper layer
+ -> tx-data from port (dsp) to record()
+* VboxPort::handler
+ -> streaming announcement to upper layer
+ -> recording announcement
+* VboxPort::message_epoint
+ -> recording audio message from upper layer
+
+
+
+*/
+