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56 Dialogic
®
Global Call IP Technology Guide — November 2007
Dialogic Corporation
IP Call Scenarios
Figure 12. SIP Early Media, Calling UA Offers SDP
3.1.3.4 SIP SlowStart Mode (Calling UA Answers SDP)
When a SIP application sets the optional SlowStart parameter, it specifies that the INVITE message
it sends will not contain SDP, so that it is up to the called UA to offer SDP which the calling UA
will subsequently answer. In SIP terminology, this is known as delayed offer.
Figure 13. SIP Early Media, Calling UA Answers SDP
gc_MakeCall( )
Calling
App
Called
App
IP
CCLib
IP
CCLib
GCEV_ALERTING
GCEV_CONNECTED
GCEV_EXTENSION
(EXTID_MEDIAINFO - TX)
GCEV_EXTENSION
(EXTID_MEDIAINFO - RX)
GCEV_EXTENSION
(EXTID_MEDIAINFO - RX)
GCEV_EXTENSION
(EXTID_MEDIAINFO - TX)
GCEV_DETECTED/OFFERED
GCEV_ANSWERED
GCEV_ACCEPT
gc_AcceptCall( )
gc_AnswerCall( )
Unidirectional media
streams are enabled
before each respective
GCEV_EXTENSION
(MEDIAINFO) event
is dispatched upon
receiving SDP answer.
Unidirectional media
streams are enabled
before each respective
GCEV_EXTENSION
(MEDIAINFO) event
is dispatched after
answering SDP.
INVITE
("m=" , "m=" , ...)
m
n
180 RINGING
("m=" , "m=" , ...)
m
n
200 OK
("m=" , "m=" , ...)
m
n
Pre condition: Both line devices are IDLE. Called party has executed gc_WaitCall().
"FastStart" is enabled.
Post condition: Call is connected.
If the 180 (or 183)
response does not
include SDP answer,
media cannot be
enabled until 200 OK.
ACK
If only one Rx coder is
specified in offer,
streaming is enabled
at time of offer.
gc_MakeCall( )
Calling
App
Called
App
IP
CCLib
IP
CCLib
GCEV_ALERTING
GCEV_CONNECTED
GCEV_EXTENSION
(EXTID_MEDIAINFO - RX)
GCEV_EXTENSION
(EXTID_MEDIAINFO - TX)
GCEV_EXTENSION
(EXTID_MEDIAINFO - RX)
GCEV_EXTENSION
(EXTID_MEDIAINFO - TX)
GCEV_DETECTED/OFFERED
GCEV_ANSWERED
GCEV_ACCEPT
gc_AcceptCall( )
gc_AnswerCall( )
Unidirectional media
streams are enabled
before each respective
GCEV_EXTENSION
(MEDIAINFO) event
is dispatched upon
receiving SDP offer.
180 Ringing
200 OK
("m=" , "m=" , ...)
m
n
INVITE [no SDP]
ACK
("m=" , "m=" , ...)
m
n
Pre condition: Both line devices are IDLE. Called party has executed gc_WaitCall().
"SlowStart" is enabled.
Post condition: Call is connected.
If only one Rcv codec
was specified in SDP
offer, unidirectional
receive stream is
enabled when SDP
is offered.
Unidirectional media
streams are enabled
before each respective
GCEV_EXTENSION
(MEDIAINFO) event
is dispatched upon
receiving SDP answer.