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Global Call IP Technology Guide — November 2007 55
Dialogic Corporation
IP Call Scenarios
3.1.3.2 H.323 SlowStart Mode
Many factors affect the opportunities for early media in H.323 SlowStart mode.
• Unless both endpoints support what is referred to as “early H.245”, early media is not possible
in the H.323 SlowStart connection mode.
• When a Global Call application specifies the optional SlowStart mode, or when one endpoint
does not support H.323 fastStart mode, media transmission cannot begin at either endpoint
until the remote endpoint has sent its Ack to the appropriate OpenLogicalChannel command.
• If the OLCAck that either endpoint receives contains a FlowControlToZero flag parameter that
is true, media transmission from that endpoint is not be enabled until a subsequent
FlowControl message is received.
• If the calling endpoint sets the MediaWaitForConnect element in the Setup message, the called
endpoint does not enable media transmission until the Connect message is sent.
Figure 11. H.323 Early Media, SlowStart Mode with Early H.245 Enabled
3.1.3.3 SIP FastStart Mode (Calling UA Offers SDP)
The SIP protocol does not define distinct “fast start” and “slow start” modes as does H.323, but the
Dialogic
®
Global Call API library uses the same FastStart/SlowStart parameter interface to allow
applications to specify whether the calling UA offers SDP in its INVITE message or whether it
allows the called UA to offer SDP, which SIP refers to as “delayed offer”. In the default “FastStart”
mode, the calling endpoint offers SDP and the called UA answers.
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
CONNECT
PROGRESS
gc_AcceptCall( )
gc_AnswerCall( )
SETUP(TCS, MSD tunneled)
ALERTING(TCSAck,
MSDAck, MSD, TCS)
FACILITY(OLCAck )
n
FACILITY
(OLCAck , OLC )
nm
FACILITY
(MSDAck, TCSAck, OLC )
m
Unidirectional media
streams are enabled
before each respective
GCEV_EXTENSION
(MEDIAINFO) event
is dispatched.
Unidirectional media
streams are enabled
before each respective
GCEV_EXTENSION
(MEDIAINFO) event
is dispatched.
Pre condition: Both line devices are IDLE. Called party has executed gc_WaitCall().
SlowStart is enabled. Tunneling is enabled.
Post condition: Call is connected.