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Global Call IP Technology Guide — November 2007
Dialogic Corporation
IP-Specific Operations
The scope of the mode setting is determined by which Global Call function the application passes
the GC_PARM_BLK to:
• gc_SetConfigData( ) sets the slow start mode as the default for the entire system (all line
devices on all board devices for both H.323 and SIP protocols).
• gc_SetUserInfo( ) with duration = GC_ALLCALLS sets the slow start mode as the default
connection mode for H.323 and SIP calls on a given line device.
• gc_MakeCall( ) with the GC_PARM_BLK in the GCLIB_MAKECALL_BLK structure sets
the slow start connection mode for the new call only.
The following code segment illustrates how to insert the parameter that specifies a slow start
connection in a GC_PARM_BLK:
gc_util_insert_parm_val(&libBblock.ext_datap,
IPSET_CALLINFO,
IPPARM_CONNECTIONMETHOD,
sizeof(char),
IP_CONNECTIONMETHOD_SLOWSTART);
If the application has previously set the default mode to the slow start mode, it can override that
default for an individual call or can reset the default to fast start mode by inserting the following
parameter element in a GC_PARM_BLK:
IPSET_CALLINFO
IPPARM_CONNECTIONMETHOD
• value = IP_CONNECTIONMETHOD_FASTSTART
Here again, the Global Call function that is used determines the scope of the setting:
• gc_MakeCall( ) with the GC_PARM_BLK in the GCLIB_MAKECALL_BLK structure sets
the fast start connection mode for the new call only.
• gc_SetUserInfo( ) with duration = GC_ALLCALLS resets the default mode to fast start for a
given line device for both H.323 and SIP protocols.
• gc_SetConfigData( ) resets the default mode for the entire system (all line devices on all
board devices) to fast start for both protocols.
4.2.2 H.323 Fast Start and Slow Start
H.323 version 2 defines a specific call connection method called fastStart, which exchanges
endpoint media capabilities much earlier in the setup process than the call connection method
defined in H.323 version 1 (a process which then became known as slow start setup). If the remote
side supports H.323 version 2 or above, fast start setup can be used; otherwise, slow start setup is
used even if the local endpoint attempts to initiate a call using fast start setup.
In H.323 slow start setup, the messages that are used to communicate each endpoint’s supported
media capabilities are exchanged using the H.245 channel that is established after the H.225 TCP
connection, and this introduces significant latency. Media streaming cannot be established until
both sides have communicated and negotiated their capabilities in multiple message exchanges.
Early media is not possible in H.323 when slow start connection is specified by either party.