4.2. Using Fast Start and Slow Start Setup

Fast start and slow start are supported in both the H.323 and SIP protocols.

In H.323, fast start and slow start setup are supported depending on the version of H.323 standard supported at the remote side. If the remote side supports H.323 version 2 and above, fast start setup can be used, otherwise a slow start setup is used. Fast start connection reduces the time required to set up a call to one round-trip delay following the H.225 TCP connection. The concept is to include all the necessary parameters for the logical channel to be opened (H.245 information) in the Setup message. The logical channel information represents a set of supported capabilities from which the remote end can choose the most appropriate capability. If the remote side decides to use Fast start connection, it returns the logical channel parameters it wants to use in the Alerting, Proceeding, or Connect messages. Fast start connection is preferable to slow start connection because fewer network round trips are required to set up a call and the local exchange can generate messages when circumstances prevent a connection to the endpoint.

In SIP, fast and slow start setup is also supported. In slow start setup, the INVITE message will have no Session Description Protocol (SDP) and therefore the remote side will propose the message set 100, 180, 200, and the call is answered with the SDP in the ACK message.

In Global Call, fast start and slow start connection are supported on a call-by-call basis. Fast start connection is used by default, but slow start connection can be forced by including the IPPARM_CONNECTIONMETHOD parameter with a value of IP_CONNECTIONMETHOD_SLOWSTART in the ext_datap field (of type GC_PARM_BLK) in the GCLIB_MAKECALL_BLK structure associated with a call.

The following code segment shows how to specify a slow start connection explicitly by including the IPPARM_CONNECTIONMETHOD parameter when populating the ext_datap field (of type GC_PARM_BLK):

    gc_util_insert_parm_val(&libBblock.ext_datap,IPSET_CALLINFO,
        IPPARM_CONNECTIONMETHOD,sizeof(char), IP_CONNECTIONMETHOD_SLOWSTART); 

In addition, the IPPARM_CONNECTIONMETHOD parameter can be set to a value of IP_CONNECTIONMETHOD_FASTSTART to force a fast start connection on a line device configured to use a slow start connection (using gc_SetUserInfo( ) with a duration parameter of GC_ALLCALLS).

Note: In SIP, only the calling side can choose fast start or slow start, unlike H.323 where both sides can select either fast start or slow start.

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