H.245 is used for exchange the codec/function ability each endpoint owned and decide what Codec/application need start in this IP call;
Audio Codec example : G.723.1, G.729A, G.711, NetCoder, GSM 6.10 or
Application Example : T.38 Real-Time Fax, T.120 Shared Board etc.
Most common used is G.723.1 and G.729A in VOIP Call;
in h.323 V2, there have Fast Start mode, means no H.245 session exist in the call, replaced with a simple and fast message embedded in the H.225 Q.931 SETUP/ALERTING/CallProgress etc. mesages to fast the connection speed, you know in normal h.245 start mode, there have about more than 10 messages exchanged between two h.323 endpoint, if the internet isn’t fast, it will take about 3-10 seconds to establish the voice connection, so currently Fast Start mode is a standard setting in most gateway;