After the formation of default bearer at the time of initial attach,UE sends SIP INVITE message to P-CSCF on this default bearer.After the media negotiation happened between both calling and called party IMS network.P-CSCF sends AAR message to PCRF with the media details(audio),its GBR(UL/DL).PCRF than convey the same information to PCEF in RAR message to install the rule for audio bearer and in turn PGW initiates Create Bearer Requests towards MME via SGW. when the Create Bearer Response reached to PGW via SGW from MME,than PGW open the gate status for both uplink and downlink packets.
I just explained the dedicated bearer set up and basic nodes involved in this.
