When a callee has been alerted by the provisional response, the chances of a session establishment failure are minimum assuming network resources are available. So in this case network reservation is the most important and mandatory task.
So precondition can be defined as a network resource reservation acknowledgement procedure by which the both the terminal users can establish a specific QCI before going to the ringing state. Depending upon the presence of the SDP in initial INVITE request, desired and local network reservation condition there are several call flows, but we will only consider the primary one.
Suppose UE-A and UE-B are two precondition enabled VoLTE clients and want to establish a media session between them. Consider UE-A as the initiator of the session.
UE-A starts the session by sending an INVITE request(SDP offer) to user B. Within the INVITE request UE-A indicates its capability for precondition by adding the precondition supported header.
Upon receiving the INVITE request, as UE-B is precondition capable, it sends a 183 Session Progress message with precondition required header and starts exchanging the required QoS parameters.
UE Initiated Dedicated Bearer Setup Request :
After sending the 183 Session In Progress response UE-B starts reserving the network resources required for this session by sending BEARER RESOURCE ALLOCATION REQUEST to MME. This message contains QoS class indicator, guaranteed and maximum bit rates for the uplink and downlink and TFT of UE-A. MME sends this towards S-GW as a GTP-C BEARER RESOURCE COMMAND. PCEF now sends a CC-Request to PCRF. PCRF is responsible for checking the subscription status from SPR.
PCRF then invokes RE-AUTH-REQUEST request to AF. RE-AUTH-ANSWER contains corresponding QoS and charging parameters and PCRF sends this information within CC-ANSWER to PCEF so that it can establish a dedicated bearer. This procedure is called UE initiated dedicated bearer setup.
Network Initiated Dedicated Bearer Setup Request :
Similarly, after receiving the 183 response by serving P-CSCF of UE-A sends Diameter AA-Request to PCRF indicating mobile using its IP address, and describes the requested media using parameters such as the media type, codec and port number, and the maximum uplink and downlink data rates. PCRF is responsible for fetching all required subscriber information from SPR and provides the AA-Answer to AF. Then PCRF defines new PCC rules and sends a Re-Auth-Request to PCEF so that it can establish a dedicated bearer. This procedure is called network initiated dedicated bearer setup.
After receiving the new PCC rules from PCRF, P-GW sends QoS parameters, uplinks TEI and TFT for the UE using CREATE DEDICATED BEARER REQUEST. S-GW forwards this to MME. MME then creates ACTIVATE DEDICATED EPS BEARER CONTEXT REQUEST with the information received from S-GW, embeds it within S1-AP E-RAB SETUP REQUEST and forwards to eNB. After receiving this request from MME, eNB sends an RRC CONNECTION RECONFIGURATION MESSAGE to the UE, which specifies the information received from MME along with the new radio bearer configuration data. The mobile configures the bearer as instructed and acknowledges with an RRC CONNECTION RECONFIGURATION COMPLETE to eNB. eNB now acknowledges with an E-RAB SETUP RESPONSE.
Parallel to the bearer setup procedure, if the 183/INVITE response contains ‘Require: 100rel’, UE-A issues a PRACK to acknowledge that it received the provisional response containing the SDP body (as well as QoS parameters). Sometimes this PRACK request can also carry the SDP offer but we will ignore that flow for the present.
Response of PRACK from UE-B. Sometimes this PRACK request can also carry the SDP answer but we will not consider that flow now.
After the resource reservation on the UE-A side, VoLTE client invokes the UPDATE request to acknowledge that required resource reservation is complete and specifies this within the SDP message body.
After the resource reservation on the UE-A side, VoLTE client invokes the UPDATE request to acknowledge that the required resource reservation is complete and specifies this within the SDP message body.
After receiving the UPDATE by UE-B, if resource reservation is complete in MT side, it then acknowledges its status within the 200 OK response.
7. 180 Ringing
After sending the 200OK/UPDATE response, if resource reservation status matches then UE-B sends 180.
If 180 response contains 100rel then UE-A acknowledges with a PRACK request.
Response of PRACK from UE-B.
If user accepts the call, UE-B sends the 200 Ok response of INVITE. As the SDP negotiation is already complete 200OK does not contain the SDP body.
In response of 200OK/INVITE caller acknowledged with ACK.
Author: Arindam Ghosh