provides the MME's call flows for HSS-based P-CSCF Restoration.
Figure 1. Call flow
for HSS-based P-CSCF Restoration
On receiving the
P-CSCF Restoration indication from the HSS, the MME/SGSN from the received IMSI
identifies the UE and finds the corresponding IMS APN. The support of this
feature by the serving SGW/PGW is determined based on the local configuration
at the MME. If the optional extension is not supported by the SGW/PGW, the MME
releases the identified PDN connection towards the UE by executing PDN
disconnection/detach procedure with NAS cause code "reactivation requested/
reattach required". Additionally, the MME/SGSN release the same PDN connection
towards the SGW/PGW by sending Delete Session message. As a result of the
release of the IMS PDN connection, the UE activates the IMS PDN connection to
select an available P-CSCF and to perform a new initial IMS registration.
Figure 2. Call flow
for HSS-based P-CSCF Restoration, continued...
The HSS-based P-CSCF
basic mechanism is optionally extended by reusing part of the "Update PDP
context/bearer at P-CSCF failure" mechanism. This in order to avoid the need to
deactivate and reactivate the IMS PDN connection. PCO-based optional extension
is based on the possibility for the P-GW/GGSN to know whether or not the UE
supports the "Update PDP context/bearer at P-CSCF failure" mechanism.
The MME sends Modify
Bearer to the P-GW for the associated PDN connection with a P-CSCF Restoration
indication. The MME provides this indication to the P-GW through the S-GW. When
Modify Bearer Request is received by the S-GW with the P-CSCF Restoration
indication, this message is forwarded to PGW. PGW sends Update Bearer Request
to the MME along with a list of available P-CSCF addresses within PCO IE to
update the destination UE.
MME sends an Update
EPS Bearer Context Request or Modify PDP Context Request to the UE, including
the PCO with the list of available P-CSCF addresses otherwise, upon reception
of Delete Bearer Request the MME sends Delete EPS Bearer Context Request to the
UE with NAS cause code "reactivation requested". When the PDN connection is
released, the UE re-activates the IMS PDN connection and selects an available
P-CSCF. If the UE has received Modify EPS Bearer Context Request, the UE as per
PCO based P-CSCF Restoration procedures, selects an available P-CSCF from the
list for IMS registration. The UE performs a new initial IMS registration.