IWF for CSFB Concept:
Traditionally support for CSFB requires an upgrade on the legacy MSCs to support the SGs interface. Additionally careful TA to LA mapping is required to avoid MT call failures when the UE falls back to a LA that is not the same as the one it registered on.
Putting all this together can be an expensive and complicated process which is where the “IWF for CSFB” comes in.
The IWF provides a number of advantages:
At the most basic level it is an interface between the MME and the legacy CS network (as shown below). On one side it communicates using the SGsAP with the MME, while at the other side it communicates using MAP to the MSCs and HLRs.
At the same time it provides a VLR function where all the LTE UEs are registered against. This is done by creating a "virtual" LA and updating the HLR accordingly. Any MT calls are thus routed to the IWF. At the same time it eliminates the need for TA - LA planning as all TAs can point towards this one "virtual" LA.
How does it works..??
For UE originating calls, the UE performs access domain selection. The service domain selection functionality decides whether the call is serviced in the CS domain or the IMS. Following domain settings are possible:
CS Voice only
CS Voice preferred, IMS PS Voice as secondary:
IMS PS Voice preferred, CS Voice as secondary
IMS PS Voice only.
As Operator X is using open UE market, so domain section can’t be controlled by Operator X and User/UE is free to select any domain.
Upon getting combined attach request (EPS/IMSI Attach) the MME will always send attach accept containing the EPS Attach Result as “Combined EPS/IMSI Attach”, if in the HSS subscriber profile configured as for CS as well as PS services (in HSS subscriber profile will have CS as well as PS domain). MME will trigger IWF (preconfigured/based on subscriber profile received form HSS) over interface.
Upon receipt of the SGsAP_Location_Update_Request from the MME, the IWF will initiate a MAP_Update_Location towards the network identified by the IMSI of the registering subscriber. Combined Attach procedure is explained in below call flow. After getting successful response from IWF, the MME will send an Attach Accept to the UE. With this approach UE is LTE attached and CS attached.
Network Components:
MME:
The CS fallback and SMS over SGs enabled MME supports the following additional functions:
Mapping Tracking Area Identity (TAI) to single configured LAI of IWF.
Maintaining of SGs association towards MSC/VLR for EPS/IMSI attached UE.
Initiating IMSI detach at EPS detach.
Initiating implicit IMSI detach
Uses IMSI hash table to distribute subscribers across multiple IWFs.
IWF:
The CS fallback and SMS over SGs enabled IWF supports the following functions:
Maintaining SGs association towards MME for EPS/IMSI attached UE.
Traditionally support for CSFB requires an upgrade on the legacy MSCs to support the SGs interface. Additionally careful TA to LA mapping is required to avoid MT call failures when the UE falls back to a LA that is not the same as the one it registered on.
Putting all this together can be an expensive and complicated process which is where the “IWF for CSFB” comes in.
The IWF provides a number of advantages:
At the most basic level it is an interface between the MME and the legacy CS network (as shown below). On one side it communicates using the SGsAP with the MME, while at the other side it communicates using MAP to the MSCs and HLRs.
At the same time it provides a VLR function where all the LTE UEs are registered against. This is done by creating a "virtual" LA and updating the HLR accordingly. Any MT calls are thus routed to the IWF. At the same time it eliminates the need for TA - LA planning as all TAs can point towards this one "virtual" LA.
How does it works..??
For UE originating calls, the UE performs access domain selection. The service domain selection functionality decides whether the call is serviced in the CS domain or the IMS. Following domain settings are possible:
CS Voice only
CS Voice preferred, IMS PS Voice as secondary:
IMS PS Voice preferred, CS Voice as secondary
IMS PS Voice only.
As Operator X is using open UE market, so domain section can’t be controlled by Operator X and User/UE is free to select any domain.
Upon getting combined attach request (EPS/IMSI Attach) the MME will always send attach accept containing the EPS Attach Result as “Combined EPS/IMSI Attach”, if in the HSS subscriber profile configured as for CS as well as PS services (in HSS subscriber profile will have CS as well as PS domain). MME will trigger IWF (preconfigured/based on subscriber profile received form HSS) over interface.
Upon receipt of the SGsAP_Location_Update_Request from the MME, the IWF will initiate a MAP_Update_Location towards the network identified by the IMSI of the registering subscriber. Combined Attach procedure is explained in below call flow. After getting successful response from IWF, the MME will send an Attach Accept to the UE. With this approach UE is LTE attached and CS attached.
Network Components:
MME:
The CS fallback and SMS over SGs enabled MME supports the following additional functions:
Mapping Tracking Area Identity (TAI) to single configured LAI of IWF.
Maintaining of SGs association towards MSC/VLR for EPS/IMSI attached UE.
Initiating IMSI detach at EPS detach.
Initiating implicit IMSI detach
Uses IMSI hash table to distribute subscribers across multiple IWFs.
IWF:
The CS fallback and SMS over SGs enabled IWF supports the following functions:
Maintaining SGs association towards MME for EPS/IMSI attached UE.
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