SRVCC functionality is required within VoLTE systems to enable the packet domain calls received in LTE to be handed over to a legacy circuit-switched (CS) voice system, such as CDMA2000 1xRTT. SRVCC for 1xRTT, also referred to as enhanced SRVCC, enables the MME to move a VoLTE UE between an LTE and a 1xRTT network with smooth, seamless handovers. The MME acts as a relay agent to ensure CDMA2000 messages received from the UE are delivered to the interworking solution function (for 3GPP2, 1xCS IWS) associated with the mobile switching center (1x RTT MSC) (or vice-versa) through the S1-AP and S102 interfaces.
By using the MME's SRVCC for 1xRTT capabilities, the operator performs handovers while maintaining existing quality of service (QoS) and ensuring call continuity that meets the critical requirements for emergency calls.
This feature is license-controlled and the commands to configure and manage the feature interfaces require a feature license key. Speak with your Cisco Representative for information about this license. For information about the commands and their use, refer to the Configuring SRVCC for 1xRTT section later in this chapter.
The MME provides the following features in support of SRVCC for 1xRTT functionality:
MSC Pool Areas: Multiple MSCs would be handled by pooling all the MSCs mapping to a particular cell for load distribution. MSC pool areas can be configured for load balancing and intelligent selection of MSC servers based on IMSI hash values. Up to 10 MSC servers can be defined per S102 service.
MSC Non-Pool Areas: MSC selection, based on local MSC configuration.
MSC Selection: If an MSC pool area has been configured, the selection logic for the pool area is based on the CDMA2000 sector cell ID (includes the MSC ID and the Cell ID) in the CDMA2000 1xRTT network
- The MME attempts to match the
MSC ID and the Cell ID:
- If the match is found in the non-pool area configuration, then the configured MSC is selected.
- If the match is found in the pool area
- then IMSI hashing is used to select the MSC.
- if no hash corresponds, then the MSC selected is the one configured for the 'non-configured-values'.
- If no MSC is found, a failure message is returned.
When the UE attaches with IMEI, the MSC configured for the non-pool area is always selected because IMSI hashing cannot be performed for that UE.
Relationships to Other Features
SRVCC for 1xRTT is related to the CSFB for 1xRTT feature. Each requires a separate license to take advantage of the separate functionality and use the configuration commands.
If licenses for both features are installed in the system and both features are configured, then the MME can use the S102 interface for both CSFB for 1xRTT and SRVCC for 1xRTT.
1xRTT SRVCC and 1xRTT CSFB calls will be decided based on the presence or absence of the CDMA2000 1xRTT SRVCC Info IEs in an Uplink S1 CDMA2000 Tunneling message. This IE should not present for a 1xRTT CSFB call. If only one feature is licensed and configured and if the above condition is not appropriately satisfied for any received call, then that call will be dropped.
The CSFB for 1xRTT feature is described elsewhere in this administration guide.