The MME supports the
following SRVCC features:
SRVCC CS-PS Handover
Continuity on PS Handover Failure: During S1-based CS-PS SRVCC handover, if
one of the following types of failures occurs
- Peer SGSN DNS query failed
- Fwd Relocation Response timeout
- Fwd Relocation Response was received
with a failure cause
then the handover
will continue for CS calls if CS handover on the Sv interface succeeds. This
means that the S1 SRVCC handover will continue as partially successful and the
handover command message will not carry any bearer related information.
Selection using DNS: As defined in 3GPP TS 29.303 V10.4.0, the MME supports
DNS-based MSC selection. In the NAPTR query response, the MME will analyze the
"Service Parameter" of "x-3gpp-msc:x-sv", and select a specific MSC from a pool
list provided in the DNS response. The provisioned weights and priorities on
the DNS server are used to load share proportionally between the MSC servers.
If DNS lookup fails, the MSC will be selected from local
configuration. If an MSC pool area has been configured, the selection logic for
the pool area will be used.
Areas: MSC pool areas can be configured for load balancing and intelligent
selection of MSC servers based on PLMN and/or IMSI hash values. Up to 24 MSC
servers can be defined per MME service. Each pool-area can optionally be
associated with a PLMN, which is the target PLMN as specified in the SRVCC
The MME attempts to select an MSC using the following selection
order: 1) Pool-area that matches the PLMN and of type hash 2) Pool-area that
matches the PLMN and of type round-robin 3) Pool-area that does not have PLMN
and of type hash 4) Pool-area that does not have PLMN and of type round-robin.
Offload: The MME allows an administrator to place one or more MSC server in
maintenance mode. This action removes the MSC server as a possible selection
Fallback on Failure: The MME automatically attempts to resend the Sv PS to
CS Request to a different MSC if: 1) no response is received (timeout) from the
MSC to a Sv PS to CS Request, or 2) any failure response is received from the
MSC to a Sv PS to CS Request.
If no alternate MSC is configured, or if the second MSC fails as
well, the SRVCC handover fails. A new MSC is attempted only for the initial PS
to CS Request. No additional configuration is needed to enable this
When an MSC is selected by DNS, and multiple results are
returned, the second MSC result will be used for fallback. In case DNS
selection returns just one MSC, the second MSC for fallback will be from local
configuration if it exists. If DNS lookup fails, the MSC for fallback will be
selected from local configuration.
Disabling MSC Fallback on Failure:
If so configured, the MME rejects handover based on the SRVCC failure cause
received from the MSC. So that
in some situations, the MME will ignore MSC fallback
procedures outlined above. If a voice call can be handed over to one of
multiple MSC IP addresses during SRVCC handover, and if the PS-CS Response from
the first MSC returns with a negative cause, and if that cause has been
included in the MME's Call-Control Profile configuration with the
msc-fallback-disable command, then the MME
fails the SRVCC HO and does not try the next available MSC. For configuration
details, refer to 'Disabling MSC Fallback Based on SRVCC Cause' in the section
Configuring an MSC Pool Area.
Other Supported SRVCC Features:
The MME implementation of SRVCC also supports:
- IMS Centralized Service call handling
as specified in 3GPP TS 29.280, enabling call flow handling for advanced
- Emergency Calls as defined in 3GPP TS
- GTP echo path
management messages as defined in 3GPP TS 29.280.
- GTP-C DSCP