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The Multimedia Telephony Service for IMS (MTSI), which is also known as Multimedia Telephony is a standardized IMS telephone service in 3GPP. The objective of defining a service is to specify the minimum set of capabilities required in the IP Multimedia sub-system to secure multi-vendor and multi-operator inter-operability for Multimedia Telephony and related Supplementary Services.

The user experience of multimedia telephony is equivalent to or better than corresponding circuit-switched telephony services. Multimedia telephony also exploits the richer capabilities of IMS. In particular, multiple media components can be used and dynamically added or dropped during a session.

The MTSI client functionality is required when SBC is deployed as:

  • MGCF providing interworking between an MTSI client and a non-MTSI client (that is, a CS UE).
  • Access SBC media gateway.


Figure : MTSI Workflow

Adaptive Multi-Rate (AMR) Support

SBC Core supports AMR Initial codec mode as per 3GPP26.114 specification. When AMR or AMR-WB is used, to avoid congestion on the link and to improve inter-working with CS GERAN, SBC limits the initial codec mode for a session to a lower mode until at least one frame-block. On enabling "Initial codec mode" while creating AMR Codec Entry, an AMR call starts with a lowest mode rate or second lowest mode rate until it receives a CMR request from the U.E to adapt the mode rate to a different value. If disabled, the AMR call starts with the highest mode rate within the negotiated mode-set.

SBC also supports the following functionality:

  • AMR to AMR transcoding for both restricted and unrestricted mode set
  • AMR to non-AMR transcoding
  • AMR attributes such as mode-change-period, mode-change-capability and mode-change-neighbor.

SBC does not support the following AMR attributes:

  • CRC
  • Robust-Sorting
  • Interleaving

The existing DSPs support CMR interpretation and updating the transmission rate according to peer's CMR requests.

The Synchronization info attribute does not transparently pass during the transcoded and pass-through calls.

For a configuration example, see Configuring AMR/AMR-WB Options.

Real-Time Transport Control Protocol (RTCP) Support

SBC Core supports RTCP bandwidth interworking by negotiating RR and RS bandwidth parameters. However, SBC does not allow using RTCP in a peer-to-peer voice call. This can be disabled at the MTSI client by setting RS=0 and RR=0 during call setup. But when a call is HELD, MTSI clients can enable RTCP to prevent the triggers of RTP inactivity.

SBC supports enabling the RTCP for point-to-point calls only when the call is HELD based on PSX configuration flag.