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The following sections describe the LMSD inter-working without tones:

Implementing LMSD When SBC Cannot Play Tones

When 180 ringing with SDP and valid Alert-Info header is received, but the SBC is not configured with tone profiles, the SBC relays the 180 ringing by storing the received SDP and forwards 180 ringing without SDP to ingress. Consider a scenario where the SBC, which is not configured with tone profile, receives Alert-Info header in the 180 ringing. It performs inter-working only when the following conditions are met:

  • Accept Alert-Info is enabled on the leg which receives 180 ringing.
  • 180 ringing is received with SDP answer.
  • Alert-Info is received in LMSD format <http:/LMSD/tone?sig-id=x>, where x = (rt).
  • Sends 180 ringing with Alert-Info header and without SDP when the SBC cannot play the required LMSD specified tone or the tone and announcement profile is not configured on both legs of a call.


    In LMSD inter-working state, if the SBC receives 18x ringing without SDP, it relays 180 without SDP and remains in the same state. The SBC passes the Alert-Info transparently, regardless of the transparency setting of Alert-Info header.

The SBC aborts a transaction by sending appropriate error response code to the IMS network and CANCEL to the other leg (MSC) when following conditions are met:

  • Egress leg IPSP flag Accept Alert-Info is enabled.

  • Initial INVITE received with SDP.

  • 180 ringing is received from the MSC with SDP.

  • Alert-Info received in LMSD format <http:/LMSD/tone?sig-id=x>, where x = ct or bt.


    If the tone is not in ct, bt or rt format, SMM is used to convert that string to rt.

When SBC is Handling P-Com-DropEarlyMedia

The P-Com-DropEarlyMedia flag signals the SBC to cut-through or not cut-through the early media (before receiving an answer), if a SDP response is received from a CDMA network. This header added by the IMS network indicates media anchoring element (SBC) not to pass the normal ring back tone being played by the MSC. The SBC processes this header and does not pass it to the egress call-leg.

The SBC accepts the P-Com.DropEarlyMedia header in the initial INVITE in the format  ̶  P-Com.DropEarlyMedia:Value, where Value is set to true or false. If the value is set to True, the SBC blocks media streams, else the media streams are passed.


If 180 ringing and 183 does not include SDP, it is passed normally.

When P-Com.DropEarlyMedia interworking is carried out in the SBC, it will continue to drop media packets till the final answer is received. Any intermediate UPDATE messages altering SDP is handled normally but the media is not cut-through at the SBC. The SBC supports media cut-through only on receiving the final 200 OK response.


Calls Originating from MSC

For calls originating from the MSC server, a 180 ringing with SDP response triggers a flag in the MSC to play a ringback tone. This causes the need for the SBC to send a 180 ringing with Alert-Info <http:/null> request to force the MSC to stop the ringback tone. To facilitate this, the SBC uses a SIP trunk group flag, stopLmsdToneOnUpdate, to accomplish the following on the leg which received the update from the MSC:

  1. Send a 180 ringing without SDP with Alert-Info <http:/null> to stop the LMSD tone.
    NOTE: The 180 ringing is sent only if the SBC has relayed the Alert-Info with the non-null URI.

  2. Send an update with the SDP offer.


This flag is disabled by default. Refer to SIP TG - Signaling - Stop LMSD Tone On Update - CLI for configuration details. 


Ignoring ISUP MIME

The SBC uses the trunk group flag, dropIsupMimeBody, to drop the ISUP MIME body received in SIP-I messages and then treat the call as a normal SIP call. If SIP-I is configured for incoming messages, the SBC adds the ISUP MIME to the outgoing messages.


This flag is disabled by default. Refer to SIP TG - Signaling - Drop ISUP Mime Body - CLI for configuration details. 

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