SBC: Max-Requested-Bandwidth is set to 0 when the 200OK is received for the RTT triggered Re-Invite (Doc ID 3079653.1)

Last updated on APRIL 08, 2025

Applies to:

Symptoms

The Max-Requested-Bandwidth is set to 0 in the AVP of Rx message, when the 200OK is received for the RTT (Real-Time Text) triggered Re-Invite.

         AVP: Max-Requested-Bandwidth-UL(516) l=16 f=VM- vnd=TGPP val=0
         AVP: Max-Requested-Bandwidth-DL(515) l=16 f=VM- vnd=TGPP val=0

Changes

The scenario is as follows:

- Normal VoLTE - VoLTE call is established. Dedicated bearer is set.
- At one point in the call, the calling party requests for RTT to change audio call to text form, where the text message (RTT) will be sent via RTP packets. This triggers Re-INVITE which contains line m=text in the SDP.
         Media Description, name and address (m): text 12345 RTP/AVP 112 111
         Media Attribute (a): rtpmap:112 red/1000
         Media Attribute (a): rtpmap:111 t140/1000
         
- A Rx communication is initiated by SBC towards PCRF where we see Media-Component-Description(517) with Media-Type val=TEXT with Max-Requested-Bandwidth = 3000
         AVP: Max-Requested-Bandwidth-UL(516) l=16 f=VM- vnd=TGPP val=3000
         AVP: Max-Requested-Bandwidth-DL(515) l=16 f=VM- vnd=TGPP val=3000
         
- The Re-INVITE is responded with 200OK.
         
- T140 is the codec set for the text, and hence media-profile for RTT was configured as follows:

media-profile
        name                                    T140 <<----------------
        subname
        media-type                            text <<----------------
        payload-type
        transport                               RTP/AVP
        clock-rate                              0
        req-bandwidth                       0
        frames-per-packet                  0
        parameters
        average-rate-limit                   375 <<----------------

The max Requested Bandwidth to be allocated throughout the call should be 8 x 'average-rate-limit' i.e. 8 x 375 = 3000

- This again generates AAR message from SBC. This AAR now contains Media-Component-Description(517) with Media-Type val=TEXT with Max-Requested-Bandwidth = 0 which is incorrect, as the bandwidth allocated is removed.
         AVP: Max-Requested-Bandwidth-UL(516) l=16 f=VM- vnd=TGPP val=0
         AVP: Max-Requested-Bandwidth-DL(515) l=16 f=VM- vnd=TGPP val=0
        
        Access ----------- SBC --------- PCRF -------- Core
         Invite  ---------->
                                         ---Rx--> PCRF
                                         <--Rx--- PCRF

                                         --------- Invite -------> Core
                                         <------- 200OK -------
               
        Re-Invite (RTT) -->
                                          ---Rx--> PCRF
                                         <--Rx--- PCRF
                                         ------- Re-Invite -----> Core
                                         <---------200OK--------           ** offer sdp comes with 112, 111 i.e. Red, t140 respectively
                                         --AAR--> PCRF                       ** This AAR sets BW to 0 in the AVP

Cause

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