Modify Bearer Request in LTE?

Question

Please help me to understand the meaning of modify bearer request and its usage?

Answer 1

There are few scenarios due to which an existing bearer need to be modified.
Bearer characteristics need to be modified from PGW to till UE.
Bearer modification can be originated by either by UE or MME or PCRF etc.
First bearer modification done at core network then at eNodeB and if everything goes successfully then
at UE.

Please refer 29.274 3GPP spec which covers GTP signalling and 36.413 for S1-AP.
If you see with respect to bearer QOS changes that's why it is mandatory parater in ERAB Modifiy Request which comes from MME to eNodeB for an UE.

Once eNodeB receives this S1AP message it do some jobs at its own end and send RRC Connection reconfiguration message to UE. In successful scenario UE sends RRC Connection Reconfiguration complete then eNodeB send ERAB-Modify Response just for confirmation.

Answer 2

Whenever there is a need of existing bearer need to be modified say QoS change i.e. more bandwidth etc. Modify Bearer message is triggered at various places.

This is what 29.274 (section 7.2.7) is saying see if it is useful -
The direction of this message shall be from MME/S4-SGSN to SGW and/or from SGW to PGW. The Modify Bearer Request message shall only be sent on the S11 interface by the MME to the SGW and on the S5/S8 interfaces by the SGW to the PGW as part of the procedures:

• E-UTRAN Tracking Area Update without SGW Change
• UE triggered Service Request
• S1-based Handover
• UTRAN Iu mode to E-UTRAN Inter RAT handover
• GERAN A/Gb mode to E-UTRAN Inter RAT handover
• E-UTRAN Initial Attach
• UE requested PDN connectivity
• 3G SGSN to MME combined hard handover and SRNS relocation procedure
• X2-based handover without SGW relocation

It shall also only be sent on the S4 interface by the SGSN to the SGW and on the S5/S8 interfaces by the SGW to the PGW as part of the procedures:

• Routeing Area Update with MME interaction and without SGW change
• E-UTRAN to UTRAN Iu mode Inter RAT handover
• E-UTRAN to GERAN A/Gb mode Inter RAT handover
• Inter SGSN Routeing Area Update Procedure and Combined Inter SGSN RA / LA Update to S4 SGSNs without SGW change
• Iu mode RA Update Procedure without SGW change
• Serving RNS Relocation Procedure
• Combined Hard Handover and SRNS Relocation Procedure
• Combined Cell / URA Update and SRNS Relocation Procedure
• Enhanced Serving RNS Relocation without SGW relocation
• UE Initiated Service Request Procedure
• Iu mode to A/Gb mode Intra SGSN Change
• A/Gb mode to Iu mode Intra SGSN Change
• Iu mode to A/Gb mode Inter-SGSN Change
• A/Gb mode to Iu mode Inter-SGSN Change
• Paging Response with no established user plane on S4
• PDP Context Activation Procedure

only on the S4 interface by the SGSN to the SGW as part of the procedures:

• RAB Assignment Procedure
• SRVCC from E-UTRAN to UTRAN or GERAN with DTM HO support procedures and SRVCC from UTRAN
  (HSPA) to UTRAN or GERAN with DTM HO support. and only on the S5/S8 interfaces by the SGW to the PGW as part of the procedures:
• Tracking Area Update procedure with SGW change
• Gn/Gp SGSN to S4 SGSN Routing Area Update
• X2 based handover with SGW relocation
• Gn/Gp SGSN to MME Tracking Area Update
• Enhanced Serving RNS Relocation with SGW relocation
• Routeing Area Update with MME interaction and with SGW change
• Inter SGSN Routeing Area Update Procedure and Combined Inter SGSN RA / LA Update using S4 with SGW change
• Iu mode RA Update Procedure using S4 with SGW change

Answer 3

MBR will be a part of control messages and fall under S11, S5/S8 interfaces. This is used to modify the bearer with respect to QOS/HANDOVER/Data path update..