In case one of any parameters associated with the PDP context changed, like the QoS as example. The node in which the PDP context is changed should initiate  'PDP Context Modification' procedure. This is in order to inform other nodes about this change.

The PDP context exists on the MS, the SGSN, and the GGSN. Therefore, those nodes can initiate the 'PDP context modification' procedure.

PDP Context Modification Procedure in 3G

1. The MS sends 'Modify PDP context request' to the SGSN, including the changed parameter.
2. The SGSN sends 'Update PDP context request' to the GGSN including the changed parameter.
3. The GGSN returns an 'Update PDP Context Response'.
4. 'Radio access bearer modification' is initiated according to the changed parameter.
5. If any change occurd in the QoS during the RAB modification, the SGSN will send 'Update PDP context request' again to the GGSN, telling it with the modified parameter.
6. The GGSN returns an 'Update PDP Context Response'.
7. The SGSN sends 'modify PDP context accept' to the MS.

PDP Context Modification Procedure in 2G

1. The MS sends 'Modify PDP context request' to the SGSN, including the changed parameter.
2. The SGSN sends 'Update PDP context request' to the GGSN including the changed parameter.
3. The GGSN returns an 'Update PDP Context Response'.
4. The SGSN may inform the BSC with information related to ongoing user data transmission, including QoS characterstics through 'BSS Packet Flow Context Procedures'.
5. If any change occurred in the QoS during the 'BSS Packet Flow Context Procedures', the SGSN will send 'Update PDP context request' again to the GGSN telling it with the modified parameter.
6. The GGSN returns an 'Update PDP Context Response'.
7. The SGSN sends 'modify PDP context accept' to the MS. updating the PDP parameters with it.
8. The MS returns 'Update PDP context accept'.