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Initiated by network to modify an EPS bearer context with specific QOS and TFT.
It can also be initiated when UE sends bearer resource allocation request or bearer resource modification request.
It can also be initiated by network to update APN of UE in case of inter system handover.
MME will initiate this procedure by sending MODIFY EPS BEARER CONTEXT REQUEST to UE. MME also includes a EPS bearer identity to identify which EPS bearer Context to be modified.

UE shall apply the received TFT for all uplink traffic to radio bearers if the modify request contains a TFT that has packet filters.
If there are no Guaranteed Bit Rate or Maximum Bit Rate values for uplink or downlink in received modification request UE shall use previous received values.
UE may reject the request with following causes

#26 Insufficient Resources
#41 Semantic errors in TFT

#43 Invalid EPS bearer Identity

Reference 3GPP TS 23.401

Dedicated EPS bearer context activation procedure is to establish an EPS bearer context with specific QOS and TFT between UE and EPC.
This procedure is initiated by the network
UE can also request for dedicated bearer by sending resource allocation request or modification request
MME will initiate dedicated EPS bearer context activation by sending ACTIVATE_DEDICATED_EPS_BEARER_CONTEXT_REQUEST

The dedicated bearer which will be associated with a default bearer will be identified by the linked EPS bearer identity field in dedicated bearer activation request.
The linked EPS bearer id also indicates which IP address, PDN the dedicated bearer is linked.
For all uplink traffic flows to radio bearer UE should use the TFT received in bearer activation request.

· UE may reject the request with causes such as

o #26 Insufficient resources

o #43 Invalid EPS bearer identity

o #41 Invalid TFT operation.

Reference : 3GPP TS 23.401