EPS Bearer Modification Procedure

•   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

• 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

Default EPS Bearer Context Activation Procedure

Default EPS Bearer Context activation Procedure

•          Default EPS Bearer Context activation Procedure is an ESM procedure initiated by network.
•          This procedure is used to establish a default EPS bearer context between the UE and EPC.
•          Initiated by network in response to PDN connectivity request from UE.
•          The default bearer context activation can be part of attach procedure. If attach procedure fails default bearer context establish also fails.
•          Default EPS bearer context doesn’t have any TFT assigned during activation, but network may anytime assign a TFT after establishment and may modify the TFT or packet filter of this default bearer.
•          Default bearer context activation procedure initiated by MME by sending Active Default EPS Bearer Context Request.
•          In case initial attach if default bearer is activated , the Activate default EPS bearer context request is sent in attach accept message.
•          If Activate default EPS bearer context request is sent in response to PDN connectivity request from UE, MME will start T3485.
•          MME will assign an EPS bearer identity in ACTIVATE DEFAULT EPS BEARER CONTEXT REQUEST.
•          MME will retrieve the PTI from PDN CONNECTIVITY request sent in the activate request from UE.
•          UE should verify the PTI (procedure transaction id) received in the message from network with the PTI sent from UE.
•          UE shall send a reject message with one of the following reject causes

o   #26  Insuffiencent Resources

o   #31  Request rejected, Unspecified

o   #95-111 protocol error

•          If UE provided APN in PDN connection request and received ACTIVATE DEFAULT EPS BEARER CONTEXT REQUEST UE shall stop T3396 if it is running for APN provided by UE.
•          If the PDN request is for emergency UE should not stop T3396 if it is running.

·         EPS BEARER :

o   Established between UE to P-GW( E-RAB +S8 bearer)

o   E-RAB is established between UE and S-GW(Radio bearer +S1 bearer)

o   S8 bearer is established between S-GW to P-GW

o   Radio bearer is established between UE and eNodeB.

o   S1 bearer is established between eNodeB to S-GW

·          Default Bearer :

o   Established while attach. Per PDN connection one default bearer will be established. Allocates IP address to UE.

o   No QOS, TFT or Guaranteed bit rate.

·         Dedicated Bearer :

o   Dedicated bearer will be linked to a default bearer.

o   QOS can be Guaranteed bit rate or Non-Guaranteed  bit rate.

o   If default bearer is deactivated dedicated bearer will also be deactivated.

 Reference : 3 GPP TS 24.301