Network Initiated Dedicated bearer

Before writing about dedicated bearers, we must be aware of the types of bearers in LTE.

There are 2 type of bearers available in LTE, “Default & Dedicated bearer”.

Default Bearer is the primary bearer that is established when UE initially attaches/latches to the network. Also, there could be additional Default bearers that could be required to access PDN for some other services(like IMS-Volte,Internet-Data).

Dedicated Bearer is the secondary bearer that is associated with any of the default bearer.

For more details refer:
http://www.simpletechpost.com/2012/05/default-bearer-dedicated-bearer-what.html

Here are the few questions to answer to better understand the context:

Which network element initiate the Network initiated dedicated bearer ?

Answer: In most of the cases, it is initiated by PCRF (Policy and Charging Rules Function) and in very few cases PGW where PCRF is not deployed.  

How Network initiated dedicated bearer gets triggered ?

Answer: PCRF initiates the dedicated bearer creation by sending diameter RAR(Re-Auth-Request) to PGW in which AVP ‘Charging Rule Install’ is present that specify the QoS parameter(QCI/ARP/GBR etc.) of the dedicated bearer to be created. PGW upon receiving RAR, sends Create Bearer Request with the received QCI/ARP to SGW. Upon receiving Create Bearer Response from SGW dedicated bearer gets created.

Refer: http://www.lte-5g-secrets.com/2012/05/lte-call-flow-beyond-pgw.html  

How PCRF decides to sends RAR with ‘Charging Rule Install’ ?

Answer: PCRF is configured with Policies in which it is specified that for certain events new bearer creation is to be triggered.

One good example is Volte(Voice over LTE) event.

Refer Volte Call flow:

https://www.rfwireless-world.com/Terminology/VoLTE-Originating-Call-Flow-Procedure.html

During VoLTE call setup procedure, P-CSCF informs PCRF through Rx interface about the Volte call that is being setup. PCRF has been configured with Policy (lets say ‘IMS-Volte-Policy’) with the information that Dedicated bearer of QCI-1(Voice call) is to be triggered towards PGW. Upon receiving Request from P-CSCF, PCRF sends RAR with Charging Rule Install named ‘Voice_Rule’ with QCI-1 along with other necessary parameters to PGW for dedicated bearer creation for Voice Call in which Voice data to be sent from source UE to destination UE.

For more details:
http://www.lte-5g-secrets.com/2012/05/lte-call-flow-beyond-pgw.html