RRC Connection Establishment in LTE

Background

The first thing UE does after switching on is to synchronize to each frequency and check whether this frequency is from the right operator to which it wants to connect to. UE does this by going through very initial synchronisation process. Once synchronized, UE reads the master information block and System information blocks to check if this is the right PLMN. Lets assume it finds that PLMN value to be correct and so UE will proceed with reading System information block 1 and System information block 2. The next step is known as Random Access Procedure in which the network for the first time knows that some UE is trying to get access and the network provides temporary resources to the UE for initial communication.

Once the Random Access procedure is successfully completed, next is RRC connection establishment procedure which configures SRB1 for UE and let UE inform the network what exactly it wants i.e. Attach, Service Request, Tracking area update etc. RRC connection establishment is 3 way handshake procedure comprising of following messages.

- RRC Connection Request
- RRC Connection Setup
- RRC Connection Setup complete

RRC Connection Request (RACH Msg3)

 Actually the RACH Msg3 is the first message of RRC connection establishment procedure. Once the UE has obtained temporary resources via MSG2 in RACH process , its now ready to send 'RRC connection request' message using UL-SCH to eNodeB. UE is identified by temporary C-RNTI assigned in RACH Msg2

• The message contains following information
• UE identity (TMSI or Random Value )
• TMSI is used if UE has previously connected to the same network. With TMSI value, UE is identified in the core network 
• Random value is used if UE is connecting for the very first time to network. Why we need random value or TMSI? Because there is a possibility that Temp-CRNTI has been assigned to more than one UEs in previous step, due to multiple requests coming at same time (Collision scenario explained later)
• Connection establishment cause: This shows the reason why UE needs to connect to network

RRC Connection Request message

RRC Connection Setup 

The RRC connection setup message contain configuration details for SRB1 so that later messages can  be transferred via SRB1. Remember the SRB2 is always configured after the security activation.

RRC Connection setup message include default configuration for SRB1 but can also include configuration information for PUSCH, PUCCH, PDSCH physical channels, CQI Reports, Sounding reference signal, antenna configuration and scheduling requests.

RRC Connection Setup message IEs layout

It is not possible in this blog to explain all the information carried by this message but an example message taken from test network is shown below 

RRC Connection Setup message 

RRC Connection Setup Complete

After receiving the RRC Connection setup message, UE complete the three way handshake procedure by sending 'RRC Connection setup complete' message and moves to RRC Connected mode. 

The message contains following information

• selectedPLMN-Identity:  This is equal to 1 if UE selects the first PLMN from the plmn-identityList included in SIB1 or 2 if the second PLMN is selected in case UE belongs to more than one PLMN
• dedicatedInfoNAS:  This IE is used to transfer UE specified NAS layer information between network and UE.

Example message is shown below

RRC Connection Setup complete message

For more LTE call flows, please check out this tool