Serving Radio Network Subsystem (SRNS) Relocation

SRNS Relocation is a procedure executed at UTRAN side where by:

• Serving RNC is shifted from Current Serving RNC to Controlling RNC, .i.e. Iu signalling connection is shifter from current SRNC to the CRNC. After this CRNC becomes the Serving RNC.
• This may happen when UE sends measurement report to change serving cell, and this new serving cell belongs to different CRNC
• On reception of Meas Report NW may decide to perform Serving Cell Change with SRNS relocation
• At times SRNC and CRNC for a UE may be same or different.

SRNC and CRNC is the terminology used with respect to a UE. 

Serving RNC (SRNC):

• Serving RNC is that RNC where the Iu signalling connection terminates between RNC and the Core NW.

Controlling RNC (CRNC):

• Controlling RNC is that RNC which provides physical resources to the UE. In other words, CRNC controls and owns the resources of the cell to which UE is connected.

To perform SRNS relocation UTRAN may include the following IEs in any reconfiguration message with the following restriction:

•  if the reconfiguration procedure is simultaneous with SRNS relocation procedure:
• if the transmitted message is a RADIO BEARER RECONFIGURATION:
• include the IE "New U-RNTI".
• else:
• include the IE "Downlink counter synchronisation info".

Downlink Counter Synchronization Info:

DL Counter Sync Contains two IEs:

1. RB with PDCP SN Info: This contains two IEs as:
• RB ID
• RPCP Seq NO Info
• This IE is included for each RB having PDCP and lossless SRNS Relocation is needed for that RB.
• In order to perform lossless SRNS relocation, UTRAN provides a PDCP Seq no from where the transmission starts after relocation is done
2. RB with PDCP Context relocation: This contains three IEs as:

• RB ID
• DL RFC 3095 context relocation
• UL RFC 3095 context relocation
• RFC 3095 is basically used for ROboust Header Compression (ROHC)
• The context reinitialization MUST be done for all contexts at the compressor.
• This parameter may for instance be used to do context relocation at, e.g., a cellular handover that results in a change of compression point in the radio access network.

UE behavior on reception of DL-CounterSynchronization Info:

• If the IE "PDCP SN Info" is included, the UE shall:
• transfer the sequence number to the PDCP entity for the radio bearer;
• configure the RLC entity for the radio bearer to stop i.e.
• Stop RLC entity for SRBs 1, 3 & 4
• Stop all RLC AM and UM entities for RBs whose PDCP SN is not included
• include the current PDCP receive sequence number and the radio bearer identity for the radio bearer in the variable PDCP_SN_INFO in the response message

      Note: RBs whose SN is Included in the IE "PDCP SN Info" is not STOPPED

• Else if PDCP SN Info is not included in the message
• Stop all RLC AM an UM entities, except SRB - 2
• SRB - 2 is re-established
• if the received message contains "ciphering mode info"
• If security is pending, then it is applied and response is sent with new ciphering configuration
• set the new uplink and downlink HFN component of COUNT-C of RB2 to MAX(uplink HFN component of COUNT-C of RB2, downlink HFN component of COUNT-C of RB2);
• increment by one the downlink and uplink values of the HFN of COUNT-C for RB2;
• calculate the START value
• include the calculated START values for each CN domain in the IE "START list" in the IE "Uplink counter synchronisation info";

After transmission of response message UE needs to wait for RLC ACK for completion of SRNS Relocation procedure in UE.

After reception of successful RLC ack UE shall:

• Configure all RBs to "Continue", except for SRB-2 if not stopped
• re-establish all RLC entities except SRB-2
• Initialize first 20 bits of HFN of the Count - C of all AM and UM RLC entities to the START value transmitted in the response message
• Initialize remaining bits of Count - C of all UM RLC entities to zero

New - URNTI:
UTRAN Radio Network Temporary Identity (URNTI), is an identity allocated to UE having an RRC Connection and identifies the UE with in sUTRAN

• Initially Allocated by UTRAN on RRC Conn Establishment
• During SRNS Relocation NW may allocate New - URNTI to the UE
• On reception of this IE UE needs to calculate START Value and include calculated START in the response message