5G (NR) RAN Learning - Uplink and Downlink RAN ​​Status Transfer

RAN Status Transfer is the process of transferring a terminal (UE)'s uplink and downlink status information from a source radio access network (RAN) node to a target RAN node in a 5G network. This typically occurs during handover or dual connectivity scenarios. During this process, the AMF transmits information about downlink data (e.g., the number of forwarded packets), along with SN status and the PDCP (Packet Data Convergence Protocol) sequence number and hyperframe number (HFN) status for both uplink and downlink data, to the target RAN.

I. Uplink RAN ​​Status Transfer aims to enable lossless handovers over NG-RAN. The transfer process uses UE-related signaling. The specific process is shown in Figure 8.4.6.2-1 below, where:

• The source NG-RAN node initiates this process by stopping allocating PDCP SNs for downlink SDUs and sending an UPLINK RAN STATUS TRANSFER message to the AMF when it deems the transmitter/receiver status frozen.
• For each DRB for which PDCP-SN and HFN status preservation is applicable, the source NG-RAN node shall include the DRB ID IE, UL COUNT IE, and DL COUNT IE in the DRB Subject Status Transfer List IE within the RAN Status Transfer Transparent Container IE of the UPLINK RAN STATUS TRANSFER message.
• For each DRB for which the source NG-RAN node has accepted an uplink forwarding request from the target NG-RAN node, the source NG-RAN node may also include the missing and received uplink SDUs in the UL PDCP SDUs IE of the UPLINK RAN STATUS TRANSFER message.

II. Downlink RAN ​​Status Transfer aims to implement NG-RAN-based lossless handover transfer procedures using UE-related signaling. The specific process is shown in Figure 8.4.7.2-1 below, where:

• The AMF initiates this procedure by sending a DOWNLINK RAN STATUS TRANSFER message to the target NG-RAN node. The target NG-RAN node performing this handover according to TS 38.300 and using a full configuration shall ignore the information received in this message.
• For each DRB in the RAN Status Transfer Transparent Container IE that is subject to the State Transfer List IE, the target NG-RAN node shall not transmit any uplink data packet with a PDCP-SN lower than the value of the UL Count Value IE.
• For each DRB in the RAN Status Transfer Transparent Container IE that is subject to the State Transfer List IE, the target NG-RAN node shall use the value of the DL COUNT Value IE of the first downlink data packet that has not yet been assigned a PDCP-SN.
• If at least one DRB in the RAN Status Transfer Transparent Container IE of the DOWNLINK RAN STATUS TRANSFER message contains the receive status of the UL PDCP SDUs IE, the target NG-RAN node may use it in status report messages sent to the UE over the radio interface.