Workflow

These steps highlight the UPF's process of handling the RedCap UE's connectivity with 5G network:

1. RAT type reception: The UPF receives the RedCap RAT type NR_REDCAP with value 27over the N4 interface.

2. RAT types processing: UPF receives the RedCap RAT type in the ratType IE (3GPP IE) only. All the other RAT types are populated in both the subscriberParams IE (proprietary IE) and ratType IE (3GPP IE).

   If UPF receives the RAT type value in both IEs, it prioritizes the 3GPP ratType IE over the subscriberParams IE, otherwise, it honors the subscriberParams IE values.

3. DDN BAR level buffering initialization: SMF creates an empty BAR and associates it with FARs during the session establishment.

4. Feature capability indication: UPF advertises its support for one of these key features to SMF and stores them in the UP function feature context:

   • DDND: The UP function supports the buffering parameter Downlink Data Notification Delay. If the DDND feature is enabled on the UPF, it delays sending PFCP Session Report Request messages by the time specified in this IE, with reference to the first packet arrival timestamp.

   • UDBC: The UP function supports the UL/DL Buffering Control buffering parameter, in the PFCP Session Modification Request. If the UDBC feature is enabled, the UPF buffers the number of downlink packets suggested by the SMF.

     This value takes precedence over any default or locally configured buffer limits.

   • DBDM (DBD): The UP function supports buffering parameters DL Buffering Duration and DL Buffering Suggested Packet Count in the PFCP Session Modification Request.

     If the DBDM feature is enabled, the UPF buffers downlink data packets for the specified duration, extending the buffering window.

     The DL Buffering Suggested Packet Count IE indicates the number of downlink data packets, when the extended buffering of downlink data packet is required in the UP function. The packets that exceed the limit are discarded.

   These features are enabled or disabled via UPF CLI configurations.

   When a RedCap UE enters the Idle mode, the SMF sends N4 modification requests containing updated BARs with specific supported features, that are, DDND, UDBC, or DBDM.

   When you enable or disable the RedCap feature after switchover, it triggers Sx Update Request.

5. Dynamic buffering: UPF honors the Suggested Buffering Packet Count or DL Buffering Suggested Packet Count IE received from SMF. If these IEs are not present, UPF considers FAR level buffering with the default value of buffering-limit far-max-packets, that is five packets.

   UPF discards any packet that exceeds the determined buffer limit are discarded, and corresponding DDN drop counters are incremented for monitoring.

   If the SMF sends the DL Suggested Buffer Packet Count, UPF overrides the Suggested Buffer Packet Count or Buffer Cap present in the UPF configuration.

   The parameter "Suggested Buffer Packet Count" suggests the UPF for BAR level buffering, else UPF follows FAR-level buffering. Therefore, UPF is always expecting a value in this parameter from SMF, for BAR-level buffering. However, this value should be non-zero to honor BAR-level buffering and extend it further.

   Since the purpose of the parameter "DL Buffering Suggested Packet Count" is to extend the buffering, is suggested in the "Suggested Buffer Packet Count", therefore the "DL Buffering Suggested Packet Count" supports "Suggested Buffer Packet Count" and cannot be sent without "Suggested Buffer Packet Count".

6. 'Idle' to 'Active' transition: When a UE transitions from the idle state to active state, the UPF automatically resets the buffering parameters stored in the BAR context.

7. EDR support: UPF updates NR RedCap or NR eRedCap RAT type in EDR.

8. Handover support: UPF supports the handover from RedCap to 4G in the Idle mode. In all the other scenarios, handover happens in the Active mode.