Reduced Capability Support

Table 1. Feature History

Feature name

Release information

Description

Reduced Capability support on SMF

2025.03.0

This feature enables the SMF to support PDU sessions establishment over the RedCap RAT.

SMF supports the connectivity of Reduced Capability UEs with the network by defining a new RAT type as RedCap.

Commands introduced:

  • user-plane-buffering-mgmt true : Enables buffering at UPF.

  • hlcom true : Enables HLCOM sessions for entire DNN.

  • policy extended-buffering extbuff { sbpc sbpc_count ddnd ddnd_count | dbpc dbpc_count dbd dbd_duration} : Configures the SBPC, DDND, DPDC, and DBD parameters.

  • priority priority_val ruledef ruledef_name actiondef actdef_name event im-entry/paging_failure : This CLI defines the event policy.

  • ruledef ruledef_name condition { rat matches redcap | hlcom is true } : This CLI defines the rule.

  • actiondef actdef_name priority priority_val action extended-buffering attributes policy extbuff : This CLI defines the action.

Reduced Capability

Reduced Capability (RedCap) is a new 5G standard, developed specifically for UEs that do not require the full capabilities of 5G. The RedCap UEs, such as wearables and industrial sensors, typically operate in low-power or idle states and transition to the active state only when data transfer is needed. For the rest of the time, the RedCap UEs are either in power-saving mode or are unreachable.

High Latency Communication

When a RedCap UE enters the power-saving modes like "Power-Saving Mode (PSM)" or "Extended Idle Mode DRX (eDRX)", it becomes unreachable for a period, leading to high latency during the initial data exchange. To mitigate this, the High Latency Communication (HLCOM) feature buffers Mobile Terminated (MT) data at the UPF, while the UE is in the power-saving state. This extended buffering ensures that the data is not lost and is available when the UE becomes reachable again.

The HLCOM is particularly beneficial for Machine Type Communication (MTC) devices (IoT devices) that frequently use PSM and eDRX to optimize energy consumption.

This feature allows the network operators to support the connectivity of RedCap UEs with the network by introducing a new RAT type in SMF, called as RedCap.

How RedCap Support works

Workflow


Note

For this feature to work

  • HLCOM should be enabled on SMF. For more details on the HLCOM configuration, see the Enable HLCOM for a RedCap session section.

  • The compliance profile should be configured with v17.5.0 for the service nsmf-pdusession.

These stages explain the process of RedCap support on SMF:

  1. RedCap PDU Session Establishment: A RedCap UE sends a PDU Session Establishment Request to AMF.

    Upon receiving the request, AMF sends a PDU Session SM Context Request to SMF and sets the ratType IE as RedCap in the message.

  2. Subscription, Policy Creation, and Charging Data requests: SMF receives the ratType IE as RedCap from AMF and forwards it to UDM, PCF, and CHF for subscription, policy creation, and charging data requests respectively.

  3. BAR creation during N4 Session Establishment: The buffering duration and packet count for HLCOM are communicated from the SMF to the UPF using Buffering Action Rules (BARs) as defined in the 3GPP TS 29.244 specification.

    SMF sends the N4 Session Establishment Request to the UPF including the ratType IE as RedCap. SMF also creates an empty BAR and associates it to the FARs during N4 Session Establishment.

  4. UE's entry in the Idle mode: SMF instructs the UPF to buffer when the UE moves to idle state. SMF sends an N4 Session Modification Request to UPF with UpdateBAR IE containing these IEs:

    Table 2. Buffering IEs in UpdateBAR IE

    Buffering IE

    Description

    DDND (Downlink Data Notification Delay)

    		 The UP function supports the buffering parameter "Downlink Data Notification Delay (DDND)".

    SBPC (Suggested Buffering Packets Count)

    The UP function supports the "Suggested Buffering Packets Count (SBPC)" is present, when the UP function indicates the support of the feature UL/DL Buffering Control (UDBC).

    UPF sends the Downlink Data Notification to SMF.

  5. UE's exit from the Idle mode: SMF sends an N1 N2 Transfer Request to AMF. SMF sets the extBufSupport IE (Extended Buffering Support) to true in the N2 PDU Setup Request message to indicate extended buffering support to AMF based on configuration and based on the UPF support.

    SMF does not set extBufSupport to true in case of paging for a control plane message.

    As the UE is unreachable, the AMF sends a PDUSessionResourceSetupRequestTransfer Response message to SMF including the status code and cause as 504: UE_NOT_REACHABLE. SMF also receives a maxWaitingTime IE (Estimated Maximum Waiting Time) in the N1N2MsgTxfrErrDetail IE. SMF starts the maxWaitingTime IE timer and stores the flow information for retry.

    SMF sends the N4 Modification Request with UpdateBAR with "DL Buffering Duration (DBD)" same as the estimated maximum wait time and "DL Buffering Suggested Packet Count (DBPC)" from the configuration, if the UPF has indicated the support for DBDM.

    For more details on the configuration, see the Configure buffering values for HLCOM RedCap sessions section.

Limitations

These are the known limitations of Reduced Capability support on SMF:

  • 17.x compliance profile is required for nsmf-pdusession, npcf-smpolicycontrol, and chf-convergedcharging services.

  • 16.9.x compliance profile is required for namf-comm services.

  • SMF supports the extended buffering (HLCOM) functionality only for the RedCap RatType and not for other RatTypes.

  • SMF supports the extended buffering only for Idle mode exit due to data trigger.

  • The RedCap PDU Session Establishment is not supported for roaming calls.

  • UPF selection based on HLCOM supported feature bits is not supported.

  • The AMF selection or discovery for namf-comm service based on HLCOM is not supported.

  • SMF will cap the DBD value to a lower value in case of encoding restrictions on the N4 interface.

  • The ratType label with NR_REDCAP value is not supported for policy_msg_processing_status stats.

Standards compliance

SMF supports 3GPP compliance profile version 17.5.0 for the service nsmf-pdusession.

Enable RedCap support on SMF

Follow these steps to enable the RedCap support on SMF:

Procedure

SUMMARY STEPS

  1. Enable HLCOM for a RedCap session.
  2. Configure buffering values for HLCOM RedCap sessions.

DETAILED STEPS

Step 1

Enable HLCOM for a RedCap session.

Step 2

Configure buffering values for HLCOM RedCap sessions.

Enable HLCOM for a RedCap session

This configuration in DNN profile is used to send ExtBuffSupport in N1N2 message.

Follow these steps to enable the HLCOM for a RedCap session:

Procedure

SUMMARY STEPS

  1. Use the CLI profile dnn dnn_profile to create an instance of the DNN profile.
  2. Use the CLI user-plane-buffering-mgmt true to create BAR and link the FARs to it.
  3. Use the CLI hlcom true to send ExtBuffSupport IE in the N1N2 message. Save and exit from the current configuration mode.

DETAILED STEPS

Step 1

Use the CLI profile dnn dnn_profile to create an instance of the DNN profile.

Example:

[smf] smf# config 
[smf] smf(config)# profile dnn data-hlcom 
[smf] smf(config-dnn-data-hlcom)#

Step 2

Use the CLI user-plane-buffering-mgmt true to create BAR and link the FARs to it.

Example:

[smf] smf(config-dnn-data-hlcom)# user-plane-buffering-mgmt true 
[smf] smf(config-dnn-data-hlcom)#

Step 3

Use the CLI hlcom true to send ExtBuffSupport IE in the N1N2 message. Save and exit from the current configuration mode.

Example:

[smf] smf(config-dnn-data-hlcom)# hlcom true 
[smf] smf(config-dnn-data-hlcom)# exit  
[smf] smf(config)#

What to do next

After enabling the HLCOM for a RedCap session, you must configure the buffering values for the HLCOM session. For more information on this, see the Configure buffering values for HLCOM RedCap sessions topic.

Configure buffering values for HLCOM RedCap sessions

These steps define the process of buffering values for HLCOM RedCap Sessions:

Before you begin

Before configuring the buffering values for HLCOM sessions, you must enable HLCOM. For more information on this configuration, see the Enable HLCOM for a RedCap session topic.

Procedure

SUMMARY STEPS

  1. Use the CLI policy extended-buffering extbuff to create an instance of the Extended Buffering policy.
  2. Use the CLIs { sbpc sbpc_count ddnd ddnd_count } { dbpc dbpc_count dbd dbd_duration } to define the SBPC, DDND, DBPC, and DBD parameters. Save and exit the current configuration mode.
  3. Use the CLI policy eventmgmt eventmgmt_name to create an instance of the Event Management policy.
  4. Use the CLI priority priority_val ruledef ruledef_name actiondef actdef_name event im-entry/paging_failure to define the ruledef and actiondef for the configured Event management policy. Save and exit the current configuration mode.
  5. Use the CLI policy rulemgmt rulemgmt_name to create an instance of the Rule Management policy.
  6. Use the CLI ruledef ruledef_name condition { rat matches redcap | hlcom is true } to define the rule. Save and exit to the Global Configuration mode.
  7. Use the CLI policy actionmgmt actmgmt_name to create an instance of the Action Management Policy.
  8. Use the CLI actiondef actdef_name priority priority_val action extended-buffering attributes policy extbuff to define the policy, extended buffering, and priority. Save and exit to the Global Configuration mode.

DETAILED STEPS

Step 1

Use the CLI policy extended-buffering extbuff to create an instance of the Extended Buffering policy.

Example:

[smf] smf# config 
[smf] smf(config)# policy extended-buffering buf1 
[smf] smf(config-extended-buffering-buf1)#

Step 2

Use the CLIs { sbpc sbpc_count ddnd ddnd_count } { dbpc dbpc_count dbd dbd_duration } to define the SBPC, DDND, DBPC, and DBD parameters. Save and exit the current configuration mode.

Example:

[smf] smf(config-extended-buffering-buf1)# sbpc 200 ddnd 1000  
[smf] smf(config-extended-buffering-buf1)# dbpc 1000 dbd 30  
[smf] smf(config-extended-buffering-buf1)# exit 
[smf] smf(config)#

Note

 

SBPC and DDND values are used during the Idle mode entry. DBPC and DBD values are used during Idle mode exit.

Step 3

Use the CLI policy eventmgmt eventmgmt_name to create an instance of the Event Management policy.

Example:

[smf] smf(config)# policy eventmgmt Redcap 
[smf] smf(config-eventmgmt-Redcap)#

Step 4

Use the CLI priority priority_val ruledef ruledef_name actiondef actdef_name event im-entry/paging_failure to define the ruledef and actiondef for the configured Event management policy. Save and exit the current configuration mode.

Example:

[smf] smf(config-eventmgmt-Redcap)# priority 1 event imentry ruledef rdRedcap actiondef adRedcap  
[smf] smf(config-eventmgmt-Redcap)# priority 2 event paging-failure ruledef rdRedcap actiondef adRedcap  
[smf] smf(config-eventmgmt-Redcap)# exit 
[smf] smf(config)#

Step 5

Use the CLI policy rulemgmt rulemgmt_name to create an instance of the Rule Management policy.

Example:

[smf] smf(config)# policy rulemgmt rm1 
[smf] smf(config-rulemgmt-rm1)#

Step 6

Use the CLI ruledef ruledef_name condition { rat matches redcap | hlcom is true } to define the rule. Save and exit to the Global Configuration mode.

Example:

[smf] smf(config-rulemgmt-rm1)# ruledef rdRedcap 
[smf] smf(config-ruledef-rdRedcap)# condition rat matches redcap  
[smf] smf(config-ruledef-rdRedcap)# condition hlcom is true  
[smf] smf(config-ruledef-rdRedcap)# exit 
[smf] smf(config-rulemgmt-rm1)# exit 
[smf] smf(config)#

Step 7

Use the CLI policy actionmgmt actmgmt_name to create an instance of the Action Management Policy.

Example:

[smf] smf(config)# policy actionmgmt ad1 
[smf] smf(config-actionmgmt-ad1)#

Step 8

Use the CLI actiondef actdef_name priority priority_val action extended-buffering attributes policy extbuff to define the policy, extended buffering, and priority. Save and exit to the Global Configuration mode.

Example:

[smf] smf(config-actionmgmt-ad1)# actiondef adRedcap 
[smf] smf(config-actiondef-adRedcap)# priority 1 action extended-buffering attributes policy buf1  
[smf] smf(config-actiondef-adRedcap)# exit 
[smf] smf(config-actiondef-adRedcap)# exit 
[smf] smf(config)#

Monitoring and troubleshooting

This section discusses the bulkstats and show commands used for monitoring and troubleshooting this feature.

Bulkstatistics

These are the statistics used to support this feature:

  • The label ratType in all the existing stats reflect value the as NR:Redcap for Redcap sessions.

  • The N4 message stats is enhanced to indicate BAR creation.

    proto_udp_req_msg_total{app_name="SMF", bar_present="true",
cluster="SMF",data_center="DC",gr_instance_id="1",instance_id="0",
interface_type="N4",message_direction="outbound",
message_name="session_establishment_req",
msgpriority="",peer_info="SMFIP:10.1.9.165:UPFIP:10.1.12.172",
sec_pdr_present="false",service_name="protocol1",status="accepted",
transport_type="origin"} 1

show subscriber namespace smf rat nr-redcap

The output of the show command show subscriber namespace smf rat nr-redcap is enhanced to support the RedCap functionality on SMF. 

[smf] smf# show subscriber namespace smf rat nr-redcap"alwaysOn": "None",
        "dcnr": "None",
        "wps": "Non-Wps Session",
        "ratType": "NR_REDCAP",
        "ueType": "NR Capable UE",
        "iwkEpsInd": true,
        "sessTimeStamp": "2025-07-09 09:09:01.550487493 +0000 UTC",
        "callDuration": "5.471171384s",
        "ipPool": "poolv4DNN4",
        "commonId": 2097190,
        "linkedEbi": 6,
        "smfIwkEpsInd": true,
        "snssai": {
          "sd": "Abf123",
          "sst": 2
        },
        "radiusEpInfo": "10.1.46.49:1812",
        "authAlg": "pap-default",
        "serverGroup": "radServerGrp",
        "authStatus": "Authenticated",
        "roamingStatus": "Homer",
        "uePlmnId": {
          "mcc": "123",
          "mnc": "456"
        },
        "ipAllocationBy": "Radius",
        "anType": "3GPP_ACCESS",
        "initialRatType": "NR_REDCAP"
      },