Understand PRA Based Differential Charging between 4G and 5G NSA Subscriber

Introduction

This document describes Presence Reporting Area (PRA) based Differential National Security Agency (NSA) online charging solution.

Prerequisites

Requirements

Cisco recommends that you have knowledge of these topics:

• PRA
• Mobility Management Entity (MME)
• Cisco Serving Gateway (SGW)/Cisco Packet Data Network Gateway (PGW)
• Policy and Charging Rules Function (PCRF)

Also,

• MME to support PRA feature enhancement to map 'S1-U IP Address' to 'PRA ID'
• PGW supports PRA trigger towards PCRF
• PCRF installs new PCC rulebase once it receives the presence-reporting-area-status as In area (0) or out of area (1) from GW

Components Used

The information in this document is based on StarOS : 21.28.mx.

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command.

Background Information

This feature is to support the requirement of differentiate the charging between 4G and 5G customers in the 5G NSA setup for prepaid subscriber (online charging).

PRA is an area defined within 3GPP packet domain for reporting UE presence within that area for policy control and/or charging reasons.
For NSA differential charging, PRA feature is used to report subscriber presence in 4G and 5G.

PRA ID Solution Overview

Expectation from/on MME:

• MME is expected to detect the UE movement from 4G to 5G coverage (gNB) and vice versa in order to build the logic to map this event with PRA reporting.
• PRA ID must be same as configured in PCRF for differential charging.
• Applicable to DCNR UE only.

PCRF subscribes to PRA event trigger:

• PGW stores PRA action and forwards to SGW

When 4G to 5G transition happens (S1 tunnel switch):

• Based on gNB transport address, MME marks the PRA ID status as OPRA (out of 5G coverage)/IPRA (in 5G coverage)
• MME communicates PRA information to SGW and SGW forwards to PGW

PGW receives PRA information from SGW and forwards to PCRF:

• PCRF changes rulebase based on PRA information
• User-plane is communicated about change of rulebase

High level setup architecture

Abbreviations

PRA	Presence Reporting Area
OCS	Online Charging System
GW	Gateway (GGSN/PGW)
PCRF	Policy and Charging Rules Function
MOP	Method of Procedure
MME	Mobility Management Entity
SGW	Serving Gateway
PGW	Packet Gateway

Possible Impacts and Considerations

• Solution proposed for Option3x mode of 5G NSA deployment only.
• Since UE movement is tracked from 4G to 5G and 5G to 4G, this tracking is informed to SGW/PGW, so high number CCR-U’s are expected.
• Proposed solution is a customization and has not been implemented globally.
• End-to-end field testing is required to be done in the network.
• CUPS UP/legacy SPGW performance are impacted due to extra signalling:
  • Throughput impact (extra signalling on SPGW + increased Sx modifications in CUPS solution)
  • Frequent toggling of UE between 4G/5G results in more signalling for PRA
  • Slowpath/Fastpath flow switches due to rulebase changes
• Cisco PCRF supports PRA feature.
• Enabling differential charging causes additional signaling on Gx interface, which can impact PCRF performance.

Flow

X2-HO – PRA Differential Charging

X2 HO with SGW Relocation - PRA Differential Charging (4g5g)

S1 HO (No SGW Relocation) - PRA Differential Charging

GnGp (GGSN to PGW) HO – PRA Differential Charging (UE Moving to 5g)

Procedure

MME End Configuration Changes

• Configure pra-profile and associate pra-profile in mme-service.
• Up to 50 IPv4 subnets and 50 IPv6 subnets can be added to pra-profile.
  As of now only pra-profile is supported.
• At any point of time, association or dissocation of pra-profile from mme-service does not give restart to mme-service.

config
  lte-policy
    pra-profile dcnr-5g-radio 5G-PRA
     gnb-s1u ipv6-prefix xxxx:xxxx:x:84a4::/64
     gnb-s1u ipv6-prefix xxxx:xxxx:2b::/48
     gnb-s1u ipv6-prefix xxxx:xxxx:4:8601::2:540d
    exit
end
config
 context s1mme
  mme-service mme
   associate pra-profile dcnr-5g-radio 5G-PRA
end

GW Configuration Changes

• Configure endcode-supported-feature cno-uli under ims-auth-service.
• cno-uli enables PRA information reporting feature.
• Configure separate RG. RG is used to report 5G usage.
  
  

configure 
 context context_name 
 ims-auth-service service_name 
 policy-control 
 diameter encode-supported-features cno-uli 
 { default | no } diameter encode-supported-features 
 end

config
active-charging service ECS
 group-of-ruledefs NPR1_5G
 group-of-ruledefs-application gx-alias
 add-ruledef priority 2 ruledef RG_5G_default_IP_ANY_PrePaid
 add-ruledef priority 40 ruledef tethering_ip_ttl_RG
exit

ruledef 5G_default_IP_ANY_PrePaid
 ip any-match = TRUE
exit

rulebase <rulbase Name>
 action priority 702 static-and-dynamic ruledef 5G_default_IP_ANY_PrePaid charging-action 5G_IP_ANY_with_Gy_PrePaid
exit
end

Notes:

• diameter encode-supported-features: Enables or disables encoding and sending of supported-features AVP.

• cno-uli: Enables PRA information reporting feature.

• no: Removes the previously configured supported features.

• default: Applies the default setting for this command.

Verification

Wireshark Capture MME

When UE moves to 5G, inside presence reporting shows as True.

When UE moves to 4G, inside presence reporting showi as False.

Wireshark Capture GW

You can see when UE moves to 5G area usage reported with RG: 623 while for 4G usage reported with RG: 403.

DRA receives presence-reporting-area-status as In area (0) when UE in 5G or out of area (1) when UE in 4G from GW.

CCR-I

When MME reports presence reporting area true, GW sends CCR-I to PCRF with Presence Reporting Area Information: Supported.

CCA-I

Here, you can see that whenever DRA receives presence-reporting-area-status as in area (0) or out of area (1) from GW, it is sending rat type as a GAN and EUTRAN respectively towards PCRF. Basis on this rat type PCRF is changing rulebase and modifying QOS for 4G and 5G.

Revision History

Revision	Publish Date	Comments
2.0	12-Aug-2026	Updated Content
1.0	11-Aug-2024	Initial Release