Roaming in a 5G Network

Roaming in a 5G Network

Due to good responses to one of my LinkedIn Post on 5G Roaming, I have decided to write an elaborative article on this topic. This article will cover Telecom Roaming in a broader manner and how it will be conceptualized in a 5G Network.

Roaming Basics (Click to enlarge)

What is Roaming?

Roaming is a very critical feature of Telcos that allows the Subscribers to use their Mobile Services outside of the Home Networks. Home Network refers to the Service Provider's Coverage Area. Services can be making or receiving Calls, Mobile Data usage, Supplementary Services like Call Forwarding, etc.

Roaming provides additional revenue streams for Operators as they can connect Outside Visitors to their Network for usage as well as allow their Home Subscribers to use the Visitor Network.

Roaming concept is quite old and already 2G/3G/4G subscribers are enjoying this feature worldwide.

How Roaming Works?

Roaming agreement and Connectivity (of Signaling and Bearer) between the Operators are required to enable this feature. Connectivity can be direct between the Operators (e.g. between neighboring countries) or it can be via an International Hub (e.g. between countries that are geographically far away).

Due to obvious reasons, Operators usually maintain Roaming agreements with more than one Roaming Operator. It allows reliability as well as robustness in the Roaming Connectivity offered to their Home Subscribers.

Concepts of Inbound and Outbound Roamers

Home Subscriber roaming in a Visiting Network: This Subscriber will be an Inbound Roamer for the Visiting Network and an Outbound Roamer for the Home Network.

Visiting Subscriber roaming in a Home Network: This Visiting Subscriber will be an Inbound Roamer for the Home Network and an Outbound Roamer for the Visiting Network.

Through Inbound Roaming, the Visiting Network Operator grants access and consumption of its resources to the Roaming Subscribers from the Partner Network.

Outbound Roaming allows Home Subscribers to access the Visiting Network Services as per the Roaming Pack provisioned by the Home Operator.

Usage of Common Mobile Services

Home Subscriber activates the International Roaming Pack based on the Region/Country of Visit. Roaming Pack gets activated in the Subscriber's package.

Home Subscriber moves to the Roaming Country and Switches ON the Phone.

Subscriber chooses either of the 2 Options to register to the Network of Roaming Operators; choose the mobile network automatically or choose an operator manually from the list.

The Roaming Operator checks the identity of the Subscriber before it latches it on the Network. It verifies with the Home Operator, whether the Foreign Subscriber has subscribed to Roaming services or not.

Roaming Operator checks with the Home Subscriber's HLR or HSS using the Subscriber's IMSI and downloads the profile in its Visiting Register (E.g. VLR of the Roaming Operator).

From the Subscriber's Subscription Profile, it knows which Mobile Services are to be allowed in its Network.

The Visiting Network updates the Home Operator's HLR/HSS with the current roaming location of UE.

As the UE is latched on the Roaming Network, the Subscriber can start consuming the Voice & Supplementary Services using the resources of the Visiting Network.

fig. Roaming Infrastructure (Click to enlarge)

For Data, GRX (GPRS Roaming Exchange) / IPX (IP Exchange) backbone comes into the picture.

This backbone enables the IP Connectivity between the Roaming Partners and allows IP data routing. Operators also utilize GRX/IPX connectivity for negotiating SLAs as per the required QoS during Roaming operations.

Other infrastructure related to Voice Service Connectivity (of Signaling and Bearer) remains intact for 2G/3G Operators.

4G is a complete IP-based network. So, the UE or Subscribers need to have the IP Connectivity throughout the time it is latched on the 4G Network. The IP Connectivity can be obtained either from Home Network or Visiting Network.

Roaming Approaches in a 5G Network

Before going to the below section, please refer to the article on Basics of 5G Architecture where I have explained the basics of related 5G Network Functions and Service Based Architecture (SBA).

Local Breakout: In Local Breakout, data traffic is routed directly from the Visiting Network (VPLMN) to the Data Network while authentication and handling of subscription data are handled in the Home Network (HPLMN). Basic roaming Policy and Charging is applied by the Visiting PCF and CHF as per roaming agreements. In this case, only signaling data is routed to the Home network.

fig. Roaming - Local Breakout (Click to enlarge)

Here, the IP Address is obtained from the Visiting Network. It means that the UE is using Radio, SGSN (for GPRS/3G) / SGW (for 4G) / AMF/SMF (for 5G), and GGSN (for GPRS/3G) / PGW (for 4G) / UPF (for 5G) of the Roaming Network for the connectivity.

Home Routed: In this case, the IP Address is obtained from the Home Network. Here, UE uses Radio & SGSN (for GPRS/3G) / SGW (for 4G) / AMF/SMF (for 5G) of the Roaming Network and GGSN (for GPRS/3G) / PGW (for 4G) / UPF (for 5G) of the Home Network.

fig. Roaming - Home Routed (Click to enlarge)

In the Home-Routed scenario, the Visiting Network data traffic is routed to the Data Network via the Home network. It provides more control to the Operators wrt offering roaming services, policy, and charging the subscribers. However, it adds an extra layer of complexity and lag to the network. Along with the signaling data, bearer data is also routed to the Home network.

In 5G Roaming, SEPP (Security Edge Protection Proxy) acts as a service relay between VPLMN and HPLMN for providing a secured connection as well as hiding the network topology. You can compare its functionalities as similar to SBC (Session Border Controller) when Voice packets are routed from the Core network to the IMS network for VoLTE service.

Simplified View of Roaming Usage and Settlements

For the purpose of simplicity, 3^rd Party Clearing House is not mentioned.

fig. Roaming - Usage, Billing, and Settlements (Click to enlarge)

Roaming Settlements happen at the Operator's level by exchanging roaming records for Inbound and Outbound Roamers. In general, these records are mediated via Clearing/Settlement House in the format agreed for the information exchange (E.g. TAP records) between the Roaming Partners.

Home Operator charges the Subscriber for Roaming via its Retail Billing. Operators sell the Roaming packs to their Subscribers to enable Charging & Usage. Depending on the Regions or Countries, Usage Charges (e.g. Incoming Calls, Outgoing Calls, Data Usage, etc.) vary and you can find the Roaming Charges on the Telecom Operators' websites.

Conclusion

There are quite a no. of advancements made in this area for easing the use and reducing the costs. Roaming has also advanced to remove dependencies on factors like Vendor providers, Devices, Mobile Generation used, etc. between Home & Visiting Networks.

Operators are upgrading their traditional OSS/BSS systems to prevent Bill Shocks, perform real-time charging of Roaming records, etc., and also trying to ease the Roaming Operations and Settlements with technologies like eSIMs, Multi IMSI SIM models, and Blockchain.

📖 Master 5G Roaming & Settlements

If you are a Network Architect or BSS Professional, understanding 5G Roaming is no longer optional - it is a core requirement for global connectivity. My book "The 5G Core: Architecture and Functions Explained" takes you beyond the basics into the technical specifics of SEPP, HR vs. LBO call flows, and the business logic of 5G settlements.

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I do hope that this article has helped you in some way to understand Roaming and its approaches in the 5G world.

Kindly share this article with your friends and colleagues. Feel free to like and comment. Happy learning.

Glossary

PLMN: Public Landline Mobile Network
HPLMN: Home PLMN
VPLMN: Visiting PLMN
SEPP: Security Edge Protection Proxy
VoLTE: Voice over LTE
EPC: Evolved Packet Core
SBC: Session Border Controller
PCF: Policy Control Function
CHF: Charging Function
IMS: IP Multimedia Subsystem
SBA: Service Based Architecture
UE: User Equipment
UPF: User Plane Function
AMF: Access & Mobility Management Function
SMF: Session Management Function
NEF: Network Exposure Function
AF: Application Function
UDM: Unified Data Management
NWDAF: Network Data Analytics Function
GRX: GPRS Roaming Exchange
IPX: IP Exchange
IMSI: International Mobile Subscriber Identity
HLR: Home Location Register
VLR: Visitor Location Register
HSS: Home Subscriber Server
SGSN: Serving GPRS Support Node
GGSN: Gateway GPRS Support Node
PGW: PDN Gateway
SGW: Serving Gateway
AUSF: Authentication Service Function
TAP: Transferred Account Procedure
OSS: Operations Support System
BSS: Business Support System

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