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) Roaming Basics
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 allows 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 InboundRoaming, the Visiting Network Operator grants access and consumption of its resources to the Roaming Subscribers from the Partner Network.

OutboundRoaming 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 HLS/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.

Roaming Infrastructure
(Click to enlarge) Roaming Infrastructure
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).

LocalBreakout: 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.

Roaming - Local Breakout
(Click to enlarge) Roaming - Local Breakout
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.

HomeRouted: 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.

Roaming - Home Routed
(Click to enlarge) Roaming - Home Routed

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, 3rd Party Clearing House is not mentioned.

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

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), NFs (Network Functions), 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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  1. Thanks for quite informative post.
    Quick question, Before 5G, there's no security consideration between Visited and Home so there can be fake Update Location (or ULR) from Visiter's MME to Home HSS. As 5G introduce SEPP and I wonder how it protects such fake messages?

    1. Dear Reader,
      Thanks for reading. As per spec 33.501 SEPP providers need to make sure to detect malformed N32 messages as well as maintain the integrity between roaming partners. Detections can be based on rules or analyzing (via real-time analytics) the signaling data exchanges for fraud detections. It will certainly evolve.

  2. In case of LocalBreakOut , Charging can be done Online or Offline ? And i beleive in HomeRouted, as the home network is involved, OCS can be connected for Charging easily. Please help to understand a bit more in this regard.

    1. Hi Nilesh,

      For Local Breakout, basic charging can be online or near realtime, as per the implementation & roaming agreement.
      For Home Routed, data is routed via Operator's Network and Home CHF is used. Hence, you can fully utilise Charging Server wrt rate plans & any rating time discounts.

      Note - Above behaviours are stated in reference to Outbound Roamers of the Home Network.


    2. Why is seemingly nobody enabling the local breakout?

      I live in Europe and use 4G and 5G roaming all the time, but am very annoyed by the performance I am getting because all traffic being sent back to home country.

      Ping times are absurd if I get far away from home, any service requiring lower latency becomes unusable.

  3. Do you have any reference article for 5G voice call service in roaming where There is CSFB required during the call. How does it happen and flow diagram or any detailed article would help. Thank You

  4. Thanks for the article. How does vSEPP selects hSEPP?

  5. In home routed roaming, ip address will be assigned by hplmn but question is who will assign the ip address for ue? smf or other function?


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