Roaming
The ability of a mobile subscriber to access services while in a visited network outside their home PLMN, enabled by inter-operator agreements and signaling via SEPP in 5G.
Roaming is a subscriber using service on a network that isn't their home operator's, made possible by agreements and signalling between the two PLMNs. 5G keeps the two classic models: home-routed, where user traffic is hauled back to the home network's UPF (so the home operator anchors the data path and breakout), and local breakout, where the visited network's UPF handles the data locally for lower latency.
What changed in 5G is the security at the border. Instead of exposing core signalling directly, inter-PLMN traffic passes through the SEPP (Security Edge Protection Proxy) at each network's edge, which protects and mediates the control-plane messages crossing between operators. Slicing adds another wrinkle: a slice in the visited network may carry a different S-NSSAI than at home, so mapped S-NSSAIs are used to line them up. When you debug a roaming subscriber, you're effectively tracing a flow that crosses two operators' cores through their SEPPs.
Frequently asked questions
- What is the difference between home-routed and local breakout roaming?
- In home-routed roaming, the visited network sends the user traffic back to a UPF in the home network, so the home operator controls the data path (useful for consistent services, charging, and lawful intercept). In local breakout, the visited network terminates the data locally on its own UPF, cutting latency and backhaul — better for things like local internet access or edge services, at the cost of less home-network control.
- What role does the SEPP play in 5G roaming?
- The SEPP sits at the edge of each operator's network and secures the control-plane signalling exchanged between the home and visited PLMNs. It protects the SBA messages crossing the boundary — handling things like topology hiding and message protection — so operators don't expose their core interfaces directly to each other. Both networks have one, and inter-PLMN signalling flows SEPP-to-SEPP.
Related terms
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