Inter-PLMN security: SEPP and TLS

Securing the Roaming Border

When a subscriber roams, signaling messages must cross between the visited and home PLMNs. In 4G, inter-PLMN traffic passed through the IPX backbone with limited security, relying heavily on trust between operators. 5G replaces this with the SEPP (Security Edge Protection Proxy), a mandatory border node at each PLMN. All inter-PLMN signaling flows through a pair of SEPPs: the visited SEPP and the home SEPP. They communicate over the N32 interface, which has two sub-interfaces — N32-c for control-plane capability negotiation and N32-f for forwarding the actual API messages.