Diameter
An AAA protocol used in 4G/EPC for signaling between HSS, MME, PCRF, and other nodes, carrying authentication, authorization, and policy/charging messages.
Diameter is the AAA workhorse of the 4G core — the signalling protocol tying together the HSS, MME, PCRF, OCS and friends. It carries authentication and subscriber data (the S6a interface between MME and HSS), and policy and charging control (the Gx interface to the PCRF, Gy to the charging system), among many others. It's the successor to RADIUS, built for the reliability and richer feature set that operator signalling needs, and it runs over reliable transport (typically SCTP).
If you work across 4G and 5G, the contrast is instructive. 5G replaced this whole family of dedicated Diameter interfaces with the service-based, HTTP/2 model — the UDM/AUSF and PCF now expose APIs rather than speaking Diameter. But Diameter is far from retired: it still runs LTE, it carries roaming signalling over the IPX network, and during 4G/5G interworking the two worlds have to talk, so interworking functions bridge Diameter and SBI. A known weak spot historically was inter-operator Diameter security over IPX, which is part of why 5G introduced the SEPP.
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