What is Diameter in LTE?
Diameter is an AAA (Authorization, Authentication and Accounting ) protocol which works at the application layer in OSI model over TCP/SCTP or TLS/DTLS (for security) protocol. Diameter is the successor of RADIUS (Remote Remote Authentication Dial In User Service) protocol that runs over UDP.
This AAA technology is a message based protocol, where AAA nodes exchange messages and receive Positive or Negative acknowledgment for each message exchanged between nodes. For message exchange it internally uses the TCP and SCTP which makes diameter reliable. Its technical specifications are given in RFC-6733 Diameter Base Protocol. (Please refer to this link to RFC-6733 for the original definition)
Advantages of Diameter compared to Radius
This AAA technology has following improvements over RADIUS:
a) More Reliable
b) Transport Layer Security
c) Fail-over Mechanism
d) Server Initiated Messages
e) Agent Support
g) Transition Support
h) Capability Negotiation
i) Roaming Support
j) Peer Discovery & Configuration
The default port is 3868 for TCP/SCTP and 5868 for TLS/DTLS.
More details on AAA in LTE
LTE Evolved Packet Core (EPC) generally have 5 nodes:
1) Mobility Management Entity (MME)
2) Home Subscriber Server (HSS)
3) Serving Gateway (S-GW)
4) PDN Gateway (P-GW)
5) Policy and Charging control entity/Function (PCRF)
These nodes interact then uses diameter based interfaces. Please see the diagram below for more detailed information on this topic:
The Dark Lines in the diagram show the major Interfaces in the LTE EPC (Evolved Packet Core). Sometimes the CSS data is not stored at HSS then S7a interface is used to communicate with the MME. S7a is also a diameter based interface.
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