Diameter protocol is a corner stone of all-IP telco networks. There are two main signalling protocols in IMS. Session Initiation Protocol (SIP) and right, the Diameter.
The main purpose of the SIP protocol is to establish a real-time multimedia session. This session goes end-to-end between the phones (or other devices) of the originator and recipient.
Internally – within the operator’s network – during the session establishment, we have to trigger many network functions. That can be because of the authentication of users, charging, allocation of resources, application of services, etc. 3GPP mandates for this purpose the Diameter protocol. The Diameter is used by both ePC and IMS for requesting/triggering additional information. These requests are sent in a form of transactions. During one IMS session setup or one IMS registration procedure we have to perform several Diameter transactions.