The Session Initiation Protocol (SIP) is used in Voice over Internet Protocol (VoIP) to handle the call-signalling functions of a VoIP call. Assured Services Session Initiation Protocol (AS-SIP) is a variation on SIP defined by the Defense Information Systems Agency (DISA) for mission-critical communication.
AS-SIP includes all the standard functions of SIP. These are the functions of a regular telephone--ringing, pick up, busy, call reject and call termination. The added functionality of AS-SIP is that its data packets are prioritised over other traffic on a network and the AS-SIP packets are encrypted.
The terminology of SIP is incorporated in AS-SIP. The calling device is called the User Agent Client (UAC) and the called device is called the User Agent Server (UAS). Each User Agent (UA) channels its calls to a proxy that handles calls for all UAs in a particular domain (for example, a company). This is called "Service Provider Architecture" with the proxy acting as service provider and adding the AS-SIP features of security. Prioritisation is handled by a second device called an "Access Router."
Although AS-SIP was developed by a US defence organisation, it is an open standard. The protocol is published and distributed by the Internet Engineering Task Force (IETF).
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