User Agent (UA ) is a logical (not necessarily physical) function that initiates or responds to SIP communications. It can be either a client or server that maintains a session status.
User Agent Client (UAC) is also a logical and physical function that initiates or responds to SIP communications.
SIP Location Server provides information via a database about a caller's possible locations for redirection and may be co-located with a SIP server.
User Agent Server (UAS) is a logical and physical function that initiates or responds to SIP communications.
Proxy Server acts like an intermediate server that is responsible for forwarding SIP requests to the destination UAS on behalf of the UAC.
Redirect Server is an optional component that does not route SIP messages. It returns a redirect via 3xx class address change in routing to UA or UAC for direct routing. SIP is designed for end-to-end signaling without intervention by a server.
Registrar Server is a UAS that accepts SIP Register requests and updates.
Back-to-Back UA (B2BUA) acts like a central PBX connection server processing incoming and outgoing messaging between other UASes. It maintains answer supervision, overseeing and participating in all dialog communications.
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