|UAC (user agent client)
||Caller application that initiates and sends
|UAS (user agent server)
||Receives and responds to SIP requests on behalf
of clients; accepts, redirects or refuses calls.
||Supports real-time, 2-way communication with
another SIP entity. Supports both signalling and media,
similar to H.323 terminal. Contains UAC.
||Contacts one or more clients or next-hop servers
and passes the call requests further. Contains UAC and UAS.
||Accepts SIP requests, maps the address into
zero or more new addresses and returns those addresses to
the client. Does not initiate SIP requests or accept calls.
||Provides information about a caller’s possible
locations to redirect and proxy servers. May be co-located
with a SIP server.