| UAC (user agent client) |
Caller application that initiates and sends
SIP requests. |
| UAS (user agent server) |
Receives and responds to SIP requests on behalf
of clients; accepts, redirects or refuses calls. |
| SIP Terminal |
Supports real-time, 2-way communication with
another SIP entity. Supports both signalling and media,
similar to H.323 terminal. Contains UAC. |
| Proxy |
Contacts one or more clients or next-hop servers
and passes the call requests further. Contains UAC and UAS. |
| Redirect Server |
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. |
| Location Server |
Provides information about a caller's possible
locations to redirect and proxy servers. May be co-located
with a SIP server. |
