User Agent Clients; User Agent Servers - Snom ONE IP Technical Manual

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User Agent Server (UAS): The UAS receives the methods, processes them,
and generates responses (e.g., it sends a 200 Ok response to indicate a success-
ful session). The UAS may issue multiple responses to the UAC.

User Agent Clients

The UAC is often associated with the end user, since applications running on systems
are used by people. The UAC can be any end-user device, such as a cell phone, mul-
timedia handset, personal computer (PC), personal digital assistant (PDA), or a soft-
phone.
The requests generated by the UAC are sent to a server (typically a proxy server) and are
known as "methods," which will be discussed later.
Note: Non-IP devices like dumb phones can also be turned into SIP UAs by using an
inexpensive analog telephone adapter (ATA) to make them SIP-aware. An ATA is a
box with one or two analog ports with RJ11 jacks used for connecting regular analog
phones to the VoIP network. Popular ones include the SIPura or Linksys PAP2.

User Agent Servers

Servers possess a predefined set of rules to handle the requests sent by clients and are
usually part of the network. There are several types of servers:
Proxy Server— Proxy servers help track down addresses of recipients whose exact ad-
dresses are not known in advance. If the proxy server cannot find the address of the
recipient, it will send the request to other proxy servers. Destinations include another
extension on the same proxy server, the next-hop proxy server in the routing table, or a
media server. SIP proxy servers use presence services to track users, which means users
can be located regardless of their physical location. Proxy servers are the most common
server in the SIP environment.
Registrar Server— A SIP registration server is responsible for registering devices. It
does this by authenticating the device with a user name and password and keeping a
table of IP addresses and extensions/phone numbers. This authentication process is
similar to logging into a web server, which requires a user name and password. The
registrations server makes it possible for users to alter the address at which they are
contactable. Registrations play an important role in the process since SIP devices that
Appendix B: SIP Overview 501