The Sip Architecture; User Agents - Snom 4S Administrator's Manual

This paragraph is intended to serve only as a basic introduction to
the SIP world. for more detailed information please refer to the available
literature on this topic.

User Agents

User agents play a central role in SIP. They are normally the
endpoints of the communication. Typical examples for a user agent are
telephones, gateway and media servers.
User agents typically fulfill the following tasks:
• they play and record audio,
• they compress and uncompress the digital audio,
• they do echo compensation if there is a need for this,
• they compensate for packet jitter and packet loss,
• they look for the destination,
• they retrieve their configuration information,
• they keep track of phones that offer a call pickup,
• they publish their state upon request,
• they determine and publish the probability of finding somebody,
• they terminate one or more identities,
• they redirect calls when nobody picks up,
• they are part of a virtual LAN,
• they search address books (LDAP),
• they search internet addresses (DNS A, DNS SRV),
• they usually include a web server,
• they send an receive instant messaging information,
• they publish network management information (SNMP),
• they behave like normal computers on the network (DHCP, DNS).
The SIP
Architecture
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