LevelOne VOI-7011 - Manual (Page 59)

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Understanding the SIP
SIP, the Session Initiation Protocol, is a signalling protocol for
Internet conferencing, telephony, presence, events notification
and instant messaging. SIP was developed within the IETF
MMUSIC (Multiparty Multimedia Session Control) working
group, with work proceeding since September 1999 in the IETF
SIP working group.
SIP enabled PBXes and/or SIP User Agents utilize the Session
Initiation Protocol(SIP) to interconnect and to establish voice
sessions between each other over an IP Network. SIP
Telephony has emerged as a viable alternative to legacy (TDM)
and fixed-line circuits for the establishment and transmission of
voice communications.