ZyXEL Communications ZYWALL USG 20 Manual page 358

Chapter 19 ALG
commands from a system running an FTP client. The service allows users to send
commands to the server for uploading and downloading files.
H.323
H.323 is a standard teleconferencing protocol suite that provides audio, data and
video conferencing. It allows for real-time point-to-point and multipoint
communication between client computers over a packet-based network that does
not provide a guaranteed quality of service. NetMeeting uses H.323.
SIP
The Session Initiation Protocol (SIP) is an application-layer control (signaling)
protocol that handles the setting up, altering and tearing down of voice and
multimedia sessions over the Internet. SIP is used in VoIP (Voice over IP), the
sending of voice signals over the Internet Protocol.
SIP signaling is separate from the media for which it handles sessions. The media
that is exchanged during the session can use a different path from that of the
signaling. SIP handles telephone calls and can interface with traditional circuit-
switched telephone networks.
RTP
When you make a VoIP call using H.323 or SIP, the RTP (Real time Transport
Protocol) is used to handle voice data transfer. See RFC 1889 for details on RTP.
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ZyWALL USG 20/20W User's Guide