Fundamentals
Voice over IP (VoIP)
97
VoIP Profiles for SIP
While a call is being established and also during an ongoing connection, SIP
terminals can negotiate amongst one another which codecs are to be used.
SIP signalling is always being conducted via the Forum 523/524 communica-
tions system. After successfully agreeing on a codec, the resulting RTP
connection is established between the terminals directly. The Forum 523/524
communications system can also serve as a proxy for the media data stream
(“RTP Proxy function”) if external SIP lines do not allow direct RTP data
connection between the terminals.
Negotiating the codecs to be used takes place as follows:
1.
One of the SIP terminals sends of list of supported codecs.
2.
The second SIP terminal deletes the codecs from the list which are not
supported and sends the shortened list back to the first SIP terminal.
The exchanged codec lists can be filtered by the Forum 523/524 communica-
tions system, e.g. to limit bandwidth based on the VoIP profile activated for
the connection or for the terminal. Please note that the negotiation regarding
codecs supported also takes place for connections between SIP terminals and
other terminals / subscribers. For example, the Forum 523/524 communica-
tions system uses a pre-defined codec list for a connection between a SIP
terminal and a TDM port:
IP system phones or DECTover IP base stations participate in codec negoti-
ation with the VoIP profile configured respectively. If no common audio codec
is determined during negotiation, it will not be possible to establish the call
connection.
Transparent codec interconnection
Transparent codec interconnection allows SIP terminals to use addition codecs
even though the desired codec is not, or not directly supported by the
Forum 523/524 communications system. Some examples are:
●
G.722 audio codec for HQ audio
●
H.264 video codec for transmitting images
Transparent codec interconnection is supported for internal SIP subscribers
and in the context of TC system networking with SIP tie line connections.
G.711a
G.729 (if possible)
RFC 2833/4733 DTMF
Summary of Contents for forum 523
Page 1: ...Forum 523 524 Mounting and Commissioning Manual ...
Page 10: ...8 ...