Fundamentals
160
Voice over IP (VoIP)
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 5004/5008/5012
communications 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 5004/5008/5012 communications 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
negotiation 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 5004/5008/5012 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 as
well as for external SIP lines and in the context of TC system networking with
SIP tie line connections.
You have to create a special VoIP profile for completely unfiltered
(transparent) codec negotiation:
1.
Open the
Telephony
:
Extended
:
VoIP profile
page in the configurator of
the Forum 5004/5008/5012 communications system.
2.
Click on the
New
button to create a new VoIP profile.
G.711a
G.729 (if possible)
RFC 2833/4733 DTMF