Version 6.6 773 October 2014
Installation & Operation Manual 43. Session Border Controller (SBC) Application
For an SDP offer to provide an extended coder list to a remote user agent, the
following prerequisites must be fulfilled:
An extended coders list has been configured for the user agent's IP Group (i.e.,
Destination IP Group)
The incoming offer contains at least one supported coder (otherwise, transcoding
can't be performed)
Both legs have available DSP's
T.38 doesn't appear in the offer
If the above prerequisites are not met, the SDP offer is sent without the extended
coders list. The coders from the extended list are added after the ones from the
original offer (decreases transcoding probability). Coders common between the
extended coders list and those in the original SDP offer are not added. Transcoding
may be performed even in scenarios when the same coder has been chosen - this
occurs if the coders use different coder parameters (e.g. rate and packetization time).
The device also supports early media, whereby the first offer\answer transaction is
finalized and the media flow starts before the SIP call is connected (before the INVITE
200 OK response).
The offer and answer options can be included in the following SIP messages:
Offer in first INVITE, answer on 180, and no or same answer in the 200 OK
Offer in first INVITE, answer on 180, and a different answer in the 200 OK (not
standard)
INVITE without SDP, offer in 180, and answer in PRACK
PRACK and UPDATE transactions can also be used for initiating subsequent
offer\answer transactions before the INVITE 200 OK response.
In a SIP dialog life time, media characteristics after originally determined by the
first offer\answer transaction can be changed by using subsequent offer\answer
transactions. These transactions may be carried either in UPDATE or ReINVITE
SIP transactions. The media handling is similar to the original offer/answer
handling. If the offer is rejected by the remote party, then no media changes
occur (e.g. INVITE without SDP, then 200 OK and ACK, offer\answer within an
offer/answer, and Hold ReINVITE with IP address of 0.0.0.0 - IP address is
unchanged).
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