AudioCodes Mediant 3000 User Manual page 412

assigned to each IP Group. Therefore, each offer destined to specific IP Groups include
this coder.
In the scenario depicted in the figure below, the IP phone on the LAN side initiates a call to
the IP phone on the WAN. The initial SDP offer (from the LAN leg) includes codec G.711
as its supported codec. Since this is sent to a Destination IP Group that is configured with
an extended coder list, on the WAN leg the device adds another supported codec G.729 to
the SDP, which is now offered to the WAN IP phone. The WAN IP phone chooses the
extended codec (G.729) in its SDP answer to the device's WAN leg. Since this codec was
not included in the original incoming offer, the device performs transcoding (between G.729
and G.711) between its two legs, allowing the streaming of media to occur.
Figure 28-4: Transcoding using Extended Coders (Example)
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
User's Manual 412
Mediant 3000
Document #: LTRT-89729