SIP call can be
classified as follow:
Early Offer
In an Early Offer,
the session initiator (calling device) sends its capabilities (for example,
codecs supported) in the SDP contained in the initial Invite (thus allowing the
called device to choose its preferred codec for the session).
For Early Offer, the
answer is sent in a response (usually 200 OK).
Delayed Offer
In
a Delayed Offer, the session initiator does not send its capabilities in the
initial Invite but waits for the called device to send its capabilities first
in 200 OK message (for example, the list of codecs supported by the called
device) thus allowing the calling device to choose the codec to be used for the
session.
For
Delayed Offer, the answer is typically sent in the ACK of 200 OK
CUCM can
support Delayed Offer in one direction and Early Offer in the other direction.
This capability can often be useful in situations where a SIP device connecting
to CUCM via a SIP trunk wishes to control the codecs offered and selected for
inbound and outbound calls.
Mixed-Offer
What
will happen if the calling device is using Delayed Offer and CUCM needs to
forward the call over SIP trunk with Early Offer config? By default the call
will fail because CUCM doesn't have media characteristics to relay them over
SIP Trunk. Solution is to insert MTP
where CUCM will use the characteristics of the MTP device over the Early Offer
SIP trunk.
This was really helpful.Cheers !!
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