Codec Freedom -- Specifications -- Redefining the Voice Codec with SDP
The ability to exchange MIME-typed content with AMQP 1.0 means that a codec negotiation with SDP can be started, even though we may still be passing the underlying call over a G.711 connection. As soon as both sides agree and have completed a break on the data link level, they can start sending with a new codec, using either the bit-inclusion protocol or the bytewise escaping protocol.
text/sdp message arrives over AMQP with Content-Disposition
it should be treated as addressed to
the receiving phone. This is usually a similar procedure to receiving a SIP
INIVITE with the SDP message attached. See
RFC 7195 for a
suitable SDP format to use in this case.
It is important to note that the acceptable codecs in an SDP offering over modified G.711 should not exceed the bandwidth available and leave some room for data communication; as a general rule, the limit of 56 kbit/sec should not be exceeded. It is always possible to add one AMQP channel, or another protocol that does not require realtime treatment, to fill the remaining space. It is also possible to define only an AMQP channel, so that the entire bandwidth is (temporarily) taken over by AMQP.