MEGACO Descriptors

Associated with each command and
response are a number of descriptors. These descriptors are effectively the
parameters or information elements associated with each command or
response. The content of a given descriptor will depend on the termination
in question.
Many such descriptors exist, but one in particular is worth noting. This
is the media descriptor, which describes media streams. It contains two
components—the termination state descriptor and the stream descriptor.
The stream descriptor is comprised of three components—the local control
descriptor, the local descriptor and the remote descriptor. This structure can
be represented as follows:
■ Media descriptor
■ Termination state descriptor
■ Stream descriptor
■ Local control descriptor
■ Local descriptor
■ Remote descriptor
The termination state descriptor indicates whether the termination is
currently in service, out of service, or in test. It also provides information
about how events detected by the termination are to be handled.
The stream descriptor is identified by a stream ID. Stream ID values are
used between an MG and an MGC to indicate which media streams are
interconnected.Within a given context, streams with the same stream ID
are connected. A stream is created by specifying a new stream ID on a particular
termination in a context.
The local control descriptor is used to indicate the current mode of the
termination, such as send-only, receive-only, or send-receive, where these
terms refer to the direction from the context to the outside world. Thus, the
term receive-only means that a termination can receive media from outside the context and pass it to other terminations in the context, but it cannot
send media to anywhere outside the context.
The local descriptor and remote descriptor are basically SDP session
descriptions related to the local end of a media stream and the far end of a
media stream respectively. Imagine, for example, a VoIP gateway (gateway
A) that is communicating with another VoIP gateway (gateway B) across an
IP network. The local descriptor for gateway A specifies the media formats
that gateway A wants to receive and the address and port number to which
that media (that is, the RTP stream) should be sent. The remote descriptor
for gateway A indicates the media formats that gateway B wants to receive
and the address and port to which that media should be sent.