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A media resource is a software- or hardware-based entity that performs media processing functions
on the data streams to which it is connected. Media processing functions include mixing multiple
streams to create one output stream (conferencing), passing the stream from one connection to
another (MTP), converting the data stream from one compression type to another (transcoding),
echo cancellation, signaling, termination of a voice stream from a time-division multiplexing
(TDM) circuit (coding/decoding), packetization of a stream, and streaming audio (annunciation).
368 Chapter 15: Media Resources
Not all the different media resources described in Table 15-1 are needed in every deployment.
Software resources are provided by CUCM services, whereas hardware features are
provided by digital signal processors (DSP). The DSP resources are hardware modules in
the gateway router or switch. The software resources are controlled by the Cisco IP Voice
Media Streaming application running on CUCM.
Table 15-1 introduces the different types of media resources.
A conference bridge is a resource that joins multiple participants into a single call. It can
accept a number of connections for a given conference, up to the maximum number of
streams allowed for a single conference on that device. The conference bridge mixes the
streams together and creates a unique output stream for each connected party. The output
stream for a given party is the composite of the streams from all connected parties minus
their own input stream. Some conference bridges mix only the three loudest talkers on the
conference and distribute that composite stream to each participant minus their own input
stream if they are one of the talkers. Software conferencing is limited to the G.711 audio
codec.
Table 15-1 Media Resource Functions
Resource Function
Voice termination TDM legs must be terminated by hardware that performs coding/decoding and
packetization of the stream. This is performed in DSPs on the gateway router.
Audio conferencing A conference bridge joins multiple participants into a single call. It mixes the
streams together and creates a unique output stream for each connected party.
Transcoding A transcoder converts an input stream from one codec into an output stream
that uses a different codec.
MTP An MTP bridges the media streams and allows them to be set up and torn
down independently.
Annunciator An annunciator streams spoken messages and various call progress tones.
Music on hold Music on hold provides music to callers when their call is placed on hold,
transferred, parked, or added to a conference.
NOTE All hardware resource limitations are well documented in the media resource
chapter of the SRND available online at http://www.cisco.com/go/srnd. It is best practice
to use the Cisco DSP Calculator, which you can find at http://www.cisco.com/go/
dspcalculator. The Cisco DSP Calculator requires Cisco.com membership access.
Media Resource Support 369
A transcoder takes the stream of one codec and converts it from one compression type to
another compression type. For example, it could take a stream from a G.711 codec and
transcode it in real time to a G.729 stream. In addition, a transcoder provides MTP
capabilities and may be used to enable supplementary services for H.323 endpoints
when required.
Two streams that use the same codec using different sampling intervals may also be
connected.
A single-site deployment usually has no need for transcoding devices.
An MTP is an entity that accepts two full-duplex G.711 streams. It bridges the media
streams and allows them to be set up and torn down independently. The streaming data
received from the input stream on one connection is passed to the output stream on the other
connection, and vice versa.
An annunciator is a software function of the Cisco IP Voice Media Streaming application
that provides the ability to stream spoken messages or various call-progress tones from the
system to a user. It is capable of sending multiple one-way Real-Time Transport Protocol
(RTP) streams to devices such as Cisco IP Phones or gateways, and it uses Skinny Client
Control Protocol (SCCP) messages to establish the RTP stream. The announcements may
be customized by replacing the appropriate WAV file.
Music on hold (MoH) is an integral feature of the Cisco Unified Communications system.
This feature provides music to callers when their call is placed on hold, transferred, parked,
or added to an ad hoc conference.