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CUCM offers software-based media resources. Start the IP Voice Media Streaming
application to activate the following:
■ Audio conferencing
■ MTP
■ Annunciator
■ MoH
370 Chapter 15: Media Resources
The following media resources are available only in hardware:
■ Transcoding
■ Voice termination
Audio conferencing and MTP media resources can also be offered by hardware media
resources. MoH is a special case. Because of the potential WAN bandwidth utilization
of MoH, the multicast streams of the server are normally scoped at the headquarters.
Survivable Remote Site Telephony (SRST) can stream one media resource at branch
locations.
The signaling between hardware media resources and CUCM most often uses SCCP to set
up and tear down calls. All audio streams from any endpoint are always terminated by the
media resources involved in the call. There is no direct IP phone-to-IP phone audio stream
with media resources involved in the call flow.
The voice-termination function is needed when an incoming or outgoing TDM call is
terminated on a gateway. The TDM leg is terminated by the Cisco IOS router’s DSP and
has to perform decoding, coding, and packetization functions.
There are two different audio streams in Figure 15-1, one inside the public switched
telephone network (PSTN), the other one a VoIP audio stream using RTP transport.
Signaling messages are exchanged between the gateway and CUCM and between the
telephony device and CUCM. The PSTN signaling is not considered in Figure 15-1.
Figure 15-1 PSTN Voice Termination
DSPs for Voice
Termination
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PSTN
PSTN Call
Audio Signaling
VoIP
TDM
Media Resource Support 371
RTP bearer traffic streams are sent from the IP phones to the conference bridge resource
mixing the audio. The conference resource mixes the audio streams and sends back a
unique audio stream to the IP phones. The audio stream must subtract the audio stream of
the person receiving the audio stream so that no echo is heard. Some conference devices,
because of processing limitations, mix only the three loudest talkers.
Signaling messages (control traffic) are exchanged among the IP phones, CUCM, and
the conferencing resource (if using a hardware resource or a version of Cisco Unified
MeetingPlace). Cisco Unified MeetingPlace is not covered in this book.
Most conference bridges that are under the control of CUCM use SCCP to communicate
with CUCM. Session Initiation Protocol (SIP) support is increasingly being added to all the
Unified Communications components.
CUCM does not distinguish between software- and hardware-based conference bridges
when it processes a conference allocation request. Allocation of conferencing resources is
covered in further detail later in this chapter. The number of individual conferences and
maximum number of participants per conference varies based on the resource in use.
Figure 15-2 illustrates that software conferencing is integrated into CUCM.
Figure 15-2 Software Conferencing
NOTE The Cisco Press book Voice and Video Conferencing Fundamentals is an
excellent resource for a more thorough understanding of audio conferencing and
videoconferencing.
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PSTN
Conference Call
Integrated
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GW
Audio Signaling
372 Chapter 15: Media Resources
A transcoder converts an input audio stream using one audio codec into an output stream
that uses a different audio codec. The transcoder in Figure 15-3 is implemented using DSP
resources in the Cisco router. Transcoders are necessary when audio streams are using
compressed audio codes (G.729 or iLBC), but the resource they are attempting to use
accepts only G.711 calls. iLBC is the Internet Low Bandwidth Codec, which operates at
15.2 kbps. Most Cisco Unify voice-mail deployments use the G.711 audio codec for voicemail
storage to guarantee high quality.
Audio streams (RTP bearer channels) are set up between the telephony devices and the
transcoder. Signaling messages are exchanged between the telephony devices and CUCM
and between the transcoder resource and CUCM. DSP resources are required to perform
transcoding. Those DSP resources are located in Cisco routers and switches.
Figure 15-3 Transcoding Media Resources
An MTP bridges two media streams and allows them to be set up and torn down
independently.
An MTP can be used as an instance of translation between incompatible audio streams, to
synchronize clocking, or to enable supplementary services for devices that do not support
the empty capability set (ECS) option of the H.323 Version 2 protocol.
Audio streams exist between telephony devices and the MTP resource. Signaling messages
are exchanged between the telephony devices and CUCM. Figure 15-4 illustrates a
hardware-based MTP.
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PSTN
Transcoded Call
G.711
Application
Server
Hardware
Transcoding
G.729
Audio Signaling
G.711
G.729
Media Resource Support 373
Figure 15-4 Hardware MTP
An annunciator is a function of the Cisco IP Voice Media Streaming application service that
provides the ability to stream spoken messages or various call-progress tones from the
CUCM system to a user.
The annunciator can send multiple one-way RTP streams to devices such as Cisco IP Phones
or gateways, using SCCP messages to set up the RTP stream. Tones and announcements
are predefined by the system. The announcements support localization and may also be
customized by replacing the appropriate WAV file. The annunciator can support G.711
a-law, G.711 mu-law, G.729, and Cisco wideband audio codecs without transcoding
resources.
Signaling messages are exchanged between telephony devices and CUCM. The audio
stream is one way, from the annunciator to the telephony device. The annunciator is a
software component of CUCM, as shown in Figure 15-5.
The MoH feature is part of the Cisco IP Voice Media Streaming (IPVMS) service running
on CUCM. This feature provides music to callers when their call is placed on hold or a
supplementary service is initiated. Supplementary services are not limited to, but include
the following: transfer, park, and conference. When a supplementary service is initiated,
the call is temporarily put on hold before the function is completed. Implementing MoH is
relatively simple but requires a basic understanding of IP unicast and multicast traffic, MoH
call flows, configuration options, server behavior, and requirements.
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IP IP
SIP
Hardware MTP
Call Using MTP
Audio Signaling
G.711
G.711
374 Chapter 15: Media Resources
Figure 15-5 Annunciator Services
Audio streams are created between telephony devices and the MoH server. Signaling
messages are exchanged between telephony devices and CUCM. Figure 15-6 illustrates
the MoH component of CUCM.