Enabling Media Services in a VoIP Network
Audio Conferencing
Audio conferencing is the cornerstone of business collaboration services offered by VoIP-enhanced service providers. Media servers perform the task of mixing multiple voice streams and transmitting the mixed stream back to the participants. The voice streams can originate as RTP streams or from TDM if the media server interfaces to the public-switched telephone network (PSTN). Complementing the basic mixing function are gain control, voice message playout, voice message record and whisper mode functions.
Gain control provides for the ability to adjust the gain of the audio conference inputs, either positively or negatively, and to do so automatically. Voice message playout, as described above, provides the capability to play messages to individual conference participants or to all participants. Voice message record can be used to record both individual participants' input and the mixed output of the conference call itself. Whisper mode allows a participant belonging to a sub-group to communicate on demand exclusively to the sub-group. A sub-group of two participants allows one of the participants to act as a "coach" to the other sub-group participants.
Media Gateway Call Processing
As referenced above, media servers often reside at access points into the network. As such, they are combined with the media gateway function. The media gateway function processes voice calls between a TDM network, such as the PSTN and an IP network. It processes a full-duplex voice call consisting of media flow in two directions, TDM to IP and IP to TDM. At the media processing level, the media gateway is the core function in network convergence. Important media gateway functions include vocoding, echo cancellation, jitter buffer management, tone detection and generation and support for key packet protocols such as RTP for voice and T.38 for fax.
Media Processing Subsystems
Media processing subsystems combine the necessary media and packet processing technology to deliver the media server functions described above. As this functionality requires very specific media and packet processing technology, developers of complete media server solutions often outsource this functionality as a subsystem that can be integrated into the complete media server solution. The media processing subsystem often takes the form of a board or mezzanine card solution that adheres to standards such as CompactPCI® or PCI Mezzanine Module. While the subsystems are delivered as hardware platforms per these standards, the processing technology exists as downloadable software that runs on DSP and RISC processors on the platform. This software-based technology provides the OEM developer with the ability to maintain, upgrade and scale their solutions.
Conclusion
IP networks are increasingly being used to deliver enhanced voice services. Unlike the legacy PSTN, elements in a VoIP network are distributed and utilize flexible signaling protocols and open application interfaces to advance the development and deployment of new voice services. Media processing subsystems provide the enabling technology to deliver both IP and TDM media streams that represent the "voice" in the voice services application. As such these media processing subsystems provide the basis for all enhanced voice services application in the IP network.
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