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Microsoft® Lync® Server 2013 Unleashed Second Edition by Randy Wintle, Alex Lewis, David Ross, Tom Pacyk

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Media and Codecs

Although SIP meets the needs for signaling information in calls, VoIP PBXs still require a method to transmit the actual media stream, whether it is audio or video. The Real-Time Transport Protocol (RTP) is used in almost every VoIP implementation and was developed specifically for transmitting audio and video traffic across networks. Encryption of the media traffic was later added in the form of Secure Real-Time Transport Protocol (SRTP), which is what Lync Server 2013 uses by default to ensure that the media cannot be intercepted and played back.

It’s important to note that RTP and SRTP only provide a standard for carrying the media traffic which can be composed of various media codecs. Think of the RTP stream as a wrapper ...

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