People who are new to Lync gets confused with all the typical Lync words like SIP, RTP, SRTP, Media Codecs etc. I will try to explain what they are in simple terms.
SIP stands for Session Initiation Protocol. It is a widely used protocol, by Lync and many IP PBXs for signalling. The signalling is all about how a call is placed, transferred or ended. This does not include the actual voice traffic (audio) in a call, which is the media.
This says that a call needs a signalling information which is fulfilled by SIP and a way of sending the voice traffic or the media. This is where all the other terms come in.
RTP stands for Real-time Transport Protocol & is used for transmitting audio and video traffic across networks. A way to encrypt the voice and video traffic was provided by SRTP (Secure Real-time Transport Protocol), which is used in Lync server 2010. Putting the pieces together, a call needs SIP (which does signalling) and RTP/SRTP (for transmitting the audio & video).
The RTP/SRTP protocols only provides a standard for carrying the media and it is the media codecs that does the actual job. If you think about it, the words that you say and video content in a call has to be converted into bits so that it can be sent through the network. The media codecs does exactly this work – encode and decode the traffic. The Microsoft proprietary codec is called RTAudio and is used in Lync. The codec used in a call determines the quality of the call and the bandwidth consumed. Another common codec (which is used in many VoIP systems) is G.711 and Lync endpoints can use it in certain scenarios.
The below image (from Google images) should summarize what we have gone through.
That’s enough to confuse you . If you have any questions, let me know via the comments.