This article describes the steps taken by Office Communicator to establish a Communicator call between an OC client sitting on a typical home network, connected to the Internet using a NAT router and another OC client placed on the company's internal network. The user initiating the call will be Alice and the data and logs are collected from Alice's computer. The main problem when establishing a media connection (audio or video) between Alice and Bob is finding a way media can travel through the intermediate network, without being blocked. This is where SDP, ICE, STUN and TURN come into the picture.
Full Article: OCS blog