With the Lync Server 2010 and 2013 Bandwidth Calculator, you can enter information about your users and the Lync Server features that you want to deploy, and the calculator will determine bandwidth requirements for the WAN that connects sites in your deployment. The accompanying User Guide describes the recommended process for estimating your WAN bandwidth needs for Lync client real-time traffic. The User Guide will be updated periodically with new information and modifications.
Download:
http://www.microsoft.com/en-us/download/details.aspx?id=19011