For the components required for enterprise voice, see
http://technet.microsoft.com/en-us/library/gg399076.aspx
Mediation server is one of the required components.
Conference is related with Front End Server or A/V conference Server (if you deployed).
For the resources used for a live conference session, see
http://technet.microsoft.com/en-us/library/jj688118.aspx
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