yes you must install Edge server; i would highly recommend you download and run Office communications Server Edge Planning tool from here:
http://www.microsoft.com/downloads/details.aspx?familyid=EC4B960C-3FE2-41BD-ABDF-AE89CFCB8C6C&displaylang=enThe Edge Planning Tool for Microsoft Office Communications Server 2007 R2 provides settings for configuring your perimeter network based on information that you provide to the tool.
Regards,
Mohammad Saeed Abdelaziz