Setup Lync for external access with only one firewall

Currently we have lync chat/im working normally internally, we would like to allow external access and mobility now.  My main question is can this be done with one firewall?  I'd prefer to use a 3 legged dmz setup, rather than two firewall to create a dmz.

If anyone has a link to a diagram of this that would be greatly helpful.
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inferno521Author Commented:
The guide has a lot of the information that I need
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