One area that is often overlooked when planning a Lync Server 2010 deployment is the impact of the Address Book on the network. Depending on how well populated the Address Book is and whether all users have pictures in the Address Book, it has the potential to become quite large. Because each user will download the Address Book in its entirety when he first attaches to Lync Server 2010, a widescale deployment of clients can have a large impact on bandwidth usage. Microsoft offers the information in Tables 26.6 through 26.8 to estimate space and bandwidth around Address Book planning.
These numbers are based on a large 80,000 user rollout and can be scaled appropriately for smaller deployments.