Some organizations use public folders heavily, while others do not. The degree to which public folders are used in your organization may have a large effect on how you plan for migration and coexistence of public folders and the data stored therein. Factors to consider include:

• The total number of public folders and the amount of data contained in them. Many organizations choose to remove unneeded folders from their public folder structure as part of their migration process.

• The number of existing replicas and where they are located. Because Exchange public folders can be easily replicated between servers, many organizations have placed public folder servers at remote sites to improve access for remote users; as part of server consolidation, these replicas will normally be re-homed elsewhere.

• The extent to which public folders are used as part of normal operations. If public folder-based applications are in use, how much are they used? Do users depend on them for their key tasks, or are they of secondary importance?

• The messaging clients to be used after the consolidation. Outlook 2003 can be configured to cache users' public folder favorites; this makes public folder access in Cached Exchange Mode seamless, but it increases the size of the OST file, and it causes additional synchronization traffic. Requests for public folder items will be automatically directed to the best available replica, which may be in a distant site across a slow or high-latency link.  

Even if your organization does not use public folders for business reasons, Exchange still uses two important system public folders that must be accounted for in your planning.

First, the Schedule+ Free/Busy folder is where Exchange stores free/busy status for individual mailboxes. Outlook publishes users’ calendar status to this folder, as does the Exchange System Attendant.

Having multiple replicas of this folder helps ensure quick and consistent access to schedule data throughout the organization; however, these replicas have to be synchronized, which can add a substantial amount of network traffic.

When a user creates a new meeting request, Outlook opens a connection to find each attendee’s free/busy data. First, Outlook retrieves the user's legacy distinguished name, which it then uses to identify the name of the free/busy folder it needs. Outlook then searches for the correct folder and message for the specified user's schedule data. This means that a single meeting request may generate multiple un-cached connections to different servers.

On the other hand, adding multiple replicas of the free/busy folder means that changes made to one replica may take time to propagate to other replicas. Adjusting the number of replicas, their location, and the replication schedule used may be necessary to ensure the right balance between minimized access time and replication convergence.

Offline Address book is one of the advantage when you make use of public folder. The OAB provides offline and Cached Exchange Mode users access to a subset of properties for all objects in the global address list.

Exchange generates updates to the OAB periodically, and Outlook automatically downloads available changes once a day (in online mode) or when the user goes online (in Cached Exchange Mode).

In general, Microsoft’s normal recommendation is to maintain OAB replicas on each server that contains user mailboxes. As you consolidate user mailboxes, you should bear in mind that when the OAB changes, a large number of clients may need to download OAB changes at once, and plan network capacity and CPU allocation accordingly.