In Exchange Server deployments, calendars are probably the most-used feature, next to email. One component that facilitates scheduling in calendars is free/busy data. To support the use of free/busy data, you need to know how Outlook's various scheduling mechanisms use that data. You also need to know how free/busy data is stored and updated. Armed with this knowledge, you can troubleshoot and resolve scheduling problems that arise.
How Scheduling Mechanisms Use Free/Busy Data
When an organizer is scheduling a meeting, the organizer needs to know when all the meeting invitees have the same block of free time. One way to find a common block of free time is to look at each invitee's calendar, which means the organizer must meet two conditions: authentication and access. Authentication is necessary to validate the organizer's account so that the system hosting the invitee's calendar knows whether to allow access to the schedule. One or more account domains hosting the organizer's and invitees' accounts handle the authentication. In Exchange Server 5.5 deployments, sites might span account domains and those domains might not have a trust relationship between them. The lack of a trust relationship prevents Exchange from authenticating organizers in other sites who are trying to view an invitee's calendar. (The transitive nature of Active Directory--AD--trusts eliminates this problem in Exchange Server 2003 and Exchange 2000 Server deployments.)
Even when a Windows NT 4.0 or an AD trust exists between domains, the access condition must still be met. To see another person's schedule, the calendar owner must grant at least Reviewer permission. Because the default permission to calendars is No access, the calendar owner must manually change this permission. Thus, an organizer might not be able to access invitees' calendars because the invitees don't know how to grant access to others. In addition, some people are reluctant to make their calendars publicly visible because they might contain personal or sensitive information.
To work around these problems, Exchange provides another view of a person's calendar--a map that specifies when a person is free or busy. This free/busy map is stored in such a way that it's accessible to anyone in the Exchange organization (more about this later). For example, take a look at Figure 1. On the left side is a day from a calendar, as you'd see it in Outlook. The right side shows a representation of what the various types of appointments scheduled for that day might look like if you were to view the free/busy map. When a meeting organizer wants to invite a person to a meeting, Outlook's scheduling tool looks at the invitee's free/busy map to determine whether the proposed meeting time is available.
Another Outlook feature that uses free/busy data is direct booking, which Outlook 2000 first introduced. This feature lets you reserve resources, such as conference rooms, without the need for a delegate to accept the reservation request. Direct booking uses the free/busy map to determine whether a resource is available. (For more information about direct booking, see "Using and Configuring Outlook Direct Booking," October 2002, InstantDoc ID 26184.)
How Free/Busy Data Is Stored
Free/busy data differs from the appointment data it represents in a couple of ways. First, when you create an appointment in your calendar, you assign attributes such as the time and topic of the appointment and the attendees' names. The appointment can even include attached items, such as a Microsoft PowerPoint presentation. The free/busy map doesn't include any details about the appointment. The map just contains one of four possible time-usage categories--Free, Busy, Out of Office, and Tentative--that generically define how that block of time is being used.
Second, unlike appointments, the free/busy map resides in a public folder. As the name implies, access to public folders isn't restricted in the same way that access is restricted to an individual's calendar. Anyone who can access the public folder can read and use its contents. Public folders can also be replicated to servers in other routing groups (Exchange 2000 and later) or to servers in other sites (Exchange 5.5). The replication lets meeting planners in all parts of the organization easily and efficiently access free/busy data.
In the public folder store, free/busy data is organized as a hierarchy of public folders. At the root level, the public folders are organized into two subtrees: IPM_SUBTREE and NON_IPM_SUBTREE. (IPM is short for interpersonal message.) IPM_SUBTREE is a visible subtree that contains the folders you can see when you expand the Public Folders directory in Outlook. NON_IPM_SUBTREE is a hidden subtree that contains folders the system uses to implement special features, such as the free/busy map.
The first folder in NON_IPM_SUBTREE is Schedule+ Free Busy. This folder contains a set of child folders, one for each administrative group (Exchange 2000 and later) or Exchange site (Exchange 5.5). To see the path for an administrative group's free/busy folder, you can use Exchange System Manager (ESM). Right-click the administrative group, select Properties, and click the General tab in the Properties dialog box. The path to that administrative group's free/busy folder will be in the Path field. For example, the path for the free/busy folder for the First Administrative Group in my Exchange deployment is /NON_IPM_SUBTREE/SCHEDULE+ FREE BUSY/EX:/o=NEULAN/ou=First Administrative Group/. The portion in italics specifies the free/busy folder's name. Although the slashes make it seem as if there are intermediate folders named EX: and o=NEULAN, everything from EX: on makes up the name of the free/busy folder.
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