As with other synchronization operations, Outlook XP and Outlook 2000 download the OAB with a background thread, so users can continue working during the download. Earlier Outlook versions devote all their energy to the download, so users can't do anything else.
What Happens Behind the Scenes
In an Exchange organization, one selected server generates, or builds, the OAB and stores it in a system public folder. By default, the selected server is the first server installed in an Exchange organization, but you can move the OAB to any server you like. A component called OABGen that runs inside the System Attendant process builds the OAB according to the schedule that the Default Offline Address List specifies. Most companies generate an update nightly.
You can use Exchange System Manager (ESM) to change the OAB generation schedule by right-clicking Default Offline Address List and selecting Properties. As Figure 2 shows, you can also change the OAB server and include other address lists. (The default is to use the GAL.) You can force an immediate OAB generation by right-clicking the Default Offline Address List and selecting the Rebuild option. This option comes in handy if you find that the public folder doesn't contain any OAB data (some bugs have caused this problem in the past) or you've added a large amount of new directory entries and want to generate a fresh OAB.
The System Attendant generates the OAB files as messages (with attachments) in a public folder called OAB Version 2. (Version 2 indicates that an earlier version exists, but the earlier version is now obsolete. You need the earlier version only if you have to support Exchange 5.0 and Exchange 4.0 servers.) The public folder holds two messages: One message contains an OAB with full details, and the other message contains an OAB with partial details. In addition, each message includes a file that contains the most recent full build of the complete OAB and a file that holds all the changes since the most recent full build.
When Outlook clients request an OAB download, Exchange 2000 provides either the full-details OAB message if the user has selected the Full Details download option, or the partial-details OAB message if the user has selected the No Details download option. Next, the Outlook client downloads the message and creates a set of temporary files in the same directory that stores the OAB files. If the user selected the Download changes since the last Send/Receive check box, the client downloads the OAB message into temporary files in the local OAB directory (typically \documents and settings\username\local settings\application data\microsoft\outlook), decompresses the information, and applies changes to the local OAB files. If the user didn't select that check box or if more than 6 percent of the total directory entries changed, the client downloads the message into the local OAB directory, then decompresses the data. Finally, Outlook renames the files to become the OAB. Take, for example, the file that contains the OAB user details. Outlook downloads this file to details.tm_, decompresses to details.tmp, then renames it to details.oab.
How to Set Up OAB Replication
If you have multiple servers in your Exchange organization, you'll probably want to create copies of the OAB on more than one server so that users can connect to a local server to fetch a copy of the OAB. Exchange 2000 uses standard public folder replication to replicate copies of the OAB folder to different servers around the organization. To minimize network traffic and speed client downloads, you can replicate copies of the OAB to a server in each administrative group. To view the current replication status in ESM, right-click Public Folders under the Folders directory, then select View System Folders. Expand OFFLINE ADDRESS BOOK, and right-click the o=org/cn=addrlists/cn=oabs/cn=Default Offline Address List folder, which is the internal pointer to the OAB Version 2 folder. Select the Replication tab, which Figure 3 shows, to view the current public folder stores to which Exchange 2000 is replicating the OAB. If necessary, use the Add button to add another server to the replication list. Replication typically proceeds at the same schedule used for other public folders.
In Figure 3, the two other folders under OFFLINE ADDRESS BOOK are legacy versions of the OAB that Exchange 5.5 sites generated. Exchange 2000 generates one OAB per organization, whereas previous versions generate one OAB per site. After an administrative group is fully migrated to Exchange 2000, you can safely remove the OAB for the original Exchange 5.5 site.
A Valuable and Undemanding Tool
Providing the OAB to users doesn't require much work. Yet, those users who spend a lot of time disconnected from the network will find the OAB an invaluable tool.
End of Article
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