Outlook Offline Address Book Download

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Microsoft Outlook has many new features and enhancements that are related to the offline address book. By default, Outlook uses a cached mode configuration. Because cached mode generates an offline address book, it's important that you understand how to configure the offline address book so that it works efficiently in your Microsoft Exchange organization.To make it easy for you to find the information that you want, this article is organized in categories in a question-and-answer format.

Q5: How frequently is the offline address book updated on the Outlook client?A5: If left constantly running, Outlook in cached mode automatically updates the offline address book on the client every 48 hours. The 48-hour time period is measured from the time that the offline address book was last downloaded successfully. For example, if you complete an offline address book download at 09:00 today, Outlook will start the offline address book download two days later at approximately 09:00. Therefore, different people will receive updates at different, random times.Note The default setting on Exchange Server 2010 and earlier versions is to generate an offline address book differential file every morning at 05:00. For a change that is made in Active Directory to reach the client computer, the following events must occur:

The change must be picked up by the Exchange computer that generates the offline address book files. This can take several hours. At worst, it can take 24 hours. This variable will be referred to as "x."

The update reaches the client machines x+y hours later. It would be rare for a client to ever experience a 72-hour delay or more unless there were some Active Directory or public folder replication issues.Note For Exchange Server 2013 and later versions, the default schedule is to process the OAB every 8 hours over a period of 1 hour. OAB generation can run at different times based on resource availability.Q6: How can you tell when Outlook will try its next offline address book download?A6: Unless you notice the last time that Outlook downloaded the offline address book, you may not know when Outlook is scheduled to try the next offline address book download. No indicator in the Outlook user interface advises you of the offline address book download schedule. Therefore, to manually force a download of the latest offline address book (assuming one is available on the Exchange computer), use one of the following methods:

Q7: How frequently are the offline address book files generated on the Exchange computer?A7: By default, Exchange Server 2010 and earlier versions generate the offline address book files every morning at 05:00. For Exchange Server 2013 and later versions, the default schedule is to process the OAB every 8 hours over a period of 1 hour. OAB generation can run at different times based on resource availability.Q8: How is the offline address book updated on the server and on the client?A8: Each day, the Exchange computer generates a full offline address book and a differential file from the previous day. The Exchange computer stores the differential file and the full file for the current day and stores only the differential files for the previous days.The following table outlines the process that Exchange and Outlook use to determine which offline address book files to download to the client.

Q9: What are typical offline address book sizes?A9: Offline address book sizes can vary from 3 megabytes (MB) to 700 MB (uncompressed). The following factors can affect the size of the offline address book:

The usage of certificates in a company. The more PKI certificates, the larger the offline address book. PKI certificates range from 1 kilobyte (KB) to 3 KB. They are the single largest contributor to the offline address book size.

Q10: Can I update the offline address book files on the Exchange computer more frequently?A10: We do not recommend that you update more frequently as this may affect client and server performance. Generating the offline address book files on the Exchange computer on a more frequent basis may cause performance problems on the server. You can see the performance impact in the following aspects:

For each update, Outlook must make a copy of the offline address book in the local computer and apply the incremental changes that are fetched from the server. If the offline address book on the local computer is 150 MB, Outlook would have to create a copy of that much data with each update.

Outlook has a "throttling" mechanism that slows down the offline address book update when user activity is detected. Depending on the size of the offline address book, the update can take anywhere from one hour to eight hours if there is continuous user activity.

If you want to increase the frequency of your updates, make sure your offline address book is reasonably small. A good size is in the range of 1 MB to 25 MB, uncompressed.To customize the generation of these files on the server, follow these steps.Exchange Server 2003 and Exchange Sever 2007:

If you have increased the generation frequency of the offline address book files on your Exchange computer, you can also increase the frequency of the client offline address book downloads. To do this:

With the custom send/receive group, Outlook will request an offline address book download from the server based on the number of minutes that you specified for the send/receive setting.Note Send/receive groups cannot be deployed. No tool or other mechanism is available to deploy send/receive group settings.Q11: Can I programmatically download an offline address book through the Outlook object model?A11: No. Offline address book downloads are not exposed through the Outlook object model.Q12: I added a new user to Active Directory, but the user does not show up in Outlook.A12: The offline address book is not up to date. Two events must occur before the changes show up in Outlook:

If you make frequent changes to the offline address book, you may want to modify the way that Outlook computers on your network access the Address Book. You can disable the downloading of the offline address book and force the Outlook computers to use the online Global Address List when they use Cached Exchange Mode.For more information about how to do this, click the following article number to view the article in the Microsoft Knowledge Base:921927 How to prevent users from downloading the Offline Address Book without disabling Cached Exchange ModeAs per this article, if offline address book (.oab) files were previously downloaded and were associated with a MAPI profile for Outlook, this setting will not have the correct effect. To use this setting, you must set the registry value, remove any .oab files from the local computer, and then synchronize.Q13: What is the effect of offline address book downloads on the network? Will offline address book downloads flood the network?A13: When a cached mode profile is started for the first time, Outlook downloads the full offline address book from the server. This can affect the network, especially if multiple clients are starting up at the same time.Other cases when Outlook clients require full offline address book downloads are described in question 14. Differential downloads are not expected to cause a huge network impact if all the clients update the differential files on a daily basis.Outlook has the following safeguards that help protect the network from excessive offline address book downloads:

In the default configuration, Outlook will only try one full offline address book download in a 13-hour period. An attempt is counted when Outlook receives a response from the server and can at least start the offline address book download.

Q14: Under what conditions will Outlook perform a full offline address book download?A14: Outlook in cached mode will perform full offline address book download when the following conditions are true:

The total size of the differential files is greater than one-eighth the size of the full offline address book. In this case, Outlook downloads the full offline address book for better performance. This condition may occur if there are lots of Active Directory objects that have a change to an attribute that is in the offline address book. For example, phone numbers are updated with a new area code, departments are added to all users in Active Directory, and new address types are added.To modify the one-eighth rule, follow these steps.Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:

In the Edit DWORD Value dialog box, type 4 in the Value box, and then click OK.This setting example (4) permits Outlook to download differential offline address book files that are up to one-fourth the size of the full offline address book.Note If the OAB Dif Divisor registry value is set to 0 (zero), Outlook will use 16 so that it does not divide by 0.

The version on the server and the version on the client do not match. There is a more recent version of the offline address book present on the server. For example, Version 3a (Unicode offline address book) is now available, and you previously downloaded a Version 2 offline address book.

When you click to select the Download offline address book check box and then click Address Book Settings, the Offline Address Book dialog box appears. In this dialog box, you can specify either No Details or Full Details. These settings affect the offline address book download. For example, if you select No Details, a no-details version will be automatically downloaded to your computer during an offline address book download.Q19: How can I determine whether my client is performing a full offline address book download?A19: To have Outlook display a dialog box that prompts you before the offline address book download starts, follow these steps.Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base: 2b1af7f3a8