Microsoft Outlook’s offline address book is a client-side copy of a user address book that’s synched against the Exchange server host. The offline address book (OAB) is often used by default because it cuts down on the amount of round trips to the server. It’s also useful for clients who spend a lot of time disconnected from the network. If you’re using Exchange Server 2007 or Exchange 2010 with Outlook 2003 or earlier, you’ll want to understand bandwidth management when synchronizing with older clients.
When the OAB is synchronized, only address book changes are pushed out to the client through a compressed file named Changes.oab. If that file is larger than half of the existing OAB, Outlook downloads a full copy of the OAB. In previous versions of the OAB’s sync protocol, the changes file’s size had to be one-eighth or larger that of the existing OAB to trigger a full address book download.
In Exchange Server 2010, Outlook 2007 and Outlook 2010 can synchronize the OAB using Web-based distribution. This method uses the Background Intelligent Transfer Service to update the book in the background, which minimizes bandwidth consumption.
Note: You must properly set up Web-based distribution on the server side before it can be used.
OAB can also be synchronized through public folder distribution; this method places synchronized files into an Exchange public folder and replicates them to clients. Outlook 2003 SP1 and earlier versions use this method. The downside to this is that public folder distribution isn’t as efficient as Web distribution, so it often chokes an Exchange server network.
If you’re forced to use an older version of Outlook, you can enable a throttling mechanism in Exchange to limit the amount of bandwidth that OAB updates allow. To do this, create or edit the DWORD value named OAB Bandwidth Threshold in the registry: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeIS\ParametersSystem.
This value is calibrated in kilobytes per second; Exchange server calculates average client bandwidth usage over 10 second intervals to determine whether or not they exceed the threshold.
Tracking OAB throttling and usage
It’s important to accurately monitor OAB throttling and OAB usage. You can use the OABInteg tool to pull usage statistics and then perform some math against the actual bandwidth the server is using. I recommend checking out the detailed breakdown of steps needed to correctly set the OAB Bandwidth Threshold entry to a suitable value.
According to Microsoft, setting the registry value to 0 allows a maximum of one client without administrative rights to download a full OAB in 10-second intervals at a time. Essentially, this allows you to take a dynamic bandwidth approach to OAB updates. In my opinion, it’s more beneficial to collect statistics about your network’s behavior before setting the bandwidth threshold.