Exchange File Distribution Service Test

An Offline Address Book (OAB) is a container that stores a collection of Offline Address Lists. Typically, users download these address lists to obtain information about other users in their organization.

Exchange server 2007/2010 introduces a new mechanism for distributing Offline Address Books that does not involve Public Folders; it instead uses HTTP(S) and the Background Intelligent Transfer Service (BITS). This new web-based OAB distribution process depends on several components working together:

  • Exchange System Attendant Service – this service runs on the mailbox server to create the OAB.
  • Exchange File Distribution Service – this service runs on CAS (Client Access) servers and is responsible for obtaining the OAB content from the OABGen server.
  • OAB Virtual Directory – This is an IIS virtual directory on a CAS server where the OAB is downloaded from.
  • Autodiscover – Autodiscover runs on a CAS server and returns the correct OAB URL for a given client connection.

The OAB is typically generated on a mailbox server by the Exchange System Attendant Service. At configured intervals (default: every 8 hours), the Exchange Fle Distribution Service (FDS) on the CAS server polls the mailbox server for new OAB files. The first poll happens when the Exchange File Distribution Service starts; so, the exact time a server polls will be different on each CAS. If polling reveals new files, the Exchange Fle Distribution Service downloads the files from the mailbox server. The copied files are stored in a web distribution folder on the CAS server. The user then connects to the AutoDiscover service via Outlook to get the closest OAB distribution URL. Autodiscover returns the URL to the CAS server. Outlook then connects with BITS to the URL provided, and downloads the OAB.

From this, we can infer that the location of the CAS server, the quality of the network link between the CAS and the mailbox server, and the polling interval are key factors that influence the speed, frequency, and overall efficiency of the OAB download performed by the Exchange File Distribution Service. Carelessly made changes to the polling interval and issues with network connectivity can therefore significantly impact the OAB distribution process, thereby delaying users access to the latest information pertaining to other users.

Using the Exchange File Distribution Service test, you can periodically monitor the OAB downloads performed by the FDS service on the CAS server and promptly capture slowdowns (if any) in the downloading process and changes in polling interval (if any).

Target of the test : A server configured with the Client Access Server role

Agent deploying the test : An internal agent

Outputs of the test : One set of results for the Client Access server being monitored.

Configurable parameters for the test
Parameters Description

Test Period

How often should the test be executed.


Indicates the IP address of the Client Access server.


The port number of the client access server. By default, this is 110.

Measurements made by the test
Measurement Description Measurement Unit Interpretation

Download task queued

Indicates whether any download tasks have been queued or not.


If one/more download tasks are in queue, this measure will report the value Yes. If no download tasks are in queue, then this measure will report the value No.

The numeric values that correspond to these measure values are as follows:

Measure Value Numeric Value
Yes 1
No 0

The value Yes for this measure could indicate that the CAS server is taking too much time to download the OAB files, thereby causing subsequent download tasks to be queued. The prolonged downloads could be due to a poor network link between the CAS server and the OAB Generation server. Slowdowns can also occur if a large number of OAB files are downloaded, or if the size of the OAB files is huge. 


By default, this measure will report the Measure Values in the table above to indicate if any download task is pending. However, in the graph of this measure, the same will be represented using the numeric equivalents only.

Download task completed

Indicates the number of OAB downloads that have been completed since the last measurement period.


Ideally, the value of this measure should be high. A very low value could indicate downloading bottlenecks that might require further investigation.

Another reason for a change in the value of this measure is a change in the polling interval. While an increase in the polling frequency, can increase the value of this measure, a decrease in the polling frequency can cause a less number of download tasks to be completed and can hence, reduce the value of this measure.

A reduction in the polling interval can cause critical updates to OAB files to be available to end users only after a long time.