Using web-based OAB distribution, the following happens:

• OAB is generated on one of the mailbox servers. Microsoft Exchange System Attendant service running on the Mailbox servers is responsible for that, and also publishes data files to a local file share on \Exchange Server\ExchangeOAB\<OABGrid>.
  

  There are internal delays for Offline Address Book to be generated and made available for download. If users experience download error: Task 'Microsoft Exchange Server' reported error (0x8004010F) : 'The operation failed. An object could not be found.', sometimes just waiting for 24 hours will resolve the issue.

• The Exchange File Distribution Service on the CAS server wakes up, and copies the files over from the Mailbox server to \ExchangeServer\ClientAccess\OAB\<OABGrid>. When the copy is completed and verified, the CAS makes the new OAB available.
  
• Outlook 2007 connects to the Auto Discover Service to get the OAB distribution URLs. Auto Discover returns the appropriate URLs to the CAS Server. Auto Discover will connect to the URL on the closest CAS server, which defaults to a CAS server closest to the user's mailbox server. If it is a tie, a CAS server is selected at random.
  
• Outlook connects with BITS to the URL Auto Discover provided and downloads the OAB.
  

A distribution point is an external Web URL on a CAS server, from which users download their OABs. This is a native Exchange 2007 concept.

A CAS web distribution pool is new concept introduced by Hosted Messaging and Collaboration solution version 4.5. It is a logical artifact that includes one or more CAS servers, each configured with one web distribution point for OAB download. Architecturally, we will recommend dedicating CAS servers for OAB download, separate from typical CAS servers configured for mail access. This deployment topology provides both service redundancy and a solution to support large scale concurrent client download.

The new web-based OAB distribution process depends on several components working together and without one of them will not function properly:

• OABGen - this service runs on the OAB Generation server to create the OAB. This must be an Exchange 2007 Mailbox server to support OAB Distribution
  
• Exchange File Distribution Service - this service runs on CAS servers and is responsible to getting 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 handles returning the correct OAB URL for a given client connection.