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Understanding the Availability Service

Applies to: Exchange Server 2010

Topic Last Modified: 2010-02-01

The Microsoft Exchange Server 2010 Availability service makes free/busy information available to Microsoft Office Outlook 2007 and Outlook Web App clients. The Availability service improves information workers' calendaring and meeting scheduling experience by providing secure, consistent, and up-to-date free/busy information. By default, this service is installed with Exchange 2010.

Outlook 2007 and Outlook Web App use the Availability service to perform the following tasks:

  • Retrieve current free/busy information for Exchange 2010 mailboxes
  • Retrieve current free/busy information from other Exchange 2010 organizations
  • Retrieve published free/busy information from public folders for mailboxes on servers that have versions of Exchange earlier than Exchange 2010
  • View attendee working hours
  • Show meeting time suggestions

Contents

Overview of the Availability Service

Availability Service Process Flow

Improvements Over Exchange 2003 Free/Busy

Information About Away Status

Performance

Distribution Group Handling

Availability Service API

Availability Service Network Load Balancing

Methods Used to Retrieve Free/Busy Information

The Availability service retrieves free/busy information directly from the target mailbox for users on Exchange 2010 and Exchange 2007 and can be configured to retrieve free/busy information for users on earlier versions of Exchange. For topologies that have Exchange 2007 or Exchange 2010 mailboxes in which all clients are running Outlook 2007, the Availability service is used to retrieve free/busy information.

Bb232134.note(en-us,EXCHG.140).gifNote:
If you have Outlook 2007 clients running on Exchange Server 2003 mailboxes, Outlook 2007 will use public folders to retrieve free/busy information.

Outlook 2007 uses the Exchange Server 2010 Autodiscover service to obtain the URL of the Availability service. For more information about the Autodiscover service, see Managing the Autodiscover Service.

The Availability service is part of the Exchange 2010 programming interface. It is available as a public Web service to allow developers to write third-party tools for integration purposes.

You can use the Exchange Management Shell to configure the Availability service. You can't use the Exchange Management Console to configure the Availability service.

The following figure illustrates the process flow for the Availability service.

Availabililty Service Process Flow

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The following table lists the improvements to free/busy functionality that Exchange 2010 and Exchange 2007 provide over Exchange 2003.

Free/busy improvements

Free/busy component Outlook 2003 running on Exchange 2003 Outlook 2007 running on Exchange 2010 or Exchange 2007

Up-to-date information

There was no guarantee that free/busy information was up-to-date. There were multiple factors that caused free/busy information to be outdated:

  • By default, Outlook only updated free/busy information every 45 minutes. Also, because of bandwidth and scalability issues, you could not decrease this interval.
  • There were latencies that resulted from public folder replication.
  • In cross-forest scenarios, there were delays when you used the Microsoft Exchange Inter-Organization Replication tool to replicate free/busy information across forests.

Free/busy information is guaranteed to be up to date within a small time period (60 seconds) on all the data retrieved.

Granularity

The four meeting states (Free, Tentative, Busy, and Out-Of-Office) were available in one stream. To retrieve appointment details, additional MAPI calls were required.

By default, free/busy information displays the start and end times for individual appointments. Additional calendar properties (such as Subject and Location) will be available through the Availability service.

Security

For any authenticated user, all free/busy data was available in a public folder. This meant that any authenticated user could delete, modify, or publish another user's free/busy information.

Free/busy information provides increased security, similar to general calendar sharing. In compliance with your company's policy, you can specify how much free/busy information to share with a specific user. Because the Availability service reads directly from a user's mailbox, a user cannot modify or publish another user's free/busy information.

Publishing frequency

Office Outlook 2003 has a 45-minute default publishing interval.

No publishing is required in an Exchange 2010 and Outlook 2007 organization.

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The Availability service also provides access to automatic-reply messages that users send when they are out of the office or away for an extended period of time.

Information workers use the Automatic Replies feature (formerly known as Out of Office) in Outlook and Outlook Web App to alert others when they're unavailable to respond to e-mail messages. This functionality makes it easier to set and manage automatic-reply messages for both information workers and administrators.

For more information, see Managing Automatic Replies.

You can use the performance counters listed under MSExchange Availability Service in the Performance Monitor tool to automatically collect performance data about the Availability service from local or remote computers that are running Exchange 2010.

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In Exchange 2010, distribution group expansion is processed on the Exchange 2010 server instead of on the Outlook client. In Exchange 2007, distribution group expansion is processed on the Exchange 2007 server. The primary benefit of moving distribution group expansion to Exchange 2010 is to provide consistent behavior for any Availability service consumer. In Exchange 2003 and earlier versions of Exchange, if the number of users in a distribution group was too large, the free/busy data for the distribution group members would not display when the group was expanded.

In Exchange 2010, the following improvements have been made to the handling of distribution groups:

  • The Availability service expands a distribution group up to only two-levels deep, regardless of the total number of distribution group members.
  • A distribution group's free/busy data can expand up to a maximum of one hundred members.

The Availability service is part of the Exchange 2010 programming interface. It's available as a Web service to let developers write third-party tools for integration purposes.

Using Network Load Balancing (NLB) on your Client Access servers that are running the Availability service can improve performance and reliability for your users who rely on free/busy information. Outlook 2007 discovers the Availability service URL using the Autodiscover service. To use network load balancing with the Availability service, you must make changes to your configuration.

The internal URL is used from the intranet, and the external URL is used from the Internet. If you want to use the same URL for both internal and external traffic, make sure that DNS is correctly configured to route internal traffic directly to the internal URL. Also, make sure that the URL can be accessed both internally and externally. For the Autodiscover and Availability services to work, DNS must be configured so that mail.<domain name>.com and autodiscover.mail.<domain name>.com point to the Network Load Balancing (NLB) array of Client Access servers, where <domain name> is the name of your domain.

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For more information, see Network Load Balancing Technical Reference and Network Load Balancing Clusters. You can also search for third-party load-balancing software Web sites.

For information, see Configure the Availability Service for Network Load Balanced Computers.

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The following table lists the different methods used to retrieve free/busy information in different single-forest topologies.

Client Mailbox retrieving free/busy information is running Target mailbox is running Free/busy retrieval method

Outlook 2007

Exchange 2010 or Exchange 2007

Exchange 2010 or Exchange 2007

The Availability service reads free/busy information from the target mailbox.

Outlook 2007

Exchange 2010 or Exchange 2007

Exchange 2003

The Availability service makes HTTP connections to the /public virtual directory of the Exchange 2003 mailbox.

Outlook 2003

Exchange 2010 or Exchange 2007

Exchange 2010 or Exchange 2007

Free/busy information is published in local public folders.

Outlook 2003

Exchange 2010 or Exchange 2007

Exchange 2003

Free/busy information is published in local public folders.

Outlook Web App

Exchange 2010 or Exchange 2007

Exchange 2010 or Exchange 2007

Outlook Web App in Exchange 2010 or Outlook Web Access in Exchange 2007 calls the Availability service API, which reads the free/busy information from the target mailbox.

Outlook Web App

Exchange 2010 or Exchange 2007

Exchange 2003

Outlook Web App in Exchange 2010 or Outlook Web Access in Exchange 2007 calls the Availability service API, which makes an HTTP connection to the /public virtual directory of the Exchange 2003 mailbox.

Any

Exchange 2003

Exchange 2010 or Exchange 2007

Free/busy information is published in local public folders.

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