The heartbeat interval is dynamically determined by the client device, such as an iPhone or Windows Mobile phone. ActiveSync clients maintain a log of interactions with the server and choose intervals that allow for persistent connections with the longest possible network timeout (the time at which the mobile carrier and/or any network devices between the client and the server will drop the connection).
By using the longest possible heartbeat interval, the client can maintain persistent open connections (those which the client has initiated but the server has not yet responded to) between the client and server without requiring active use of the connection and thus conserving battery life on the device.
Understanding Exchange requirements
As anyone who has administered Exchange knows, there are a number of variables and options in determining the best configuration for an Exchange environment. Factors such as firewall and proxy server configurations, internal and external DNS, the optional use of front-end and back-end servers, the Active Directory forest and domain topologies, and the versions of Exchange and Windows Server used all impact the ultimate design of an Exchange environment.
Other major factors include the use of SSL, whether self-signed certificates or a certificate authority are used (and how they're implemented), which authentication options are used, and which virtual directories on the Exchange server are secured.
In many cases, the variations among unique Exchange environments don't have a huge impact on clients. However, the iPhone is not a particularly forgiving Exchange client, it seems. There are numerous threads on Apple's discussion forums about issues preventing successful communication or sync between the iPhone and Exchange servers. In some cases, administrators report problems trying to integrate iPhones even in environments that already include other ActiveSync mobile devices such as Windows Mobile phones.
Although some admins have pointed fingers at Apple, saying that the company has created a buggy implementation of ActiveSync, the problems in many cases appear to relate to overall network and Exchange environment configuration, or environments that don't meet the specific requirements that Apple has listed for the iPhone. Apple also seems to have designed its ActiveSync implementation to require rather strict adherence to Microsoft's guidelines for mobile device support. (Links to guidelines, documentation, and other resources from Microsoft and Apple are included at the end of this article.)
Unfortunately, Apple's documentation contains very limited details about those guidelines, which means a very solid understanding of and experience with Exchange and its support for mobile devices is a must. Before trying to add iPhones to your network, do your homework and ensure that your Exchange environment meets Apple's stated requirements as well as Microsoft's recommendations for supporting mobile devices via ActiveSync, particularly if you have not worked with mobile device support before. I've included some valuable resources, along with some advice to help avoid commonly reported problems, at the end of this article.