Exchange 2003 SP2's Direct Push Technology - 21 Nov 2006

Business users have come to rely on having access to email anywhere and anytime. Exchange Server 2003 has long provided features to support mobile email users, including an Always Up-To-Date (AUTD) capability, which works in tandem with Exchange ActiveSync (EAS) to automatically synchronize a wireless mobile device to an Exchange user's mailbox data. New features in Exchange 2003 Service Pack 2 (SP2) significantly enhanced Exchange's mobility functionality, particularly in the areas of security and synchronization through Direct Push technology.
EAS: The Basis for Direct Push
EAS has been part of Exchange 2003 since the original Exchange 2003 release to manufacturing (RTM). EAS is a synchronization protocol based on HTTP and XML that's optimized to deal with high-latency and low-bandwidth networks as well as low-capacity clients (i.e., clients that have low memory, storage, and CPU). Among the benefits of EAS are that it works with the four main data types (contacts, tasks, email, calendar; is built into Exchange; doesn't require software installed on the desktop PC; and provides a familiar setup process through Exchange System Manager (ESM).

Other useful features in EAS include options for filtering, truncation, smart reply, and forwarding. These features are all designed to save bandwidth for the mobile device because they let you allow only a certain amount of email to be downloaded (e.g., 3K, 5K, headers only). The smart reply and forwarding features prevent the device from having to download an entire message just to reply to it. Instead, you add your reply content on the device, then as the mail passes through the email server, Exchange adds the rest of the original message to the reply, again saving bandwidth. EAS also allows attachment blocking with the option to allow downloading of attachments on request.

How Direct Push Works
Before Exchange 2003 SP2, you had two choices for synchronizing a mobile device with a mailbox: You could manually configure EAS on the mobile device to issue synchronization on a scheduled basis, or you could use Exchange's AUTD technology. With AUTD, the Exchange server sends an email message to an SMTP-to-Short Message Service (SMS) gate- way, which then sends an SMS message to the device to tell it to initiate synchronization. The problem with scheduled synchronizations is that you can't schedule them for intervals of less than five minutes, which means you won't always have the latest information on your device. Another problem is that, especially if your mobile operator is outside of North America, you'll be charged each time a new session is established—which occurs each time new data travels over the wire.

The idea behind AUTD is good, but the technology hasn't worked well in reality, at least not in Europe where few mobile operators provide the necessary SMTP/SMS gateway that AUTD requires. Microsoft IT became aware of this problem when it deployed Exchange 2003–based mobile messaging in the worldwide Microsoft organization, which prompted it to rectify the problem in Exchange 2003 SP2's Direct Push technology. DirectPush (aka AUTD v2) is an IP-based synchronization technology and provides these benefits:

To use Direct Push, you need a device that supports Windows Mobile 5.0 with the MSFP installed. (For a list of Windows Mobile 5.0–enabled devices, see Jason Langridge's blog at http://blogs.msdn.com/jasonlan/default.aspx.) It's important to note that, although the DirectPush feature is enabled by default in mobile devices that have the MSFP installed, mobile devices that don't have the MSFP installed can still perform synchronizations by using either the manual or scheduled methods or via AUTD. This will no longer be the case in Exchange Server 2007, which will no longer include the original AUTD SMS-based functionality, only DirectPush. Once you've upgraded to Exchange 2003 SP2, a handy feature lets a device that previously used the old AUTD sync automatically switch over to using Direct Push after the upgrade.

How exactly does Direct Push work? DirectPush maintains an HTTP Secure (HTTPS) connection between the Exchange server and the mobile device, a session that's kept alive by using heartbeats. In this way, the Exchange server can notify a mobile device whether or not a change has occurred in the device's associated mailbox; if a change occurs in the mailbox, the server can initiate synchronization. Since the device keeps an open session to the Exchange server, you might think the connection could become rather expensive. However, the device simply sits and waits for a response; it doesn't send or receive any data while it's in this pending state—so you won't incur data charges. Because the mobile device doesn't sync unless there's a change in the mailbox, as is the case with scheduled or manual syncs, the device uses less power—again saving on money as well as battery life. Additionally, any data synchronized between the mailbox and mobile devices is compressed by using GNU zip (gzip) compression.

Economical, Up-to-Date Access
Direct Push is the latest evolution of the AUTD technology that's been in Exchange 2003 since its release. As you've seen, Direct Push lets a mobile device continuously ping the Exchange server and automatically sync with the server only when new mail comes into the user's Exchange mailbox. DirectPush ensures that Windows Mobile 5.0–device users have similarly up-to-date access to mail, calendars, and contacts as they have in the office—at an economical cost.