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With Exchange 2003 Service Pack 2 and Windows Mobile 5.0 AKU2, Exchange is now able to push e-mail directly to the Windows Mobile device.
Technically this is solved through a long-time-out TCP request. The client (the mobile device) opens a request to the server but does not require an immediate response. The server responds only if anything has changed on the client's exchange datastore, or if the TCP request timeout has occurred. If the request times out the client sends another request, typically every 15 minutes. Sometimes network routers do not permit such a long TCP request life, and the interval is then automatically decreased at the cost of higher data traffic.
After the server notified the client that a change is pending the client initiates the ActiveSync connection. This is also done when the mobile device is in standby mode. Depending on your notification settings you can see through a blinking LED that new items have arrived. The ActiveSync client then issues another long-time TCP request. This is the reason why you will always see ActiveSync as an active program on the mobile device when Exchange ActiveSync is enabled.
Security: Special care should be exercised when establishing a DirectPush? connection. If possible, use a SSL encrypted connection only. If you do not use an SSL encryption all data sent and received is not encrypted and may be captured by others without your knowledge.
How to set it up
Your exchange server needs to run Windows Server 2000 or newer, and Exchange 2003 SR1 or newer. The server needs to be reachable from the internet via ports 80 and 443. It also needs to have a certificate (either from an official certificate issuing authority, or a private certificate). The private certificate needs to be installed on any Pocket PC that you want to use for DirectPush. Public certificates are verfied from the certificate issuing authority.
After that open ActiveSync setup on the Pocket PC, specify the external server address, and your Active Directory credentials.
So DirectPush is the newer version of Exchange ActiveSync.
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