Once you've configured Direct Push technology on your Exchange 2003 SP2 server and selected your security settings, it's time to configure Direct Push on your mobile devices.

These instructions assume that your handheld mobile devices are running either Windows Mobile 5.0 with the Messaging and Security Feature Pack (MSFP) or Windows Mobile 6.0.

If your mobile devices are running Windows Mobile 5.0, you must determine whether MSFP is installed before continuing. To do so, check the mobile device's version number by clicking its Start button -> Settings -> About to view the device's version information, as shown in Figure C.

Figure C: Verify a Windows Mobile 5.0 device's version information. 

The emulator shown in Figure C is running Windows Mobile 6.0. But if it were running Windows Mobile 5.0, you would look at the build number to tell whether or not MSFP was installed. If the last three digits of the build number are 2.0.0 or higher, then the mobile device has MSFP installed. If MSFP is not installed, you will have to contact the device's manufacturer for an update before continuing with the configuration instructions below.

Configuring Direct Push on a Windows Mobile device

1. Begin the Direct Push configuration process by going to your mobile device's Start menu -> Programs -> ActiveSync to view the message displayed in Figure D.

   Figure D: The ActiveSync screen.
    

   

2. Click the Set up Your Device to Sync With It link, and you will be taken to the screen shown in Figure E. Where the screen asks for the server's address, enter the URL of your OWA server, minus the HTTP or HTTPS prefix.

   Figure E: You must enter your OWA server's URL.
    

   

3. On this screen, there is also a checkbox that tells the mobile device that the Exchange server requires an HTTPS connection. If you decide to use HTTPS, make sure your SSL certificate is valid and that it matches the domain name specified within the URL you are entering. Direct Push will not work otherwise.

4. After entering the necessary URL and HTTPS information, tap the Next button.

5. As shown in Figure F, this screen simply asks for the user's authentication credentials. Make sure you must select the Save Password checkbox here. If this checkbox is not selected, the mobile device will not have any way to authenticate with the Exchange Server. (When your users change their domain passwords, they will have to manually change the stored password on their mobile device to match. Passwords are not automatically synchronized.)

   Figure F: You must allow the device to store the user's password.
    

   

6. Don't worry about clicking the Advanced button right now. The advanced configuration options simply allow you to configure event logging and to choose the connection that you want to use if multiple connections exist.

7. Click Next to determine what types of data you want to synchronize, as shown in Figure G. You can enable or disable the synchronization of contacts, calendar, email and tasks by selecting or deselecting the corresponding checkboxes.

   Figure G: Choose the types of data that you want to synchronize.
    

   

8. Instead of clicking Finish to complete the configuration process, now select the Email option and then click the Settings button to choose how much email is synchronized with the mobile device. You can also set message size limits here and control whether or not attachments will be downloaded, as shown in Figure H.

   Figure H: You can limit the amount of email that is stored on the device.
    

   

9. Click on the Advanced button on this screen to set options for encrypting or signing outbound email. In order to use these settings though, you must have a certificate that can be used with the mobile device.

10. Click Finish and the mobile device will begin its initial synchronization. During this process, you may be prompted to enter a password for the mobile device.

11. When the synchronization process completes, there is one last thing that you need to do to make ActiveSync work properly. On the ActiveSync screen, click the Menu button (found in the lower, right corner of the screen) and choose the Schedule option, as shown in Figure I. By default, the mobile device is scheduled to synchronize every 10 minutes during peak times and every four hours during off-peak times. In order for Direct Push to behave properly, both of these settings must be changed to As Items Arrive.

    Figure I: Configure mobile devices to synchronize data as items arrive.
    

    

Instead of using SMS messages, Microsoft Direct Push technology works by maintaining a constant HTTPS connection between mobile devices and Exchange Server. Because the connection is constantly available, new email messages can be delivered to mobile devices nearly instantaneously.

The pros and cons of Direct Push HTTPS connections

Having a constant HTTPS connection raises a few concerns. For starters, some mobile devices are unable to receive voice calls while data is being transmitted or received. Another common concern is that sending and receiving data consumes as much battery power as voice calls do.

Although these are some serious issues, Microsoft has taken steps to minimize the impact of sending and receiving data. Direct Push will not completely drain a mobile device's battery in a couple of hours. The mobile device does maintain a constant HTTPS connection with the Exchange Server, but it is not constantly sending or receiving data.

This is possible because the HTTP and HTTPS protocols were designed for distributed networks, and do not assume that HTTP or HTTPS messages will be delivered and responded to instantaneously.

Instead, there is a timeout value associated with an HTTP or HTTPS session. When a sender transmits a packet, it doesn't really matter how long it takes to receive a response, so long as the response comes before the session times out. Direct Push takes advantage of this by allowing the mobile device to go to sleep between packets.

Direct Push heartbeat messages

Direct Push uses heartbeat messages to keep a session alive between Exchange Server and mobile devices. A heartbeat is nothing more than a periodic transmission that keeps the session open and allows the mobile device to check to see if synchronization is necessary.

The process begins when the mobile device initiates a session with Exchange Server. Upon doing so, the mobile device transmits a heartbeat message to the server at a predefined interval. At this point, one of three things will happen:

1. The Exchange server will respond with new synchronization data. In this case, the new data is synchronized with the data that's stored on the mobile device.

2. The Exchange server will respond with an HTTP 200 OK message. This means that there wasn't any new data to be synchronized. More importantly, it means that the session did not time out.

   When the mobile device receives this response it may try to dynamically adjust its heartbeat interval so that there will be a longer period of time between heartbeats. Remember that the longer the period of time between heartbeats, the lower the battery consumption. Longer heartbeat intervals also decrease the chances that heartbeats will be disruptive to attempted voice calls.

3. The session times out before the heartbeat interval is reached. When this happens, Exchange automatically lowers he heartbeat interval in an effort to prevent the Direct Push session from timing out.

It's easy to see how a Direct Push session can be kept alive by transmitting a heartbeat and waiting for a response. However, based on my description above, it might seem like a lot is left to chance. For example, how does Exchange Server know how much to adjust the heartbeat interval? Or what if something happens that disrupts the communication process?

In reality, nothing is left to chance. Microsoft has built quite a bit of intelligence into the dynamic adjustment of Direct Push heartbeats.

Microsoft knows that there are factors that can cause heartbeats to occur abnormally. For example, if the Exchange server were to suddenly become busier than expected, then the server might not be able to respond to a heartbeat message before it expires. That being the case, Exchange Server is designed so that successive roundtrip heartbeats must occur before it adjusts the Direct Push heartbeat interval.

Another way that Microsoft has built intelligence into Direct Push is that Exchange Server is smart enough to know that it should not adjust the heartbeat interval if the heartbeat expires due to a known cause.

For example, if a user is talking on the phone when Exchange Server transmits a heartbeat response, the mobile device will never receive the response and the heartbeat will time out. However, Exchange Direct Push knows that the timeout only occurred because the user was on the phone -- so the heartbeat interval is not adjusted.

Direct Push heartbeat registry keys

The Direct Push heartbeat is controlled by four registry keys:

• HeartbeatDefault
• HeartbeatIncrement
• HeartbeatMin
• HeartbeatMax

These values are stored within the registry at:

HKEY_LOCAL_MACHINE\Software\Microsoft\ActiveSync

Keep in mind that editing the registry is dangerous, because making incorrect modifications can destroy Windows and/or your applications. Always make a full system backup prior to making any registry modifications.

HeartbeatDefault refers to the initial heartbeat interval before any adjustments are made. By default, the value for this registry key is set to 480 seconds, or eight minutes.

The HeartbeatMin registry key refers to the minimum amount of time between heartbeats. The minimum heartbeat interval is the same as Microsoft's default heartbeat interval; 480 seconds. Microsoft recommends that you do not adjust this value because lowering it can cause an excessive number of heartbeats to occur -- this will cause the mobile device to consume a lot more battery power. According to Microsoft, lowering this value nets a very small performance gain, which is not substantial enough to warrant the impact on battery life.

The HeartbeatMax value is the maximum amount of time between heartbeats. As I explained earlier, Exchange Server will dynamically increase the amount of time between heartbeats as the mobile device is able to prove that it can handle longer heartbeat durations without timing out.

There has to be a cutoff point though to prevent Direct Push from increasing the heartbeat interval indefinitely. By default, Microsoft sets the maximum heartbeat interval to 1,680 seconds (28 minutes). While researching this tutorial, I found posts on the Internet from administrators who claim to have been able to increase the HeartbeatMax value to 45 minutes, but Microsoft recommends that you do not exceed the default value.

The default value is designed to be just below the network timeout period of most networks. If you set the HeartbeatMax value too high, the mobile device will gradually increase its heartbeat period to the point that the heartbeat times out. When this happens, Exchange Server will lower the heartbeat interval to an acceptable level, but data will remain unsynchronized until this point is reached. It is better to leave the default value in place and prevent timeouts from occurring in the first place.

The most interesting of the four registry keys is HeartbeatIncrement. This key determines how much the heartbeat interval is adjusted at a time. By default, the HeartbeatIncrement value is set to 300 seconds (five minutes). This is also Microsoft's recommended value.

You can adjust the HeartbeatIncrement registry key, but doing so is a little tricky. If you set the heartbeat increment too high, Exchange Server will make large adjustments to the heartbeat interval that could potentially result in timeouts occurring (if the network's timeout threshold is unknown or the HeartbeatMax value has not been set accordingly).

If the heartbeat increment is set too low, then there is less danger of causing a timeout. But, it may take Direct Push a long time to adjust the heartbeat interval to an optimal value. This wouldn't be a big deal except that it means that the mobile device is consuming more battery power than is necessary during the adjustment period.