how to modify the time out for http(s) connection to exchange -sonicwall tz170

i have Iphone 3GS, I configure active sync in my exchange 2003 server, i am getting this log,
The average of the most recent [200] heartbeat intervals used by clients is less than or equal to [540].  Make sure that your firewall configuration is set to work correctly with Exchange ActiveSync and direct push technology. Specifically, make sure that your firewall is configured so that requests to Exchange ActiveSync do not expire before they have the opportunity to be processed.  For more information about how to configure firewall settings when using Exchange ActiveSync, see Microsoft Knowledge Base article 905013, "Enterprise Firewall Configuration for Exchange ActiveSync Direct ---------
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I got this solution
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Even if you don't have ISA, you might have to check settings on the Firewall. I have extracted a bit information from the above KB article which might be useful for you,

To resolve this issue, modify the firewall time-out values for HTTP(S) connections to the Exchange server to be greater than the default time-out limit of eight minutes.

Note This connection is not referring to the Connection Timeout field that is in the IIS MMC snap-in. Alternatively, modify the minimum heartbeat interval. We recommend that the firewall time-out value be set to a minimum of 15 minutes for the Exchange Direct Push Technology Always-up-to-date (AUTD) feature to perform optimally.

We recommend that you increase the firewall time-out values for HTTP(S) requests to the Exchange Server Microsoft-Server-ActiveSync virtual directory to provide a richer, "always-up-to-date" experience. The method that you use to increase the firewall time-out values depends on which firewall product you use. Refer to the firewall documentation for information on about how to increase the firewall time-out values.
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so how to do this in sonicwall firewall tz 170?