My experience is that the WAN latency shouldn't be higher then 35-40ms. With everything higher, BES will not work reliably. The latency from BES to SQL can be much higher, around 150-200ms. So your planned configuration with the BES local to the Exchange server is the way to go. By using the central SQL your migration from the remote to the local BES will be very straight forward with a "move user" within the Blackberry console.

For some more informations, have a look at View Document.

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