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I have been trying to find bandwidth usage information how the technologies compare.

I understand that ActiveSync is instant, and new changes get "pushed" to subscribers. But IMAP/CalDAV/CardDAV is synced on a timed basis. So that means it must use some bandwidth to check if there is anything to sync even when there is nothing to sync.

So It got me wondering, over time, how does Microsoft's ActiveSync compare to these other open source protocols in regards to bandwidth usage. Which would be more efficient?

I assume ActiveSync has to keep a constant open connection through which to "push" new changes through, but I don't know enough about this protocol.

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ActiveSync and IMAP are quite similar. Both ActiveSync and IMAP IDLE maintain a dormant TCP session for inbound push notifications. IMAP IDLE handles 'push' email as well FYI.

Both are quite conservative with bandwidth usage. Bandwidth consumption in practice will be more determined by configuration parameters than protocol choice. Setting things like 'only download first 50k of email' or 'only download attachments in X state' will make large differences in consumption.

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