Typically a normal i Tunes based restore is sufficient, which is why that is fairly rare.Finally, there are some cases where IPSW is necessary to create custom configured firmware for future restores.
Also, for users with many of the same i OS devices, having a single IPSW downloads let’s you conserve bandwidth by applying a single update to many devices.
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To use the latest version of IPSW firmware files to update or restore an i OS device, you’ll want to be sure you have the latest version of i Tunes installed as well.
Going the IPSW route can take longer to update than using the newer OTA (over the air) method, the reason being that OTA uses smaller delta updates than the larger IPSW files, which are the full sized downloads and are often literally 10x the download size.