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We won't be seeing Google's modular smartphones hitting store shelves until next year, but the company is certainly storming ahead with their development. On the Project Ara website, the Module Developer Kit (MDK) has been released, containing a wealth of information about the project and how manufacturers can go about designing their own modules.
At launch there will be two frame (or "endo") sizes available - a 2x5 mini and a 3x6 medium - with a third 4x7 large size coming in a future release. These endos will accept a range of standard 1x1, 2x1 and 2x2 modules based on the layout of the dividers on the frame, which Google likes to call the "spine" and "ribs". While third parties will be able to create modules, Google will be in complete control of the design and layout of endos.
The MDK details specific guidelines that modules must adhere to, such as how far they can extend beyond the frame and how thick they can be. More leeway is given to camera modules, which can be thicker, and some sensor modules (such as a pulse oximeter) which need to stick out from the bottom of the handset.
Google has also outlined flexible plans for how batteries can be used in Ara devices. Not only will you be able to use one or more batteries in the endo, but it will also be possible to hot-swap them without powering off the device.
It appears prospective buyers will be able to use some sort of marketplace to pick out the endo and modules they desire, but this has yet to be detailed. With Google targeting a Q1 2015 launch though, there's still plenty of time to release further information.