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By Chris De Herrera 
Copyright 1998-2007
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Redfly Under the Hood
By Chris De Herrera, Copyright 2008
 Version 1.00  Created 11/2/2008

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While I have not opened my Redfly, I have been thinking about how the Redfly works. Here are my observations on how the Redfly interacts with Windows Mobile.

TCP/IP Connectivity

All connectivity with the Redfly from the Windows Mobile device is sent over a TCP/IP connection. This TCP/IP connection appears to be based on the Windows Mobile device being the DHCP server and using the RNDIS (Remote Network Device Independent Specification). The Windows Mobile device uses 169.254.2.1 and the Redfly uses 169.254.2.2 just like ActiveSync or WMDC does. So older Windows Mobile devices that do not support RNDIS or the Advanced Network Connection option will not work.

Redirecting Video to the Redfly

From what I can decipher by observing how the Redfly works, the Redfly is an extension of the Windows Mobile device. This extension for the screen is redirecting the graphics display interface (GDI) that the GWES (Graphics, Windowing and Events) application on the Windows Mobile device usually displays locally to the Redfly. Then the Redfly interprets the GDI commands which display the user interface. So in order to play a video, the amount of bits required to be sent across the USB or Bluetooth connection is very large (800 x 480 x 16 = 175 mb/s for 30 FPS). Plus the Windows Mobile device’s CPU has to generate the larger 800 x 480 GDI commands to send to the Redfly. So faster cpu Windows Mobile devices will work better with the Redfly than slower ones. I found that the 400 MHz cpu in the Tilt generally worked well however there were some lags with moving the cursor from time to time.

In addition to video, Celio added something that Windows Mobile does not have by default – a mouse cursor. The x and y coordinates of the cursor have to be sent from the Redfly to the Windows Mobile device as part of the GDI information in order to allow you to interact with Windows Mobile.

USB Storage on the Redfly

Finally the Redfly extends the file system by allowing you to plug in a USB storage device. When you plug in the storage device, it is seen by Windows Mobile as if you plugged in a flash card into your device. To me this means that all file i/o is being sent from the Redfly to the Windows Mobile device and the Windows Mobile device is maintaining the file allocation table. Since this direct access can present a risk of corrupting or truncating files, I understand why Celio highly recommends that users use a USB cable to connect to the Redfly from their device. I would extend this recommendation by adding that the user should wait 30 seconds after saving a file to allow Windows Mobile and the Redfly to completely write the information on the storage device.

Redfly Performance

The combination of the GDI display, keyboard, mouse and storage device information can be large. So in order to optimize the performance, I expect that Celio is using compression in order to optimize the performance especially since most Windows Mobile devices are USB 1.1 clients (10 mb/s). However compression would increase the cpu utilization of your Windows Mobile device and this approach would favor devices with faster cpus for better performance.

Other Articles on the Redfly

1. Redfly "Deep Dive Review
2. Redfly Screen Shots
3. Redfly SoftMaker Screen Shots - TextMaker, PlanMaker and SoftMaker Presentations - More functionality than Word Mobile. Excel Moble and PowerPoint Mobile
4. Redfly Pictures
5. Redfly USB Peripherals

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