Windows Mobile 6.5 Screen Mirroring via ActiveSync

Real-time screen mirroring between a Windows Mobile 6.5 device and a PC relied on a client-server architecture powered by Microsoft ActiveSync's communication protocols. While ActiveSync itself handled synchronization and connectivity rather than native video streaming, it provided the underlying Remote Application Programming Interface (RAPI) and virtual networking pipeline that allowed mirroring tools to capture, compress, and transmit the device’s display buffer to a desktop monitor.

The ActiveSync Transport Layer

ActiveSync established a point-to-point communication channel over USB or serial connections using Remote NDIS (RNDIS) or Point-to-Point Protocol (PPP). This connection assigned virtual IP addresses to both the desktop and the mobile device, effectively creating a dedicated local network. In addition to general TCP/IP traffic, ActiveSync initialized RAPI (Remote API), a proprietary remote procedure call mechanism that allowed the host PC to execute commands, manage the mobile file system, and inject binaries directly into the mobile operating system's memory.

Client-Server Architecture

Because Windows Mobile 6.5 did not feature a native broadcast switch, screen mirroring required dedicated utilities such as Microsoft's Remote Display Control (CERHost/CERDisp), SOTI Pocket Controller, or MyMobiler. When connected, the PC software used ActiveSync's RAPI layer to silently install and launch a small background daemon (server agent) on the Windows Mobile device. This mobile agent established a persistent TCP/IP socket connection back to the PC client application through the ActiveSync virtual network adapter.

Frame Buffer Capture

Once running on Windows Mobile 6.5, the server agent tapped directly into the Windows CE Graphics Device Interface (GDI) or queried the display hardware abstraction layer (DirectDraw/DirectX). The agent bypassed standard high-level window managers to scrape the raw frame buffer from the display driver memory. Because mobile processors of that era had limited computing power, scraping full-color uncompressed frames at native resolutions (such as QVGA or WVGA) would rapidly saturate the CPU and the USB link.

Compression and Differential Encoding

To achieve near real-time performance without crippling the device, the mobile agent utilized differential screen scraping. Instead of capturing and transmitting the entire frame continuously, the software tracked which screen regions had changed since the previous cycle (dirty rectangles). The agent applied lightweight compression algorithms—primarily Run-Length Encoding (RLE) or proprietary delta compression—to only those altered blocks of pixels before packing the data into network packets.

Desktop Rendering and Input Redirection

The PC client application received the compressed packet stream across the ActiveSync transport, reassembled the dirty rectangles, and blitted the updated graphical elements onto a desktop-rendered device skin.

The pipeline was completely bi-directional. When a user clicked or typed within the PC window, the desktop application translated the mouse coordinates into absolute touch screen coordinates and the keystrokes into standard virtual-key codes. These inputs were packaged and sent across the ActiveSync link to the mobile agent, which injected them into the Windows Mobile 6.5 input subsystem using native APIs like mouse_event and keybd_event, enabling seamless real-time remote control.