Windows Mobile 6.5 Tethering Protocols Explained

Windows Mobile 6.5 provided mobile broadband sharing capabilities primarily through its built-in "Internet Sharing" utility, allowing computers and other devices to access the phone’s cellular data. This article explores the specific protocols and standards used to achieve tethering on Windows Mobile 6.5 devices, focusing on the native implementations over USB and Bluetooth, as well as the workarounds used for Wi-Fi sharing.

The "Internet Sharing" Application

In Windows Mobile 6.5, the primary interface for tethering was the native Internet Sharing tool (which replaced the older "Modem Link" software found in legacy versions). Internet Sharing streamlined the tethering process by directly bridging the phone’s active cellular data connection (GPRS, EDGE, 3G, or HSPA) to an external device through specific transport layers and networking protocols.

USB Tethering Protocols

USB tethering was the fastest and most stable method available on Windows Mobile 6.5. It leveraged two primary protocol implementations:

  • RNDIS (Remote Network Driver Interface Specification): RNDIS was the default and most efficient protocol used by Windows Mobile 6.5 for USB tethering. Developed by Microsoft, RNDIS encapsulates standard Ethernet frames over a USB connection. When connected to a host computer (particularly Windows XP, Vista, or 7), the phone presented itself as a virtual network interface card (NIC). The host automatically assigned an IP address via DHCP from the mobile device, allowing low-latency, plug-and-play internet access without requiring explicit dial-up commands.
  • CDC-ACM / Emulated Serial COM (Modem Tethering): Prior to the universal adoption of RNDIS, and as a legacy fallback, Windows Mobile could operate as a traditional USB modem. This used standard USB Communication Device Class (CDC) Abstract Control Model (ACM) specifications. The host PC communicated with the phone using standard Hayes AT commands over an emulated COM port, establishing the connection using Point-to-Point Protocol (PPP).

Bluetooth Tethering Protocols

Windows Mobile 6.5 provided wireless tethering primarily through Bluetooth, which relied on the Microsoft or Broadcom (Widcomm) Bluetooth stacks depending on the hardware manufacturer. Two distinct Bluetooth profiles were supported:

  • Bluetooth PAN (Personal Area Network) Profile: The PAN profile was the preferred wireless method in Windows Mobile 6.5. It operated over the Bluetooth Network Encapsulation Protocol (BNEP), which functions similarly to an Ethernet connection over an L2CAP link. The mobile device assumed the role of a Network Access Point (PAN-NAP), while the host computer joined as a Personal Area Network User (PANU). This setup allowed standard TCP/IP networking with lower overhead and higher throughput compared to dial-up profiles.
  • Bluetooth DUN (Dial-Up Networking) Profile: For compatibility with older systems, Windows Mobile 6.5 supported the DUN profile. DUN bridges the cellular data connection using the RFCOMM protocol (which emulates an RS-232 serial cable) to transport AT modem commands and PPP packets. The host computer dialed an emulated number (typically *99# or *99***1#) to initiate the connection. While widely compatible, DUN suffered from higher protocol overhead and slower data transfer rates than PAN.

Wi-Fi Tethering and Protocol Limitations

Standard Windows Mobile 6.5 did not include a native infrastructure-mode Wi-Fi hotspot (SoftAP) protocol stack out of the box, as the operating system's network architecture lacked native support for routing packets between a cellular adapter and a Wi-Fi card in access point mode.

However, tethering over Wi-Fi was achieved through OEM enhancements and third-party software:

  • 802.11 Ad-Hoc (IBSS) Mode: Early Wi-Fi tethering solutions on Windows Mobile 6.5 relied on Independent Basic Service Set (IBSS) mode. The phone established a peer-to-peer Wi-Fi network, relying on internal Network Address Translation (NAT) software to route packets between the ad-hoc wireless interface and the cellular radio.
  • Proprietary SoftAP Implementations: Certain manufacturers, notably HTC with tools like the "Wi-Fi Router" application, wrote custom Network Driver Interface Specification (NDIS) drivers. These drivers allowed the Wi-Fi hardware to broadcast an 802.11b/g beacon in true infrastructure mode, operating a lightweight DHCP server and standard NAT engine to serve multiple client devices simultaneously.