CDF is a standard for creating Active Channels in IE4. It is based on the Extensible Markup Language (XML). For the complete specifications, see the Microsoft Internet SDK Web site http://www.microsoft.com/msdn/sdk/inetsdk/help/. As mentioned previously, Mobile Channels adds a channel to the existing IE4 channels. It calls for additional tags to extend CDF for optimizing the mobile-channel performance on a Windows CE-based device. Mobile Channels uses these additional tags to navigate through CDF files and to reduce the storage space required on the device.