8.2 Framework

While this chapter starts with a short review of the complete HAVi specification, the focus is only on the Level 2 GUI API, also known as HAVi L2 GUI, which was adopted by most application-layer iTV standards (as opposed to transport standards).

8.2.1 The Home Network

The home network allows a single device, or a group of devices communicating amongst themselves , to deliver a service to a user or an application. When it is necessary for a device to interact with a user , a GUI for the device may be presented on a device with display capabilities. Therefore, each device has, at a minimum, enough functionality to allow it to communicate with other devices in the system.

During the course of interaction, devices may exchange control information and data in a peer-to-peer fashion. This ensures that, at the network communication layer, no one device is required to act as a master or controller for the system.

8.2.2 Control Model

The HAVi control model makes a distinction between controllers and controlled devices, where the controller is a logical master imposing a control structure on a controlled device using a peer-to-peer communication model. A controller is a device that acts as a host for a controlled device. A controlled device and its controller may reside on the same physical device or on separate physical devices. A controller is said to host a Device Control Module (DCM) for the controlled device. DCMs are a central concept to the HAVi architecture and the source of flexibility in accommodating new devices and features.

For example, an intelligent television in the family room might be the controller for a number of interconnected devices. A controlled device could contain Java bytecode that constructs a user interface for the device and allows external control of the device. When these devices are first connected, the controller obtains the user interface and control code. An icon representing the device may then appear on the television screen, and manipulating the icon may cause elements of the control program to actuate the represented device or devices in prescribed ways.

8.2.2.1 DCM Characteristics

The first DCM characteristic is how the DCM is obtained by the controller. Embedded DCMs are part of the resident software on a controller. Uploaded DCMs are obtained from some source external to the controller and dynamically added to the software on the controller.

The second DCM characteristic is whether a DCM is platform (controller) dependent. Native DCMs are implemented for a specific platform, that may include machine code for a specific processor or access platform specific APIs. Bytecode DCM are implemented in Java.

The third DCM characteristic is their functionality or range of use. Standard DCMs provide the standard HAVi APIs only. Proprietary DCMs provide vendor-specific APIs in addition to the standard HAVi APIs.