THE MAGAZINE FOR COMPUTER APPLICATIONS
Circuit Cellar Online offers articles illustrating creative solutions
and unique applications through complete projects, practical
tutorials, and useful design techniques.

KNOCK! KNOCK! "WHOýS THERE?"

Using UPnP to Respond to Inquiries
by Edward F. Steinfeld

Start ý UPnP Overview ý Functions ý Device Addressing, Discovery, and Description ý Control, Event Messaging, and Presentation ý UPnP Forum ý Software Tools ý Hardware ý Easier Deployment ý Sources and PDF

CONTROL

To initiate device action, a control point sends a control message using the definitions from the device description document. To exchange information the control layer uses a World Wide Web Consortium (W3C) draft standard, Simple Object Access Protocol (SOAP). SOAP is defined as a "lightweight, XML-based protocol for exchange of information in a decentralized, distributed environment." The device completes the action and responds using SOAP.

EVENT MESSAGING

The event messaging layer is a basic push model, where control points listen for notifications of UPnP device state changes. Actually, it is a little more complex. To get event messages, control points subscribe to event messages for a specific service within a device. On a network, there can be multiple control points and multiple UPnP-enabled devices. A control point might want to listen to multiple services but not all. The subscription and unsubscription of events allow control points to be selective. When a service within a device has an event, it sends that event (Notify) to all current subscribers. This way all subscribers have current knowledge of the state of the device.

Event messages use an extension to HTTP defined by the IETF draft standard, General Event Notification Architecture (GENA). GENA is defined to send and receive notifications using HTTP over TCP and UDP.

PRESENTATION

Presenting information and control to you is accomplished by the presentation function. Presentation requires getting an address, discovery, and description. The description document provides the device URL for the presentation page. There are no constraints on the use of the web page obtained from the device. The device description document provides a URL for the initial presentation page for the device. An unlimited number of linked presentation pages are permitted.

PREVIOUS • NEXT