How Does UPnP Work?
A UPnP enabled network consists of three main elements: Devices, Services and Control Points.
Devices
A Device can be any example of UPnP-enabled equipment. Consider a modern mobile phone. This acts as a container which holds a number of services (explained below).
The device may also include what is known as a nested device. This could be another device that is built into the main device. A common example found in phones is the digital camera.
At the heart of any UPnP device is a file called the Device Description Document. This file is written in XML so it can be viewed by any device or computer to discover the device properties. These properties hold valuable information such as the device's name and other identification details, plus a complete list of Services that the device supports.
Services
A Service is the set of functions that a Device supports. It is supported by two main values: an Action and a State.
Action
- If we take the example of an UPnP-enabled mobile phone, an Action could be "receive a call" or "send a file".
State
- A State describes the current status of a device. This can change depending on the situation. A typical example would be that the device's status is busy if a telephone is already taking a call.
- As mentioned in the Device section, the available Services are listed in the Device Description Document. Each Service on the Service List links to its own XML file which provides a detailed description of the Service.
A typical UPnP device will have three functions that deal with Services:
- State Table: This takes the current state of the Service, e.g. Busy or Waiting, and records it. It will be updated when the State value changes.
- Control Server: This takes Action requests and acts upon them. It will update the current State value of the Service.
- Event Server: This is responsible for letting other devices (i.e. their services) know when the State of the Service changes, e.g it can now accept incoming connections.
Control Points
The final element in a UPnP network is what is known as a Control Point.
A control point discovers and then controls other devices it finds on the network. It retrieves the Device Description Document file and the Service list from each device.
An animation is provided on the next page to demonstrate how these all work together.
