Hardware Reference
In-Depth Information
Control
Having described the controls to which this device will respond, it is then possible for remote controlling applications
to send messages to the device. These are wrapped with the Simple Object Access Protocol (SOAP), meaning that
you'll be looking at a lot of XML!
Event Notify
With the control layer being used to receive incoming messages, it only makes sense that the even notify layer is able
to send them. This happens when a state on the device changes. The specifics of these events vary according to device,
so when a “control point” first subscribes to the device, a list (again, in XML) is sent back, which details the events
to which the control point can listen. It then makes a request to subscribe to these events, and so initiates a push
mechanism between device and controller.
Presentation
Ultimately, there is a layer whereby two devices have agreed that the media at a specific URL wants to be played,
and that it should be played on a specific device. No indication is given in the specification on which media
formats should be supported by the presentation layer, and therefore which files are compatible. This is “by
design,” and it is something that the Digital Living Network Alliance has tackled. (For more on this, see later in
this topic.)
Because the presentation layer is separate from the control and server, the architecture provides us with one very
sneaky built-in advantage! That is, it is easy to stream one signal to two different presentation components. On the
surface, this provides a basic “whole-house audio” system, or a means to watch the same program in the living room
and continue watching it in the kitchen. Indeed, this is possible. But because the presentation components do not
need to match in terms of functionality, this means that you can have a UPnP-enabled TV in the lounge but only a set
of suitable speakers in the kitchen. In this way you can continue to listen to the program without being distracted by
the visuals—something that may be of benefit if you're working in the kitchen.
Authentication
This is the seventh layer in our six-layer stack. That is to say . . . there is no authentication! Although this exemplifies
our decision to not open ports and to our use of local network addresses, it does introduce a problem. In this case, the
problem is not a technical one but a management one. If anyone on your local network can access your media, can
you trust the people on your network? For the most part, these are members of your family who (should) conform
the social norms and coercions necessary to stop them abusing such access. However, you do only have a large grain
control mechanism, that is, removing their network access entirely, should this fail.
There are extensions to UPnP (such as the Device Security Service, and UPnP-UP (User Profile) and protocols
that are still not widely supported.
If you need authentication, however, and you can't be sure that your younger children won't take an interest
in your certificate 15/18/X-rated films, then you might want to resort to Samba with password-protected
areas for your media distribution. It is less well supported, and less flexible, than UPnP, but it would provide
peace of mind.
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