UPnP™
Architecture
The UPnP architecture is a set of open standards
and technologies for transparently connecting appliances, PCs,
and services by extending the Plug and Play concept to support
networks and peer-to-peer discovery, configuration and control.
The UPnP Forum
is committed to device interoperability in a number of areas and
has established working groups for Home Automation and Security,
Internet Gateways, Imaging and Printing, Audio/Video, Mobile Devices
and Appliances.
The UPnP architecture is based on simple,
open protocols such as those defined by the Internet Engineering
Task Force (IETF) that have a proven track record of enabling
real-world multiple-vendor interoperation. TCP/IP has been implemented
by many vendors on many diverse computing platforms, yet these
implementations are able to communicate reliably and without a
massive multiple-vendor test effort. When browsing the Web using
HTTP, users move seamlessly between different server operating
systems and different Web server implementations without any client-side
code or configuration changes. The Internet itself, in spite of
its huge size, is not managed by any single authority. Wire protocols
are proven technology. The UPnP architecture uses the Internet
open protocol model. Device vendors in the UPnP architecture are
not constrained to implement any single vendor's technology, and
are assured that they will interoperate with the largest number
of other vendors' products.
The UPnP architecture's focus on explicit
open wire protocols means that it is language and operating system
neutral. The advantages of this approach to developers are obvious;
the choice of language and operating system gives them the flexibility
to choose the best platform for their device and still be confident
that their products will be able to interact with other UPnP devices,
regardless of platform. For example, it is possible for a developer
using Windows to interact with services on non-Windows devices
by invoking Windows-based APIs, object models, or even XML-enabled
data-driven programming using familiar languages and tools, regardless
of the UPnP implementation on the device providing the service.
UPnP Components
The UPnP architecture has five components,
Discovery, Description, Control, Eventing, and Presentation. The
first component of UPnP networking is Discovery. When a device
is added to the network, the UPnP discovery protocol allows that
device to advertise its services to control points on the network.
Similarly, when a control point is added to the network, the UPnP
discovery protocol allows that control point to search for devices
of interest on the network. The message that is exchanged in both
cases is a discovery message containing a few, essential specifics
about the device or its services, such as the device type, identifier,
and a pointer to more detailed information. The UPnP discovery
protocol is based on the Simple Service Discovery Protocol (SSDP).
The second component of UPnP networking is
Description. After a control point has discovered a device, the
control point needs to find out more about the device. For the
control point to learn more about the device and its capabilities,
or to interact with the device, the control point must retrieve
the device's description using the URL provided by the device
in the discovery message. The URL points to a UPnP description
document that is expressed in XML and includes vendor-specific
information such as the model name and number, serial number,
manufacturer name, URLs to vendor-specific Web sites and URLs
for control, eventing, and presentation.
Devices may contain other logical devices,
as well as services. The UPnP description document includes a
list of other embedded devices and a description of the available
services. For each service, the description includes a list of
the actions that the service responds to and the arguments for
each action. The description for a service also includes a list
of variables that reflect the state of the device. These variables
are described in terms of their data type, range, and event characteristics.
The third component of UPnP networking is
Control. After a control point has retrieved the description of
the device, the control point can initiate actions to the device.
To do this, a control point sends a suitable control message to
the control URL for the service. Control messages are expressed
in XML using the Simple Object Access Protocol (SOAP). Control
actions are like function calls, and the device service will return
any action-specific values in response to the control message.
The effects of the action will also be reflected by changes in
the variables that describe the run-time state of the device.
The fourth component of UPnP networking is
Eventing. The UPnP description document for a device service includes
a list of variables that reflect the state of the device. The
device publishes updates when these variables change, and a control
point may subscribe to receive this information. The subscriptions
are established using the General Event Notification Architecture
(GENA). The device publishes updates by sending event messages.
Event messages contain the names of one of more state variables
and the current value of those variables. These messages are expressed
in XML and sent using HTTP. A special initial event message is
sent when a control point first subscribes. This initial event
message contains the names and values for all evented variables
and allows the subscriber to initialize its model of the state
of the service. To support scenarios with multiple control points,
eventing is designed to keep all control points equally informed
about the state of the device. This means that all subscribers
are sent all event messages, and subscribers receive event messages
for all evented variables that have changed.
The fifth component of UPnP networking is
Presentation. The Presentation capabilities of a device allow
HTML-based management of the device by the end user using a standard
Web browser. A control point can determine the entry Presentation
URL from the device description document, retrieve the entry page
from the URL, load the page into a browser, and start the user
management of the device.
An additional component of UPnP networking
is the application layer. The capabilities of a device are defined
by specific device and services templates that provide the framework
for the Description, Control and Eventing network components.
A device vendor can prepare these templates by themselves or work
with other vendors within the UPnP Forum to prepare industry standard
device and service templates. Current UPnP Working Committees
that have prepared standard template definitions include the Internet
Gateway, Printing and Imaging, and Audio/Visual Working Committees.
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