Added javadocs to the overview.html file to explain the overall software structure a bit better.
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<html>
<body>
-<img src="doc-files/onos-tiers.png" alt="ONOS architecture tiers"><br>
-ONOS architecture is strictly segmented into protocol-agnostic system core and
-protocol-aware providers tiers.
+<p>
+ ONOS architecture is strictly segmented into a <em>protocol-agnostic system
+ core</em> tier and the <em>protocol-aware providers</em> tier as shown in
+ the figure below:<br>
+ <img src="doc-files/onos-tiers.png" alt="ONOS architecture tiers">
+</p>
-<em>More information to come later...</em>
+<p>
+ The <em>ONOS core</em> is responsible for tracking information about the
+ network environment and distributing it to the applications either
+ synchronously via query or asynchronously via listener callbacks. The
+ core is also responsible for persisting select state and synchronizing state
+ among the cluster peers.
+</p>
-</body>
\ No newline at end of file
+<p>
+ The <em>protocol-aware providers</em> are responsible for interacting with
+ the network environment using various control and configuration protocols
+ and supplying such sensory data to the core. Some providers may also need to
+ accept control edicts from the core and apply them to the environment
+ using the appropriate protocol-specific means.
+</p>
+
+<p>
+ The following diagram describes a general structure of each ONOS subsystem:
+ <br>
+ <img src="doc-files/onos-subsystem.png" alt="ONOS subsystem structure"><br>
+ For example, the device-subsystem comprises of a core
+ {@link org.onlab.onos.net.trivial.device.impl.SimpleDeviceManager DeviceManager},
+ which exposes a north-bound
+ {@link org.onlab.onos.net.device.DeviceService} through which applications or other core components
+ can learn about the global infrastructure device inventory and through
+ which they can also subscribe for asynchronous {@link org.onlab.onos.net.device.DeviceEvent}
+ notifications via the {@link org.onlab.onos.net.device.DeviceListener} mechanism. A set of
+ administrative actions can be performed via {@link org.onlab.onos.net.device.DeviceAdminService},
+ e.g. setting mastership role, removing a decommissioned device.
+</p>
+
+<p>
+ On the south-bound side, the core {@link org.onlab.onos.net.trivial.device.impl.SimpleDeviceManager DeviceManager}
+ exposes a {@link org.onlab.onos.net.device.DeviceProviderRegistry} through which any number of
+ {@link org.onlab.onos.net.device.DeviceProvider} entities can register and in turn obtain a
+ {@link org.onlab.onos.net.device.DeviceProviderService}. Device and port information can then be
+ supplied to the core by each provider through the provider service issued
+ to them. When a provider unregisters, its {@link org.onlab.onos.net.device.DeviceProviderService}
+ will be invalidated and can no longer be used for interacting with the
+ core.
+</p>
+
+<p>
+ Within the core, the tasks of indexing, persisting and synchronizing the
+ global device and port state with the cluster peers falls on the
+ {@link org.onlab.onos.net.trivial.device.impl.SimpleDeviceStore DeviceStore}.
+</p>
+
+<p>
+ Similar structure applies to the link subsystem, host subsystem and others.
+</p>
+
+<p>
+ <em>More information to come later...</em>
+</p>
+
+</body>
+</html>
