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     <li><a href="#connecting-to-a-protocol-adapter-directly">Connecting to a Protocol Adapter directly</a></li>
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             <h1>

               Connecting Devices
             </h1>


 <p>One of the most important features of Eclipse Hono™ is to abstract away the specific communication protocols
 used by devices. This page describes the different ways of how devices can be connected to Hono.</p>
 <!-- more -->
 <p>Before a device can connect to Hono and upload data or receive commands from downstream applications,
 it needs to be <a href="/hono/docs/1.1/concepts/device-provisioning/">provisioned</a> to the system.
 As part of device provisioning, the device is associated with the <em>tenant</em> that it belongs to and gets
 assigned a logical identifier which is unique within the tenant.</p>
 <p>Devices can be generally partitioned into two groups: devices which natively support the Internet Protocol
 (IP) for communication and devices that don&rsquo;t.</p>
 <p>Devices falling into the former group can connect to Hono directly using any of the IP based protocols supported
 by Hono&rsquo;s protocol adapters. Devices from the latter group often use radio based or serial line communication protocols
 that are limited to a local area and require a <em>gateway</em> in order to connect to one of Hono&rsquo;s protocol
 adapters via IP.</p>
 <p>The diagram below shows a device that supports the MQTT protocol and connects directly to Hono&rsquo;s MQTT protocol adapter
 and another device that uses Bluetooth LE for connecting locally to a gateway which then connects to Hono&rsquo;s MQTT adapter.</p>
 <figure>
     <img src="device-types.svg"/>
 </figure>

 <h2 id="connecting-to-a-protocol-adapter-directly">Connecting to a Protocol Adapter directly</h2>
 <p>The most straight forward scenario is a device connecting to one of Hono&rsquo;s protocol adapters directly via IP based
 network infrastructure. For this to work, the device needs to use a communication protocols supported
 by one of the adapters and needs to be able to use the resource endpoints exposed by that particular protocol adapter
 as described in its <a href="/hono/docs/1.1/user-guide/">user guide</a>.</p>
 <p>In this case the connected device&rsquo;s identity will be resolved as part of authentication during connection establishment.
 For this to work, a set of credentials needs to be provisioned for the device which needs to be appropriate for
 usage with one of the adapter&rsquo;s supported authentication schemes.</p>
 <h2 id="connecting-via-a-device-gateway">Connecting via a Device Gateway</h2>
 <p>In some cases, a device may not be able to directly connect to one of Hono&rsquo;s protocol adapters.
 An example is a device that uses a serial bus or radio waves for local communication.
 Such devices can be connected to a protocol adapter by means of a <em>device gateway</em> which acts on behalf
 of the device(s) when communicating with Hono. A device gateway is often implemented as a (small) hardware box
 close to the devices, running some gateway software which translates hence and forth between the device and one
 of Hono&rsquo;s protocol adapters.</p>
 <p>From the perspective of a protocol adapter, the gateway looks just like any other device having its own device
 identity and credentials.</p>
 <p>The following diagram illustrates how a gateway publishes data on behalf of a device that uses Bluetooth for local
 communication with the gateway.</p>
 <figure>
     <img src="device-via-gateway-auth.svg"/>
 </figure>

 <ol>
 <li>The device establishes a Bluetooth connection with the gateway.</li>
 <li>The gateway sends an MQTT CONNECT packet to Hono&rsquo;s MQTT adapter to establish an MQTT connection.
 The packet contains the gateway&rsquo;s credentials.</li>
 <li>The MQTT adapter determines the tenant from the <em>username</em> contained in the CONNECT packet and retrieves
 the hashed password that is on record for the gateway from the Credentials service.</li>
 <li>The Credentials service returns the hashed password.</li>
 <li>The MQTT adapter checks the password and accepts the connection request.</li>
 <li>The device sends some sensor readings via Bluetooth to the gateway.</li>
 <li>The gateway forwards the sensor data in an MQTT PUBLISH packet to the MQTT adapter.
 The topic name contains the identifier of the device that the gateway acts on behalf of.</li>
 <li>The MQTT adapter invokes the Device Registration service&rsquo;s <em>assert Device Registration</em> operation to
 check if the gateway is authorized to act on behalf of the device.</li>
 <li>The Device Registration service confirms the gateway&rsquo;s authorization.</li>
 <li>The MQTT adapter accepts the sensor data from the gateway and forwards it downstream.</li>
 </ol>
 <p>Note that the device itself is not authenticated by the MQTT adapter in this case. The responsibility
 for establishing and verifying the device identity lies with the gateway in this setup.
 It is therefore not necessary to provision credentials for the devices to Hono.</p>
 <p>The <a href="/hono/docs/1.1/api/management/">Device Registry Management API</a>&rsquo;s <code>/devices</code> resource can be used to
 register gateways and devices. The gateways that are authorized to act on behalf of a device can be set by means
 of the device&rsquo;s <em>via</em> and <em>viaGroups</em> properties. This is useful in cases where a device may <em>roam</em> among multiple gateways.</p>
 <p>When sending commands to a device, Hono needs to determine which of the authorized gateways should be used to forward
 the command message to the device. For this purpose, Hono&rsquo;s protocol adapters keep track of the <em>last known gateway</em>
 which has acted on behalf of each device by means of the <a href="/hono/docs/1.1/api/device-connection/">Device Connection API</a>.</p>


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