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The protocol is an implementation of the MQTT 3.1.1 standard (October 2014). MQTT protocol is a client/server protocol of a subscribe/publish type. It is simple, has little overhead, and is easy to implement. It is used for M2M communication (Machine to Machine) and in the IoT context (Internet of Things). The MQTT server is also called the MQTT broker.
D2000 KOM implements the client part of the protocol. The protocol is implemented on a TCP/IP line.
For the transfer of LoRaWAN of LoRaWAN data encapsulated within the MQTT protocol, see LoRaWan protocol description.
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Note: communication with the cloud liveobjects.orange-business.com via websockets WebSockets (wss://liveobjects.orange-business.com:443/mqtt) was also tested. The program https://github.com/jimparis/unwebsockify.git was used as a WSS wrapper. This program started with the following parameters:
./unwebsockify.py --port 1883 --listen 172.16.0.1 wss://liveobjects.orange-business.com:443/mqtt
The D2000 KOM process connected to address 172.16.0.1 on port 1883. The WSS wrapper connected to the defined URL and wrapped the MQTT communication data in a websocket WebSocket envelope.
Each PUBLISH message contains a topic (Topic), data (Payload), and level of confirmation (QoS). PUBLISH messages can be sent both by the client and the server. The clients at the beginning of the communication will use the SUBSCRIBE message to indicate what topics (parameter of Topic Filter protocol) they are interested in.
| Kotva | ||||
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- QoS_0 - PUBLISH message is not confirmed, it may be lost
- QoS_1 - PUBLISH message is confirmed by the other side's PUBACK, it may be duplicated
- QoS_2 - PUBLISH message is confirmed by the other side's PUBREC which is then confirmed back by the PUBREL message and that one by a final PUBCOMP message.
The level of confirmation of the messages sent by the D2000 KOM process is defined by the protocol parameter Publish QoS. The D2000 KOM process considers the writing of the output tag to be successfully finished depending on the QoS:
- QoS_0 - after the data is successfully sent via the TCP connection
- QoS_1 - after receiving PUBACK
- QoS_2 - after receiving PUBCOMP
The MQTT communication starts with the CONNECT message sent by the client (D2000 KOM). The message contains User Name, Password, and other parameters, from which only Clean Session Flag and Client ID can be modified (parameter Will Flag is not used, as well as Will QoS and Will Retain, parameter Keep Alive is set to 0). The server replies with a CONNACK message with a return code that contains information about the success of the connect operation.
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Forced disconnection: If all stations on the line are in the simulation mode or the communication is stopped for them, the line will be disconnected (the communication socket will be closed). If the simulation is disabled for at least one station and the communication is not stopped for it (the Parameters tab of the Station type object), the line will be connected again.
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- Communication protocol "MQTT Client Protocol".
- Station address: the station address corresponds to the Topic field in the PUBLISH message received from the MQTT server. The address can be a specific Topic, a regular expression, a # character representing all Topics, or a topic .* representing all Topics that are not suitable for other stations. The processing priority is as follows:
- If there is a station with address # on the line, all messages are directed to its I/O tags and no further search is performed.
- Otherwise, all other stations on the line are searched (with the exception of the .* address). If the Topic matches the address of a station, the message is directed to that station and no further search is performed.
- Otherwise, all other stations on the line are searched (with the exception of the .* address), and their address is evaluated as a regular expression. If the Topic matches the station address, the message is directed to that station and no further search is performed.
Stations are searched in descending order (by station address), so more specific terms go first (e.g., status/battery before status/batt.*) - Finally, if there is a station with a .* address, the message is addressed to it.
- Polling parameters on the Time parameters tab - recommended value is Delay=0.
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Station protocol parameters
Communication station - configuration dialog box - "Protocol parameters" tab.
They influence some of the optional parameters of the protocol.
Table 2
| Keyword | Full name | Meaning | Unit | Default value | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Station Will Topic | Will topic of the device. If this parameter is set and a message with the same topic is received, the station will go into a communication error (StHardErr) and the values of the I/O tags will be invalidated. In this way, it is possible to emulate the standard behavior that occurs when there is a communication error with the device (even if the communication between the D2000 Kom process and the MQTT broker is functional). | ||||||||
| Station Will Payload | Content of the Will message. If this parameter is set and a message with the same topic as defined by the Station Will Topic parameter is received, the Payload must also be the same. If this parameter is an empty string, matching the topic with the Station Will Topic parameter is sufficient. |
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Possible value types of I/O Possible value types of I/O tags: Ci, Co, TxtI, TxtO, Qi, Ci, Co, Ai, Ao, Di, Do, TiR, ToR, TiA, ToA.
| Type of I/O tag | Address | Description | ||||||
|---|---|---|---|---|---|---|---|---|
| I/O tags for reading data sent by MQTT server through PUBLISH message. Note: values of I/O tags are set by the D2000 KOM process in the order IN_TOPIC, IN_DATA and IN_ID. It is not necessary for configuration to contain all three I/O tags. | ||||||||
| TxtI |
| Topic (Topic) of received PUBLISH message. | ||||||
| TxtI |
| Data (Payload) of received PUBLISH message. | ||||||
| Ci |
| Identifier of a packet (Packet Identifier) of PUBLISH message that depends on the level of validation (QoS). For messages sent with QoS_0, the identifier is zero, for QoS_1 and QoS_2, it is a positive 16-bit number. Note: if the MQTT server sends also messages with the QoS_0 level of validation and the ACK_ID I/O tag is configured, then we recommend activating the option New value when changing time in the Filter tab, so that repeated writing of the value 0 will cause a new value that differs only in a timestamp to be generated. | ||||||
| I/O tags for parsing JSON messages | ||||||||
TxtI, TxtO, Qi, |
| If Payload Type=JSON, the message is parsed as JSON data. The json_address value specifies the name of the JSON field whose value is to be assigned to the I/O tag. For other examples, see the description of the LoRaWAN protocol's Envelope type I/O tags. | ||||||
| I/O tag to confirm the received data to the MQTT server. | ||||||||
| Co |
| If an output I/O tag with ACK_ID address is defined, the D2000 KOM expects confirmation of the processing of each message by writing a copy of the value of the IN_ID tag. Only after, it sets values from the next received PUBLISH message (if it was received in the meantime) into the IN_TOPIC, IN_DATA, and IN_ID I/O tags (in this order). In the case of the QoS_0 level of confirmation, it is, therefore, necessary to repeatedly set the value of the I/O tag ACK_ID to 0. If the I/O tag ACK_ID does not exist, the values are written into the IN_TOPIC, IN_DATA, and IN_ID I/O tags immediately after the PUBLISH message is received and processed. Note: for the messages received with the QoS_0 level of validation, no confirmation is sent to the MQTT server, only the values of the received PUBLISH message will be published. | ||||||
| I/O tags for sending values to the MQTT server through a PUBLISH message. Note: in order for the D2000 KOM process to send the PUBLISH messages to the MQTT server, both I/O tags must be defined within one station. | ||||||||
| TxtO |
| The topic of the PUBLISH message being sent. | ||||||
| TxtO |
| Data (Payload) of the PUBLISH message being sent. Note: sending the message is performed out as a result of writing to the OUT_VALUE I/O tag (i.e. if the Topic does not change then it is sufficient to set the OUT_TOPIC point once - e.g. by using default value). | ||||||
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