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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 subscribe/publish type. It is simple, has little expense overhead and is easy to implement. It is used for M2M communication (Machine to Machine) and in the IoT context (Internet of Things).
D2000 KOM implements the client part of the protocol. The protocol je implemented on a TCP/IP linkline.
For LoRaWAN transfer of LoRaWAN data transfer encapsulated within the MQTT protocol, see LoRaWan protocol description.
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 client 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.
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- QoS_0 - PUBLISH message is not being confirmed, it may be lost
- QoS_1 - PUBLISH message is being confirmed by other side's PUBACK, it may be duplicate
- QoS_2 - PUBLISH message is being confirmed by other side's PUBREC and that which is then confirmed back by the PUBREL message and that one by a final by the PUBCOMP message.
The level of confirmation of the messages sent by the D2000 KOM process is defined by the parameter of protocol parameter Publish QoS protocol . D2000 KOM process considers the writing of the output tag to be successfully terminated finished depending on the QoS:
- QoS_0 - after successful data sending through the data is successfully sent via TCP connection
- QoS_1 - after receiving PUBACK
- QoS_2 - after receiving PUBCOMP
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The MQTT communication starts with the CONNECT message sent by client (D2000 KOM). Message contains (User Name), Password and other parameters, from which it is possible to set uponly Clean Session Flag and Client ID can be modified (parameter Will Flag is not used, as well as Will QoS and WillRetain, parameter Keep Alive is set to 0). Server replies with CONNACK message with a return code that contains information about the connection success informationof connect operation.
Then client sends SUBSCRIBE message with filter of topics (Topic Filter parameter), specifying which are of interest topics it is interest in, and with the required maximum level of confirmation (parameter Subscribe QoS).
The server responds with a return code that contains an information on about the success and maximum QoS that was assigned to the requested topics.
That is followed by a common Then follows a phase of communication, during which both the client and the server send PUBLISH messages (the client with any topic, the server with topics relating to the filter of topics of the received SUBSCRIBE message) and confirm them according to the value of the QoS parameter of the received PUBLISH messages.
If the server does not send a message for longer than Ping Interval seconds, the client sends the PING request message, to which the server must respond with the PING response message (within the time specified by the Reply Timeout parameter).
If parameters change on the line, the connection is closed and re-created.
The communication has been tested towards with the MQTT server www.TheThings.network.
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| Parameter | Description | Unit / size | Default value | ||||||
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| Enabling Activates detailed statements debug information about sending and receiving values. | YES/NO | NO | ||||||
| User name used in CONNECT message to connect to the MQTT server. | - | |||||||
| Password used in CONNECT message to connect to the MQTT server. | - | |||||||
| The name of one topic or a multiple topic filter sent within the SUBSCRIBE message. Using the filter the MQTT client specifies topics, within which it wants to receive messages. Note: topics are hierarchically sorted, a slash (/) is used as the separator, a plus (+) is used as a one-level mask, a hash (#) character is used a mask for multiple levels. Examples of filter: a/b , level1/+ , # , +/+/+/up | - | # | ||||||
| The desired maximum level of validation (QoS) sent within the SUBSCRIBE message. The MQTT server can then send PUBLISH messages with such or lower level of validation confirmation (but not higher). PUBLISH messages sent by the MQTT server will be validated confirmed by the D2000 KOM process according to the level of validation confirmation specified in them. The higher the level of validation confirmation, the more messages between the client and the server are exchanged (1 at QoS_0, 2 at QoS_1 and 4 at QoS_2). | QoS_0 QoS_1 QoS_2 | QoS_1 | ||||||
| Unique client identifier (Client Identifier) sent within the CONNECT message. The tested MQTT server (thethings.network) returned an error if the Client ID was blank and Clean Session Flag=NO. | - | |||||||
| Parameter Clean Session Flag of the CONNECT message. The No value means that the server uses the current session state (connection) - e. g. after collapse and recovery of the TCP connection. This means that all unverified unconfirmed PUBLISH messages are sent with QoS_1 and QoS_2 are resent (and optionally also QoS_0, depending on the implementation). The Yes value means that the session is re-created and any unverified unconfirmed PUBLISH messages are not repeated. | YES/NO | NO | ||||||
| Level of confirmation (QoS) used to send PUBLISH messages through the D2000 KOM process. Sending the PUBLISH message is the outcome of writing into the output tag with OUT_DATA address. The higher the confirmation level, the more messages between the client and server is sent are exchanged (1 for QoS_0, 2 for QoS_1 and 4 for QoS_2). | QoS_0 QoS_1 QoS_2 | QoS_0 | ||||||
| If the MQTT server did not send a any message during the specified time interval, the D2000 KOM process sends a PING request and waits for a PING response response (until time Reply Timeout). A value of 0 turns off sending the PING request messages. Parameter allows detection of TCP connection collapsefailure. | sec | 60 | ||||||
| If the MQTT server does not respond to the SUBSCRIBE, UNSUBSCRIBE, and PING requests within the required time or the D2000 KOM process fails to retrieve any read a complete message (and only part of it is retrievedread), the D2000 KOM process declares an error, closes the connection, and opens it again. Value 0 turns off the timeout. | sec | 20 | ||||||
| Timeout waiting at of a single reading of from TCP connection. D2000 KOM repeats reading of spontaneous data Max. Wait Retry times and if no data is loadedread, the timeout reading is declared and the reading is completed timeouted and finished (and may be followed by a further reading or writing). By lowering Wait Timeout and Max. Wait Retry parameters, it is possible to achieve a faster writing response of D2000 KOM process for writing at the expense of a higher CPU load when the MQTT server has no data. | sec | 0.100 | ||||||
| Number of repetitions of reading from TCP connection. See description of the Wait Timeout parameter. | - | 3 |
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