Class C and Multicast

Class C and Multicast

Class C end devices continuously listen for downlink messages. This allows applications to send messages to devices at any time, instead of having to wait for a Class A uplink. When combined with multicast groups, this allows applications to send immediate downlinks to many devices at the same time.

This guide shows how to enable or disable Class C for an and device, and how to work with multicast groups.

Enabling and Disabling Class C

In order to send Class C downlink messages to a single device, enable Class C support for the end device when creating or updating it with the --supports-class-c flag.

For example, when enabling Class C for an existing device:

$ ttn-lw-cli end-devices update app1 dev1 --supports-class-c

This will enable the Class C downlink scheduling of the device. That's it! Downlink messages are now scheduled as soon as possible.

To disable Class C scheduling, reset with --supports-class-c=false.

Note: Class C downlink scheduling starts when the end device confirms the session. This means that the device should send an uplink message after receiving the join-accept in order to enable Class C downlink scheduling.

Class C message settings

The Things Stack supports optional settings for Class C downlink messages: the downlink path and the time to send the message.

The downlink path is defined by one or more gateways IDs. The Network Server and Gateway Server schedules only on the specified gateways in the specified order. This is useful for multicast (where no downlink path is known because there is no uplink). A scheduling attempt on can fail when the gateway is not connected, if there is a scheduling conflict or if duty-cycle regulations prohibit transmission. See the Example below.

The time to transmit is an absolute timestamp in ISO 8601 format to send the message. This requires gateways either with GPS lock, or gateways that use a protocol that provide round-trip times (RTT). See the Example below.

Multicast group

It is also possible to create a multicast group to send a Class C downlink message to a group of end devices. A multicast group is a virtual ABP device (i.e. shared session keys), does not support uplink, confirmed downlink nor MAC commands.

When creating a device, you can specify in the Console and CLI whether it's a multicast group.

Show CLI example ```bash $ ttn-lw-cli end-devices create app1 mc1 \ --frequency-plan-id EU_863_870 \ --lorawan-version 1.0.3 \ --lorawan-phy-version 1.0.3-a \ 00E4304D \ A0CAD5A30036DBE03096EB67CA975BAA \ --session.keys.nwk-s-key.key B7F3E161BC9D4388E6C788A0C547F255 \ --multicast \ --supports-class-c ```

Note: A multicast group cannot be converted to a normal unicast device or the other way around.

Note: Since multicast does not support uplink, the Network Server does not know a downlink path. Therefore, you need to specify a downlink path when scheduling downlink message.


Command-line interface only

The instructions below use the command-line interface (CLI).

First, create a multicast group:

$ ttn-lw-cli end-devices create app1 mc1 \
  --frequency-plan-id EU_863_870 \
  --lorawan-version 1.0.3 \
  --lorawan-phy-version 1.0.3-a \ 00E4304D \ A0CAD5A30036DBE03096EB67CA975BAA \
  --session.keys.nwk-s-key.key B7F3E161BC9D4388E6C788A0C547F255 \
  --multicast \

Then, schedule the following message to the Application Server MQTT server or HTTP webhooks:

  "downlinks": [{
    "frm_payload": "vu8=",
    "f_port": 42,
    "priority": "NORMAL",
    "class_b_c": {
      "gateways": [
          "gateway_ids": {
            "gateway_id": "gtw1"
          "gateway_ids": {
            "gateway_id": "gtw2"
      "absolute_time": "2019-07-23T13:05:00Z"