Multicast Groups

It is also possible to create a Class B or Class C multicast group to send downlink messages to a group of end devices. A multicast group is a virtual ABP device, where multiple physical devices share the same DevAddr and session keys. It does not support uplink, confirmed downlink nor MAC commands.

We define some user parameters that will be used below:

APP_ID="app1"
DEVICE_ID="dev1"
FREQUENCY_PLAN="EU_863_870"
LORAWAN_VERSION="1.0.3"
LORAWAN_PHY_VERSION="1.0.3-a"
DEV_ADDR="00E4304D"
APP_SESSION_KEY="A0CAD5A30036DBE03096EB67CA975BAA"
NWK_SESSION_KEY="B7F3E161BC9D4388E6C788A0C547F255"

Make sure to modify these according to your setup.

Class B and Multicast

Since there are no uplinks in multicast groups, there is no MAC layer communication between the end device and The Things Stack. Therefore, it is necessary to specify the ping slot periodicity by setting the following parameter:

mac-settings.ping-slot-periodicity.value

CLI example:

ttn-lw-cli end-devices create $APP_ID $DEVICE_ID \
 --frequency-plan-id $FREQUENCY_PLAN \
 --lorawan-version $LORAWAN_VERSION \
 --lorawan-phy-version $LORAWAN_PHY_VERSION \
 --session.dev-addr $DEV_ADDR \
 --session.keys.app-s-key.key $APP_SESSION_KEY \
 --session.keys.nwk-s-key.key $NWK_SESSION_KEY \
 --multicast \
 --supports-class-b \
 --mac-settings.ping-slot-periodicity PING_EVERY_4S

See the MAC Settings guide for more information about configuring MAC layer parameters.

Creating a Multicast Group

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

CLI example:

ttn-lw-cli end-devices create $APP_ID $DEVICE_ID \
  --frequency-plan-id $FREQUENCY_PLAN \
  --lorawan-version $LORAWAN_VERSION \
  --lorawan-phy-version $LORAWAN_PHY_VERSION \
  --session.dev-addr $DEV_ADDR \
  --session.keys.app-s-key.key $APP_SESSION_KEY \
  --session.keys.nwk-s-key.key $NWK_SESSION_KEY \
  --multicast \
  --supports-class-c # or --supports-class-b

Please note that 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 a downlink message.

Example

Command-line interface only

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

Learn how to install the CLI

First, create a multicast group:

ttn-lw-cli end-devices create $APP_ID $DEVICE_ID \
  --frequency-plan-id $FREQUENCY_PLAN \
  --lorawan-version $LORAWAN_VERSION \
  --lorawan-phy-version $LORAWAN_PHY_VERSION \
  --session.dev-addr $DEV_ADDR \
  --session.keys.app-s-key.key $APP_SESSION_KEY \
  --session.keys.nwk-s-key.key $NWK_SESSION_KEY \
  --multicast \
  --supports-class-c

Then, schedule the following message to the Application Server MQTT server or HTTP webhooks. A downlink path must be specified because none is known, as there is no uplink in multicast:

{
  "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"
      }
    }
  ]
}

How to update MAC Settings of a Multicast device?

Changing MAC settings of a Multicast device (virtual ABP device) requires performing the below steps:

Change MAC settings.
Reset the Multicast device at the Network Server end as explained here to apply above mentioned changes.

← Class C Settings Downlink Queue Operations →