code: func RequestPingSlotChannel(devEUI lorawan.EUI64, dr, freq int) storage.MACCommandBlock {
  	return storage.MACCommandBlock{
  		CID: lorawan.PingSlotChannelReq,
  		MACCommands: []lorawan.MACCommand{
  			{
  				CID: lorawan.PingSlotChannelReq,
  				Payload: &lorawan.PingSlotChannelReqPayload{
  					Frequency: uint32(freq),
  					DR:        uint8(dr),
  				},
  			},
  		},
  	}
  }
docs: // RequestPingSlotChannel modifies the frequency and / or the data-rate
  // on which the end-device expects the downlink pings (class-b).