public function __toString()
 {
     $output = __CLASS__ . " (length: " . strlen($this->getFrame()) . ")" . PHP_EOL;
     $output .= "|- Status    : " . $this->displayStatus() . PHP_EOL;
     $output .= "|- NwkAddr     : " . $this->displayNwkAddrOfInterest() . PHP_EOL;
     if ($this->getStatus() == Status::SUCCESS) {
         $output .= "|- Active EP: " . implode(", ", array_map(function ($ep) {
             return Buffer::displayInt8u($ep);
         }, $this->getActiveEpList()));
     }
     return $output;
 }