docs: def __sendCommand(self, cmd):
          """send specific command to reference unit over serial port
  
          Args:
              cmd: OpenThread_WpanCtl command string
  
          Returns:
              Fail: Failed to send the command to reference unit and parse it
              Value: successfully retrieve the desired value from reference unit
              Error: some errors occur, indicates by the followed specific error number
          """
          logging.info('%s: sendCommand[%s]', self.port, cmd)
          if self.logThreadStatus == self.logStatus['running']:
              self.logThreadStatus = self.logStatus['pauseReq']
              while self.logThreadStatus != self.logStatus['paused'] and self.logThreadStatus != self.logStatus['stop']:
                  pass
  
          ssh_stdin = None
          ssh_stdout = None
          ssh_stderr = None
          try:
              # command retransmit times
              retry_times = 3
              while retry_times > 0:
                  retry_times -= 1
                  try:
                      if self._is_net:
                          ssh_stdin, ssh_stdout, ssh_stderr = self.handle.exec_command(cmd)
                      else:
                          self._sendline(cmd)
                          self._expect(cmd)
                  except Exception as e:
                      logging.exception('%s: failed to send command[%s]: %s', self.port, cmd, str(e))
                      if retry_times == 0:
                          raise
                  else:
                      break
  
              line = None
              response = []
              retry_times = 20
              stdout_lines = []
              stderr_lines = []
              if self._is_net:
                  stdout_lines = ssh_stdout.readlines()
                  stderr_lines = ssh_stderr.readlines()
                  if stderr_lines:
                      for stderr_line in stderr_lines:
                          if re.search(r'Not\s+Found|failed\s+with\s+error', stderr_line.strip(), re.M | re.I):
                              print "Command failed:" + stderr_line
                              return 'Fail'
                          print "Got line: " + stderr_line
                          logging.info('%s: the read line is[%s]', self.port, stderr_line)
                          response.append(str(stderr_line.strip()))
                  elif stdout_lines:
                      for stdout_line in stdout_lines:
                          logging.info('%s: the read line is[%s]', self.port, stdout_line)
                          if re.search(r'Not\s+Found|failed\s+with\s+error', stdout_line.strip(), re.M | re.I):
                              print "Command failed"
                              return 'Fail'
                          print "Got line: " + stdout_line
                          logging.info('%s: send command[%s] done!', self.port, cmd)
                          response.append(str(stdout_line.strip()))
                  response.append(WPAN_CARRIER_PROMPT)
                  return response
              else:
                  while retry_times > 0:
                      line = self._readline()
                      print "read line: %s" % line
                      logging.info('%s: the read line is[%s]', self.port, line)
                      if line:
                          response.append(line)
                          if re.match(WPAN_CARRIER_PROMPT, line):
                              break
                          elif re.search(r'Not\s+Found|failed\s+with\s+error', line, re.M | re.I):
                              print "Command failed"
                              return 'Fail'
  
                      retry_times -= 1
                      time.sleep(0.1)
  
                  if retry_times == 0:
                      raise Exception('%s: failed to find end of response' % self.port)
                  logging.info('%s: send command[%s] done!', self.port, cmd)
                  return response
          except Exception, e:
              ModuleHelper.WriteIntoDebugLogger('sendCommand() Error: ' + str(e))
              raise