at_command_refactoring.rb

def self.send_at_command(serial, command)
	3.times do
    result = serial.command("#{command}\r", 200)
    if result.to_s.match(/\OK/)
      break [true, ""]
    else
      [false, result]
    end
  end
end

def self.select_attach_network(imsi_id)
	cmd_at("/dev/mux1", 115200, 8, "N", 1, 20000) do |serial|
		result, reason = self.send_at_command(serial, "AT+CMEE=2") # Enable errors
		return reason unless result
    #...
	end
end