Hello Seong soon JANG;
It sounds as if your AV1515 is in need of troubleshooting and if necessary RMA for repair. I researched your serail number and found it to be under warranty until 12/01/2017. I would browse to the website link that I have listed below to start a new case so Avocent Technical Support can take care of this for you.
community.emerson.com/.../reqsupt
Best regards;
Danny