AOS R13.9.0 has been released for the NetVanta 6000 Series as a support-controlled release. If you have a need to upgrade to or install AOS R13.9.0, open a ticket with our Customer Care Center (or using this link) and a product support engineer can address this request to acquire controlled release software.
Unless you require a feature included in the AOS R13.9 firmware release, ADTRAN recommends using AOS firmware version R13.7.1 for the most stable experience currently available.