According to Official Guinness Records,
The highest capacity fibre optic cable providing a commercial service has a capacity of 8 Tbit/s, and is operated by Türk Telekomünikasyon A.Ş between the cities of Istanbul and Ankara. The capacity was verified on 11 June 2013.
For a complete list of 2013 records, please visit 2013 Guinness Records in Turkey.