The Hokkaido-Sakhalin Cable System (HSCS) is a 500km undersea cable system between Ishikari, Hokkaido in Japan and Nevelsk, Sakhalin in Russia, jointly built by TTK and NTT, ready for service on July 3, 2008

The HSCS has a inital design capacity of 640 Gbps.

In 2018, the HSCS was upgraded with 200 Gbps DWDM technology for a total system capacity up to 5.4 Tbps, and activated the first 100 Gbps channel between Tokyo and Amsterdam. 

With the seamless combination of the HSCS and TTK's terrestrial backbone across Russia to Europe, it forms a low latency route, Europe-Russia-Japan (ERJ), with latency (RTD) of 144ms between Tokyo and Amsterdan.