@gemlog Well.... I think you want to Sousa (匝瑳) city in Chiba, not Souka (草加) city (this is Saitama pref., near Chiba.)
I remembered that Chiba pref. has gateways to submarine cable. They are in Minamiboso city, Chikura and Maruyama town.
I don't know what cable is used to send our packets, but I am excited that we are connected each other with long long cable!
http://www.submarinecablemap.com/#/landing-point/chikura-japan