In the Tokyo subway, you will notice pretty quickly that several public access points Wi-Fi is available, thanks to the many signs like the above picture displayed around the corridors of stations.
Ideal to eg share a photo of the Tokyo subway with your friends on Facebook do you say? Nay! These Wi-Fi hotspots are actually reserved for those who already have an Internet connection at home or mobile phone subscription with data, ie to persons residing in Japan.
With these Wi-Fi networks ubiquitous in the subway stations of the city, not surprising that many users (as I do) have constantly staring at the screen of their smartphone.