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Here's some more information regarding on the outage (I would like to applaud Ayu on showing respect to the people affected by the outage...):

NTT DoCoMo reports wireless problems in Tokyo area

By Juro Osawa

TOKYO (MarketWatch) -- NTT DoCoMo Inc. (9437.TO) said Monday that problems with its mobile communications network are affecting 1.72 million cellphone users in the greater Kanto region, which includes Tokyo.

DoCoMo, Japan's biggest mobile carrier with a market share of about 50% by subscriber, said the problems started around 23:30 GMT. DoCoMo handset users in the affected regions are having difficulties making calls or sending messages, the company said.

DoCoMo said it hasn't been able to fix the problems yet, and is still trying to figure out what caused them.

As of the end of April, DoCoMo had a little more than 58 million subscribers in Japan.

The company said its communications network seemed to be congested, but because there is no apparent reason for the increase in traffic, it suspects there may be problems with the network infrastructure.

DoCoMo, like other major Japanese carriers, reported disruptions to its communication network immediately after the March 11 earthquake and tsunami. The company said most other cases of network problems in the past also involved natural disasters such as typhoons.

Source: MarketWatch