NTT Docomo limits nationwide outage to 5G SA subscribers

Daniel Okafor

NTT Docomo said on November 5 that a nationwide outage had made voice calls, data and SMS difficult for subscribers using 5G standalone (5G SA) since about 11:18 a.m. The operator's fourth update, issued at 3 p.m., said 4G users, other 5G users, mobile virtual network operators (MVNOs) and emergency calls remained unaffected.

What exactly failed?

Docomo's fourth update limited the disruption to 5G SA subscribers. Voice calls, data communications and short message service were difficult to use across Japan.

At 3 p.m., users of conventional 5G non-standalone (NSA) saw no impact. The same status showed no impact for 4G subscribers or mobile virtual network operators leasing Docomo's network. The same 3 p.m. status said Japan's 110 and 119 emergency calls remained available.

Docomo's updates disclosed no affected-subscriber total, traffic-loss figure or throughput measurement. The operator described the geographic reach as nationwide but offered no restoration forecast.

The affected service combines a dedicated 5G core network, known as 5GC, with 5G base stations, while conventional NSA reuses 4G equipment and SA completes communications using only the 5G network.

Docomo said selecting 4G may avoid the impact. Docomo said: "We sincerely apologize to our customers for the great inconvenience."

How did Docomo define the disruption?

Docomo's initial and second updates described the entire Docomo network and MVNOs as affected, but the third update at 2 p.m. narrowed the reported disruption to 5G SA subscribers, one hour before the fourth update.

The fourth update at 3 p.m. made no further change to that scope. It said restoration work continued, although the cause remained under investigation and no recovery time was available.

The first notice did not confirm the start time. Docomo's official sequence was:

• Shortly after 1 a.m.: Docomo Online Shop and My docomo were suspended during emergency maintenance after the operator cited suspected unauthorized access.
• Around 11:18 a.m.: The nationwide mobile-service disruption began.
• 1 p.m.: The second update disclosed the approximate onset time.
• 2 p.m.: The third update isolated 5G SA and advised a 4G workaround.
• 3 p.m.: The fourth update left the scope unchanged and said restoration continued.

The 2 p.m. revision makes the accuracy of Docomo's initial assessment a subject for future review. The November 5 sequence also calls into question the operator's information-disclosure and restoration systems.

What should affected customers do?

Docomo advised customers to select 4G in the phone's network settings. Because Docomo said a restart or airplane-mode toggle after selecting 4G may restore calls, data and SMS, the workaround may help without repairing 5G SA.

This remains a customer-side workaround, not a restoration of 5G SA. The guidance depends on 4G remaining unaffected, as Docomo's 3 p.m. status showed.

At 3 p.m., Docomo had provided no restoration timetable and had not identified the cause.

Why were the customer portals suspended?

Docomo's portal security response began shortly after 1 a.m., when it suspended the Online Shop and My docomo while investigating what it called a high possibility of unauthorized access from outside.

Docomo said it had "determined that there is a high possibility that unauthorized access was made from outside." A Docomo spokesperson said the operator was still confirming whether the mobile-service outage was related to the unauthorized access.

Both portals remained suspended at 3 p.m. No confirmed link between the two problems appeared in the updates by 3 p.m.

What are the enterprise implications?

Docomo began offering 5G SA to corporate customers on December 13, 2021, and the service is also used for corporate-dedicated devices and closed-network connections.

Those enterprise uses created a possibility of disruption to companies' business systems during the November 5 incident. The potential exposure extended beyond ordinary consumer handsets. The November 5 updates did not quantify potentially affected enterprise connections.

The 2 p.m. scope revision will shape scrutiny of information disclosure and restoration systems. Emergency calls remained unaffected, maintaining access to 110 and 119 during the incident.

The next test for Docomo is whether its next update supplies a restoration time and identifies a cause without another change in reported scope.