Singtel is conducting a thorough investigation into what caused an islandwide outage of its fibre broadband service for nearly 24 hours over the weekend.
The outage, which left residential customers and some businesses unable to access the Internet, started at 8.46am on Saturday.
Singtel said it managed to fully restore its services at 8.25am yesterday.
Mr Yuen Kuan Moon, chief executive officer of Singtel's Singapore consumer business, yesterday apologised for the disruption and said the telco is taking steps to review its processes to prevent a repeat incident.
When asked about what caused the outage, Mr Yuen told The Straits Times the telco's immediate priority was to fully restore services.
"We will now conduct a thorough investigation into what triggered the disruption - specifically why our servers could not send IP addresses to customers' modems to enable broadband connectivity," he said.
"Once there is more clarity, we will share our findings with the regulators and work with them to ensure measures are in place to prevent any such recurrence."
Singtel had earlier ruled out a distributed denial of service or DDoS attack, which works by getting thousands of infected computers to overwhelm a target site, which then cannot cope with the spike in traffic.
Mr Aloysius Cheang, the Asia-Pacific managing director of global computing security association Cloud Security Alliance, listed two likely reasons for the outage.
One, a problem with Singtel's domain name system (DNS) may have prevented users from accessing websites. DNS maps Web addresses to a machine-readable string of numbers to connect Internet users to websites.
Two, one of Singtel's network switches may have malfunctioned, he said.
"It's not uncommon for a telco to take a long time to isolate the cause of the problem," Mr Cheang said, adding that it is difficult to tell whether it was a human or hardware error.
Singtel said all affected customers will be given a 10 per cent discount on this month's broadband subscription.
Affected broadband customers who are also Singtel mobile service users will have their local mobile data charges waived for both yesterday and Saturday.
Financial consultant Debbie Lim, 34, a Singtel fibre broadband subscriber whose mobile phone line is under another telco, said: "What about people like me who are not Singtel mobile users? (I spent) all day at home and had to use the mobile data from another telco company to surf the Internet."
She added that she was upset with Singtel's customer service, as she had called its hotline on Saturday evening but was put on hold for more than an hour due to a high volume of calls. The line was later cut off.
Student Charles Tan, 21, who is both a Singtel broadband and mobile subscriber, is satisfied with the compensation. "(The outage) didn't really affect me that much because I wanted to use the Internet mainly for games and for surfing," he said. "I just had to find other sources of entertainment."