This caused some computers to get delayed responses from those servers, and an older version of Skype’s Windows software improperly processed the responses, crashing Skype for about 20 per cent of users. In a blog post, chief information officer Lars Rabbe said the 24-hour outage that cut service for nearly all of Skype’s users stemmed from a problem in a version of Skype’s software for computers running Microsoft’s Windows operating system. Rabbe said the issue, which began last Wednesday, started when a group of servers running offline instant messaging overloaded. A software glitch caused Skype’s major outage last week, the internet calling and messaging service said.

Users pay for services such as making calls from a PC to a landline or cell phone. Subscribers will be credited with a week’s extra subscription service. Other internet-based calling services that compete with the traditional phone system also have problems with consistent service. Skype’s software offers a range of free services, including the ability to make voice or video calls and send instant messages to other Skype users. Skype has indicated that it wants to list its shares on the Nasdaq Stock Market. The Luxembourg-based company said customers who pre-pay for service or are on pay-as-you-go plans will receive an e-mail with a voucher for 30 minutes of free calling to landlines anywhere in the world. AP Computers that crashed included numerous “supernodes” – computers Skype likens to phone directories, helping users connect with each other – which resulted in a much larger outage as other available supernodes couldn’t handle all the user traffic. Skype has since returned to operating normally. That was 10 per cent less than the usual traffic for the time of day, as some people still could not log on. Earlier this year, AT&T Inc.’s Internet-based “U-verse” phone system went down for several hours, affecting 1.15 million customers. On average, 124 million people use Skype each month, though the total number of registered users is more than four times that. A year ago, eBay sold its majority stake in the business for about $2 billion to an investor group that includes Skype’s founders. By Thursday afternoon, things had improved to the point where about 21 million users were logged in, said CEO Tony Bates. Skype’s popularity around the globe stems in large part from the free or cheap calls it provides. In a video posted on the Skype blog, Bates said the problems “completely took almost every user offline.”

The service went down for almost all of its users starting at midday Eastern time on Wednesday. At that point, voice calling, video-chatting and text-based instant messaging were working for most users, Bates said, but other features, such as offline instant messaging and group video calls, were still down.

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