Apple Inc. is launching the iMessage on Wednesday that will allow users of devices running on the company's mobile operating system to chat SMS-style for free and without limit.

The iMessage, which was introduced in Apple's Worldwide Developers Conference in June, runs like BlackBerry's Messenger, where text messaging does not pass through wireless networks.

Carriers charge text messaging. Sending costs about 20 cents and the receiver is also charge the same amount. But since SMS sent through iMessage will pass through a wireless Internet data connection for iPhones, iPads and iPod Touches, the service will be free for users of the Apple devices.

While the iMessage would be very much welcomed by mobile users, wireless carriers see it as a threat to their business. Texting generate significant amount of income for U.S. wireless carriers like Verizon Wireless. Craig Moffett, an analyst at Sanford C. Bernstein, estimated the industry revenue to be $20 billion per year, according to The New York Times and CNET.com.

For the three biggest carriers in the U.S., the revenue comes from SMS plans for network subscribers. Verizon charges $5 for 250 SMS, $10 for 500 messages and $20 for unlimited texting. AT&T charges the same for unlimited texting or 10 cents per text. Sprint has unlimited texting bundled in its data service plan worth $10 per month.

Meanwhile, Microsoft smartphones will soon have a similar free, unlimited SMS function. It plans to incorporate features of Skype and GroupMe into its Windows Mobile-run devices when it completes the acquisition of the companies running the free Internet voice and video calling and group messaging services.