The demand for the iPhone 4S has been strong, ever since Apple Inc. started shipping the latest iteration of its iPhone series. In fact, market experts are expecting Apple to ship over 70 million units of the iPhone in 2011.

One thing seems universal from our extensive checks and conversations with colleagues - the iPhone demand still seems strong. Our checks still show regular daily stock-outs of the 4S (especially for AT&T models) at Apple stores nationwide, Barclays Capital Markets analyst Ben Reitzes wrote, in a note to clients.

Internationally, reports suggest there is strong demand, especially in China - Apple's largest and most important growth market. Meanwhile, the analyst said Apple may launch iPhone 5 in the September 2012 quarter.

We wouldn't be surprised to see iPhone 5 launched in the September 2012 quarter featuring a new design and 4G LTE capabilities, Reitzes said.

This upgrade cycle should be even bigger than that of the 4S, given even more speed and features linked to iCloud.

In addition, the analyst said the iPhone 5 could also incorporate Near-Field Communication (NFC) capabilities - something he believes fits well with the credit card customers already accessed by Apple through iTunes. NFC allows for simplified transactions, data exchange and connections with a touch. A smartphone or tablet with an NFC chip could make a credit card payment or even serve as a keycard or an ID card. Globally, 100 million people use mobile payment opportunities outside the U.S. However, only 3.5 million use the technology in the U.S.

The iPhone, which revolutionized the smartphone market, is reportedly the bread and butter among Apple's range of products. The first iPhone was unveiled by Steve Jobs, then CEO of Apple, on Jan. 9, 2007, and released on June 29, 2007. The 5th generation iPhone, the iPhone 4S, was announced on Oct. 4, 2011, and released on Oct. 14, 2011, two days after the release of iOS 5.0, Apple's Operating System for handheld devices.

For the recently concluded fourth quarter, Apple said iPhone sales surged 21 percent to 17.07 million units. For the first quarter, Reitzes expects Apple to sell 30.78 million iPhone units and expects to sell 122 million units in fiscal 2012.