subscribers use SMS, 15% use MMS, and 11% access to the mobile Internet [4]. In 2006, the total SMS traffic in the United States market rose to more than 158 billion mes- sages and the volume of MMS reached 2.7 billion messages [4]. The average monthly SMS and MMS volume per subscriber in 2006 were, respectively, 58 and 0.99. In China, SMS is SO popular that 92% of the subscribers are active SMS users covering all age groups [8]. At the end of 2006, about 15.7% were MMS users, and this percentage reached 28% in 2007. There are about 60 million active WAP users accounting for 11.0% of the mobile subscriber [8,9]. In 2006, 430 billion SMS messages were sent, and this number rose to 529 billion in 2007 [10]. The volume of MMS rose from 5.5 billion in 2006 to 14.5 billion in 2007 [5,6]. The average monthly SMS and MMS volume per subscriber in 2006 were 84 and 1.07. And these numbers reached 98 and 2.40 in 2007. In South Korea, it was estimated that in 2007 about 70% of South Korea mobile subscribers use SMS and 30% use MMS. In 2007, approximately 23 million people accounting for 54% of total cell phone users experienced wireless Internet through mobile phone [11]. In 2007, the total SMS traffic in South Korea market came to more than 91 billion messages and the average monthly SMS volume per subscriber in 2007 was 190. Comparative Study on the Mobile Data Service Development 13.3.3 Profitability The profit margins of all kinds of mobile data services have not been measured accurately. But it is commonly believed that mobile data services, especially the services that do not need broad bandwidth, such as SMS and ringtones download- ing, have much higher margin than voice services, as voice services becoming commoditized. They are powerful weapons to maintain and improve the average revenue per user (ARPU) of mobile services. The contribution rate of data services in the U.S. mobile service revenue is rela- tively low at 13.5% in 2006 (see Figure 13.2). But the increasing data service ARPU compensated the declining voi