W-SIM (Willcom-SIM) is a SIM card developed by Willcom which, in addition to standard SIM functions, also has the core components of a cellular telephone (PHS), such as the radio receiver/transmitter, built inside. It is currently used in some terminals (listed below), which do not have radio modules.
The W-SIM core module is an extended version of a SIM card, containing not only user data but also all the transmission technology needed for a mobile device to work.
A terminal device developer can build a customized terminal without the design and development of a radio module, and users can change to a different radio module W-SIM without changing the terminal itself.
Similar to traditional handsets made by WILLCOM, W-SIM terminals can be used in Japan, including global roaming with Taiwan and Thailand. For Mainland China, China Netcom had W-SIMs available, though its PHS network is due to be phased out this year. In addition, a GSM version called the CM-G100 had been developed for use in other countries.
Read more about W-SIM: WILLCOM Branded Terminals, Terminals Branded By Other Than WILLCOM