The most common digital cellular system in the world. GSM is used all over Europe, plus many countries in the Middle East, Asia, Africa, South America, Australia, and North America. GSM's air interface is based on narrowband TDMA technology, where available frequency bands are divided into time slots, with each user having access to one time slot at regular intervals. Narrow band TDMA allows eight simultaneous communications on a single radio multiplexor and is designed to support 16 half-rate channels. GSM also is the only technology that provides incoming and outgoing data services, such as email, fax, and internet surfing. GSM makes use of a SIM card that allows memory portability between dumb GSM phones.