On January 7, 2003, Apple Computer introduced AirPort Extreme, based on the 802.11g specification. AirPort Extreme allows data transfer of up to 54 Mbit/s, and is fully backwards-compatible with the thousands of existing 802.11b (AirPort) base stations in coffee shops, retail stores, offices and homes. All of Apple's current computer models, with the exception of the XServe, have a slot to insert an AirPort Extreme card, and all models of PowerBook and iBook now ship with a card as standard. The Extreme cards, however, do not work in older Macs -- the Airport bus cannot support the new faster transfer rate. However, an Airport Extreme base station can communicate both with newer 802.11g-based devices and the older 802.11b AirPort cards.