a zone-based fare system, so it requires passengers to tag on when boarding and tag off when alighting; in contrast, San Francisco’s Muni has a flat fare structure so it only requires that passengers tag on when boarding. Other uses Clipper cards are accepted by Bay Wheels, the Bay Area's bikeshare system, as well as some electronic bicycle lockers operated by BikeLink. For each of these systems, the Clipper card is used not for payment but only as a key; users must have a credit or debit card linked to their Bay Wheels or BikeLink account, and usage fees are charged to this linked payment card, not deducted from the Clipper card's stored value. These systems are not compatible with mobile wallets such as Google Pay or Apple Pay; only physical Clipper cards may be used. Beginning in 2013, a few parking garages in the Bay Area accepted Clipper for payment as part of a pilot program. Funds used for parking were kept separate from those used for transit. This program was discontinued effective September 1, 2017. Technology Clipper cards contain an NXP Semiconductors MIFARE DESFire (MF3ICD40) or MIFARE DESFire EV1 (MF3ICD41) integrated circuit inside the card. The card operates on the 13.56 MHz range, putting it into the Near-Field Communication category. Because the card uses NFC technology, any NFC-enabled device can read the serial number, travel history, and current balance on the card. However, data cannot be written to the card without the proper encryption key, preventing unauthorized access to funds on the card. The former TransLink cards, while still functional on the fare system readers, do not conform to MIFARE and are unreadable by 13.56 MHz readers. Because Clipper operates in multiple geographical areas with sporadic or non-existent internet access, the fare collection and verification technology needs to operate without any networking. To accomplish this, the Clipper card memory keeps track of balance on the card, fares paid, and trip history. This also means if funds are added to the Clipper account via the internet, funds will not show up on the Clipper card until it has been tagged at an internet-enabled (or recently synchronized) Clipper payment terminal. Buses and other vehicles without internet access will have to return to a service station in order to synchronize with Clipper's servers. During synchronization, the payment collection device will upload to the server data about any fares collected, and will