Germany have been able to pay for bus tickets by passing their Nokia phones over a smartcard reader installed on the buses. Other applications for RFID enabled mobiles include swapping electronic business cards between phones, and using a mobile to check in at an airport or hotel. Two RFID enabled devices may also be used to enable peer-to-peer transfer of data such as music, images or for synchronizing address books. Graphical tags A graphical tag consists of an image on a marker, which can be read by a mobile telephone camera. There are a number of competing systems, including open standards like Quick Response QR Codes, Datamatrix, Semacodes (based on Datamatrix), and barcodes; or proprietary systems like ShotCodes. The design of such coding schemes needs to be rich enough to include much information and robust enough for the tag to be readable, even when partly obscured or damaged: tags might be on the outside of buildings and exposed to wear and the weather. Graphical tags have a number of advantages. They are easy to understand and cheap to produce. They can also be printed on almost anything, including t-shirts. Barcodes are a particularly attractive form of tagging because they are already very widely used, and camera phones can easily read them. SMS tags An SMS tag comprises a short alphanumerical code, which can be printed on a marker or chalked on a wall. The Short Message Service is then used to send the code and return a message. Yellow arrows are an example of this form of tagging. Virtual tags In a virtual tagging system there is no physical tag at a location. Instead a URL as a meta-object is associated with a set of geographical coordinates. Using location-based services a mobile phone that enters a particular area can be used to retrieve all URLs associated with that area. The area can be set as a few metres or a much wider area. Hardlink A hardlink is an alphanumeric combination such as an object's common name or part number that when entered into a cell phone's web browser, targeting a hardlink database, returns information that may have been stored about the target object. It is one of several methods of object hyperlinking including graphical tags (2D barcodes), SMS tags and RFID tags. The hardlink method establishes a reference link between a physical world object and a .mobi web page just