The grandfather of the IRREAL system is, of course, the ParcTAB system implemented in the early 90ies at Xerox PARC [7]. The ParcTABs were small hand held devices that kept up a network connection through infrared transmission and were explicitly tracked by the infrared cell with which they were communicating at any given time. They were providing services such as note taking, email and file browsing and had an elaborate infrared network infrastructure that basically hasn't been surpassed until today.
When hand held computers eventually became commercially available, systems such as the MOBIS [4] and HIPPIE [5] museum guides were developed. These systems use the idea of infrared markers placed in important locations. The self-contained markers broadcast an ID by which the mobile device can retrieve information that was authored for this ID and thus effectively for this area of space. Retrieval is done either from a database stored on the device (MOBIS) or via a radio network connection (HIPPIE). In the first case all information has to be installed on the device itself and therefore the size of the information space is limited by memory size. Also, information cannot be changed or updated once the user is on her way. In the second case an additional radio network has to be installed, effectively extending the amount of available information, but rendering the mobile devices heavier and more expensive and causing them to draw more power.
The main idea behind the IRREAL system is to use infrared senders to broadcast all information, so neither a large database nor a radio network card have to be installed on the mobile devices. These on the other hand use their builtin infrared ports as receivers without sending back information. While this working principle was caused mainly by technical necessities, it provides several advantages as shown in the next sections.