Polling-type transmission systems are used in many situations when some kind of identification is desired. Escort memory systems in material handling and production, container and vehicle identification, identification of objects when sorting etc may be mentioned as examples. Another example is personal identification, for example, with the aim to give admittance to a department, having restrictions on the right to enter, only to persons with authority, and at the same time to obtain a registration of everyone entering.
Some of today's transmission systems operate in such a way that the interrogator activates the transponder, and the transponder then starts transmitting information to the interrogator. If there are more than one transponder within range for reading and writing, respectively, problems arise. One possible situation is that the signal from one of the transponders dominates over the other, and the interrogator may interpret the dominating signal only. Information signal from the rest of the transponders fail to be received. Another possible course is that the transponders within range of the interrogator disturb each other, so that no reading occurs. As a result none of the information from the transponders within range is received.
One solution to the problems described above is to give the transponders individual addresses, and to selectively address the transponders. The number of addresses must be limited so that the successive addressing of the transponders do not last too long. In systems with many transponders it may be necessary to assign the same address to more than one transponder. There is a risk that more than one transponder having the same address will be present within range of the interrogator, and similarly to the situation described above with more than one transponder within range of the interrogator, information from some transponders fails to be received.