How many types of UID exist in contactless RFID card systems? UID standard for Unique IDentifer, which is a common expression defined for RFID IC selection and activation. The standard defines single, double and triple size UIDs which consist of 4, 7 and 10 Bytes respectively.
Single Size UID (4 byte)
The Single Size UIDs (unique ones) ended since the number of usable IDs is limited to approximately 3.7 billion pieces only. Therefore the UID at this range is not unique in some cases, like NUID (FNUID and ONUID), RID. It is recommended to using Double Size UIDs (7-byte) to guarantee the uniqueness of RFID card.
A 4 byte NUID (Non-Unique IDentifier) is an identifier which may be assigned to more than one RFID chip over the production time of a product so that more then one RFID card with the same identified may be deployed into one particular RFID system. The probability to have 2 PICCs on one PCD at the same time with the same NUID is still extremely low. The MIFARE Plus® card or MIFARE Classic® card with Single Size NUID can be activated like a usual Single Size UID card. There are two types NUID:
Fixed but non-unique ID (FNUID): The 4-byte UIDs with UID0 = xFh are fixed identifiers just like unique ones. However the same UID might be used for several PICCs, so that RFID card systems cannot rely on the uniqueness of such a PICC identifier.
Re-used UID (ONUID): The very old Single Size UIDs will be re-used, which means the same UID might be used for several PICCs. Therefore that RFID card systems cannot rely on the uniqueness of such a PICC identifier.
The Random ID (RID, 4 byte)
A single size UID with UID0 = 0x08 indicates a Random Identifier. The Random ID (RID) is dynamically generated, when the PICC powers up. Deselecting a PICC does not reset the RID, but a field reset does.
Double Size UIDs always contain a manufacturer code in the UID0. With the double size UIDs each manufacturer can theoretically use up to 2.8 * 1014 UIDs.
Triple size UID (10 byte)
Triple Size UIDs always contain a manufacturer code in the UID0. Currently there is no PICC using a triple size UID. However it is mandatory that every PCD supports Triple size UIDs according to ISO/IEC 14443.