There are only certain frequencies in the radio spectrum where tags are allowed to operate and depending on the frequency, tags can have different operating characteristics. This slide shows part of the radio frequency spectrum, unfortunately most of this finite resource was allocated long before RFID. So RFID is now limited to three main bands; Low Frequency (LF) band between 120 and 134 kHz, High Frequency (HF) band at 13.56 MHz and Ultra High Frequency (UHF) band between 850 and 960 MHz. Associated with each band are International standards which will be reviewed later.