The present invention provides a single circuit and a method which can identify the WLAN standard of a packet or the modulation type of a packet. The single circuit includes a shift register, N determinators and decision logic. The shift register is configured to receive several periods of the preamble of the packet. Each of the N determinators is configured to determine whether the WLAN standard, or the modulation type, of the packet matches one of the N WLAN standards, or the N modulation types. Decision logic is configured to decide the WLAN standard, or the modulation type, of the packet.