In addition, it should be further noted that, because a process of classifying terminal devices based on service features is negotiated by the terminal device and the core network device, and is transparent to the network device, when the network device does not support classifying terminal devices based on service features, both the terminal device and the network device may determine the to-be-monitored signal in a fallback mechanism. In other words, reference may be made to a method in a current technology, to perform a modulo-N operation on identifiers of terminal devices, divide the terminal devices into groups based on remainders obtained after the modulo-N operation, and then determine the wake up signal that needs to wake up the terminal device, namely, a to-wake-up signal, based on a relationship between a group and a wake up signal. Alternatively, when the network device supports classifying terminal devices based on service features, but the configured mapping relationship between category information and a wake up signal does not include the first category (for example, the first category of the terminal device is the medium probability, but the network device configures wake up signals only for two categories: the high probability and the low probability), the terminal device needs to determine, in a fallback mechanism, the to-be-monitored signal based on the first category and a category that is actually configured by the network device. In this case, the fallback mechanism may be falling back to determining the wake up signal by using only the identifier of the terminal device in the foregoing descriptions, or falling back in a fixed category direction, namely, in a configured sequence, by the terminal device and the network device by using an agreed-on method. For example, if the network device configures the high probability and the low probability, but first category information of the terminal device is the medium probability, the first category information falls back to the high probability or the low probability, and a wake up signal corresponding to the high probability or the low probability is determined as the wake up signal that needs to wake up the terminal device, namely, the to-wake-up signal.