## 2 Related Works  
In the current research on channel allocation strategies in CRNs, the internal classification of SUs is often considered. In [18], a channel allocation strategy was proposed to classify SUs according to real-time requirements, in which the priority of SU1 packets was higher than SU2 packets, and the performance index analysis and system behavior optimization were carried out for SU2. In [19], SUs were divided into two levels according to delay sensitivity, and two different active spectrum switching strategies were derived for two types of SUs. In [20], SUs were divided into real-time traffic SU1s with high priority and non-real-time traffic SU2s with low priority to offer better performance for SU2s without influencing SU1s. From [18; 19; 20], we can find that it is widespread to classify SUs, but in the literature, scholars rarely classify PUs. When a CRN system is faulty, the transmission success rate of packets generated by users is significantly reduced. In this case, PUs with higher real-time requirements should be given priority. Therefore, it is necessary to consider the classification of PUs. In conventional CRNs, channel preemption usually occurs between PUs and SUs. Similarly, the preemption principle between different levels of PUs should be considered after the hierarchical processing of PUs. There are two standard methods to deal with it: complete preemption and non-preemption [21; 22]. When considering the internal priority of the SUs, complete preemption means that the channel of the SU possessing a lower priority can be preempted by the SU possessing a higher priority, and the principle of non-preemption means that the data transmission of the SU possessing a lower priority cannot be interrupted by the SU possessing a higher priority [23]. In [24], the authors found that complete preemption of PUs to SUs could reduce the waiting time of PUs, and full non-preemption of PUs to SUs could reduce the waiting time of SUs. In [25], to diminish the delay time of SUs, the authors put forward a hybrid preemptive/non-preemptive resume priority model. From [24; 25], we conclude that a mix of complete preemption and non-preemption between users should be considered, depending on the different scenarios. Therefore, we can set different preemption rules to apply to PUs of different levels. When the system state is normal, PUs of different levels are equal and do not preempt each other's channel. When the system is affected by the failure, to ensure the data transmission of PUs with high real-time requirements, PUs who are with high real-time requirements can interrupt the data transmission of PUs who are with low real-time requirements. Moreover, we note that audio, video, or real-time services have high real-time requirements, so we can think of them as the packets with the highest priority. Due to the high priority, their real-time service transmission needs can be guaranteed. A CRN system can break down for various reasons, and many researchers have studied the problem according to the wholly stopped service area of the server. On account of the complicacy of modern operating systems, system failure may not terminate service entirely to users [26]. In [27], a partially failed and interruptible setup/shutdown strategy was proposed, and the system maintained a low service in the case of failure. In [28], the authors studied the equilibrium strategies in the almost observable and almost unobservable M/M/1 queues with partial failure, and the service rate in partial failure states was lower than the standard service rate. In [29], the equilibrium behavior of customers in Markov queues with setup times and partial failure was considered. When the system was faulty, services were provided at a low speed, and no new users were received. In [30], the authors assumed that the PU packets' transmission would be maintained at a low speed under the condition of system failure and would be resumed at a high speed after the failure was repaired.