## 2 Preliminaries  
### Related Works  
According to key problems in recent M2M communications, there are many studies being carried out to alleviate RAN overload and network congestion [11]. These studies include applying the slotted aloha scheme, the pull-based scheme, the MTC device back-off scheme, and the access class barring (ACB) scheme, among which ACB scheme is currently regarded as the major solution in M2M communications. The key of ACB scheme is to let the eNodeB broadcast a parameter to all MTC devices. When an MTC device tries to initiate a transmission, it generates a random number between 0 and 1, and compares the generated number with the ACB factor broadcast by eNodeB. If the number is less than the ACB factor, the MTC device proceeds to access the eNodeB. Otherwise, it needs to backoff temporarily.  
For existing research on ACB scheme, most of the studies emphasize the estimation of random access load and the dynamic adjustment of ACB parameters. Reference [16] utilized a PID controller to adaptively adjust the ACB factor. A Markov-Chain-based traffic-load estimation scheme according to the network collision status is developed by [17]. Reference [18] presented two dynamic access class barring (D-ACB) algorithms for fixed and dynamic preamble allocation schemes to determine the ACB factors, providing an effective method of reducing total service time.  
Except for ACB scheme, rational allocation of RACH resources is also an effective solution to tackle the RAN overload [19; 20; 21]. In [19], the authors proposed a dynamic RACH preamble allocation scheme based on the value of the ACB factor. A two stage resource allocation scheme is presented by [20]. By setting two layer ACB scheme, the proposed scheme can remarkably improve resource efficiency. Reference [21] studied the scene where M2M devices coexist with H2H devices, M2M User Equipments (UEs) form coalitions and perform relay transmission with an objective to reduce network congestion.  
As different M2M devices have various QoS requirements, a Multiple Access Class Barring (MACB) mechanism is presented by [22]. The main idea of MACB mechanism is to set distinguishing access priorities for different services. Reference [23] proposed an Extended Access Barring (EAB) mechanism to enhance the performance of ACB scheme. The basic idea of both MACB and EAB schemes is that the delay-tolerant devices are not permitted to access the network while the delay-sensitive ones are enable to request access attempts as long as MACB is activated in case of network congestion. However, an evident drawback of MACB is that it does not realize the partition of RACH resources. Moreover, it is worth noting that in real systems, the number of delay-sensitive devices is less than delay-tolerant ones. It ignores the communication quality of delay-tolerant devices.  
Thus, we need an intelligent solution for efficient resource management between coexisting delay-sensitive and delay-tolerant services to address the aforementioned problems. In this paper we propose an optimal scheme that combines an ACB scheme and RACH resource separation for two given clusters which are divided according to the delay requirement of different devices. The originality of our work is that our scheme adjusts the ACB factor and the number of preambles allocated to two clusters dynamically from the perspective of delay-sensitive services. Simulation results show that our proposed scheme shows good performance in meeting the delay requirements as well as increasing the utilization rate of the RACH resources allocated to them. High utilization rate ensures that more resources are left for delay-tolerant devices.