# Dynamic RACH Partition for Massive Access of Differentiated M2M Services
## 2 Related Work  
Notably, the M2M communication system is a large-scale network with diverse applications and a massive number of interconnected machines. There are mainly two standards bodies pushing the standardization process of M2M communications: the Third Generation Partnership Project (3 GPP) and the European Telecommunications Standards Institute (ETSI), which specified their respective M2M communication architectures. ETSI defined the service-oriented M2M network architecture that comprises the device-and-gateway domain and the network domain, but without the underpinning of particular wireless technologies [10]. Furthermore, the ETSI-M2M does not indicate the specifications for M2M area networks and details for the access and core networks. The 3 GPP-M2M focuses on enhancing the cellular wireless networks. Consequently, the typical smartM2M and oneM2M architectures specified by ETSI are inclined to provide M2M services independent of the underlying networks. That is to say, the access overload issue is less serious in the ETSI-M2M architecture, although the management will be hard. In contrast, 3 GPP categorizes MTC devices as a special type of cellular users with a low rate and priority, and the MTC devices need to connect to the base station for data transmission. Therefore, the centralized control is ready. However, when the population of MTC devices gets large, the congestion problem could be extremely severe. Since the applications of M2M networks typically require security and privacy, the centralized architecture still attracts the major attention. Consequently, in this paper, our efforts will be dedicated to M2M under the 3 GPP architecture. It is worth mentioning that in the early stage of proposals for MTC, direct connections between MTC devices, as well as multi-hop transmissions across MTC devices were also suggested [11]. However, these types of connections also often come with quality-of-service (QoS) requirements, such as delay requirements [12; 13]. Then, these functions have been gradually carried out by the device-to-device (D2D) communications with cooperation and cognition capability [12; 13; 14; 15], where adjacent devices can connect directly by reusing the cellular users' spectrum in either an underlay [12; 16], an overlay [17] or a hybrid style [13]. Multi-hop communications over D2D networks has also been attracting research attention [16; 18]. Recently, comprehensive studies have been launched over the last few years to explore the RAN overload issue for M2M communications [19]. As suggested in [20], a huge volume of signaling and data flow will be yielded, easily causing severe congestions in random access network. Similar to wireless sensor networks (WSN) [21], massive M2M devices with burst data within a short period of time may also produce massive accesses [22], which result in radio access networks' (RAN) overload issue. As M2M continues to burgeon rapidly, it is worth researching RAN overload control towards future ubiquitous IoT. There are several research outcomes of solving the congestion problem [23]. Among all available solutions, ACB is recognized as an effective yet simple mechanism to regulate access in LTE/LTE-A networks [24; 25]. Sixteen classes are defined, and several of them are reserved for high-priority cases. However, the access schemes for coexisting services with different priorities have not been specified. Towards this issue, several schemes have been proposed to adapt system parameters to the varying M2M network statuses. The work in [6] discussed several methods for modified ACB approaches: extended access barring (EAB), dynamic access barring (DAB) and cooperative ACB. The EAB scheme deals with differentiated services, which are divided into clusters with respective ACB parameters. However, different clusters still share the same access resources. The DAB scheme focuses on the dynamic adjustment of controlling parameters for ACB, while not addressing the differentiated services.