# A Cooperative MAC Protocol for a M2M Heterogeneous Area Network
## 4 M2M Area Network Design  
This section reviews the design requirements of M2M area networks using short-range wireless networking standards. M2M area networks will directly serve end devices which are spread over a geographical area. In order to collect data from distributed devices, it is necessary to provide seamless connectivity from devices to data servers. Direct connectivity from data devices to data servers will not be feasible due to the high density of nodes and transmission power requirements. It will be useful to relay data from devices via local gateways to the server using multi-hop links. Since end devices will generate infrequent data bursts, appropriate network topologies and MAC protocols need to be selected to offer seamless connectivity with maximum transmission efficiency at a minimum cost.  
A low power wireless sensor network architecture is considered to be the most suitable for M2M area networks due to its lower cost and energy requirements. Wireless sensor networks generally use one of several network topologies for data transmission. The main network topologies used are: star, peer-to-peer, tree, and mesh [17]. The star topology is the most commonly used topology for small-size sensor networks, whereas the tree and mesh network architectures extend the communication range. The star topology uses a coordinator to exchange data among connected nodes; in wireless sensor networks the star topology is also referred to as the cluster topology where the coordinator becomes the cluster head. The transmission range of the cluster topology can be extended by using the cluster tree topology. In the cluster tree topology, multiple clusters communicate with each other either via cluster heads or routers as shown in Figure 2. The cluster tree topology could be a suitable architecture for M2M area networks as the routers or gateways could be placed close to end devices reducing transmission power consumption and avoiding radio channel impairments.  
Figure 1: Functional architecture of a M2M network based on the ETSI specification.  
The medium access control protocol is another critical design issue for M2M area networks [18]. An M2M area network could be considered as a local and/or personal area network where one of three different classes of MAC protocols is used. These protocols are: scheduled access, random access, and polling [17]. Scheduled access and polling protocols generally require more control signaling support compared to random access protocols and are generally not suitable for low-power area networks. High levels of signaling increases power consumption and computational complexities. On the other hand, random access protocols generally have lower signaling requirements at the expense of QoS degradation, particularly at high traffic load. Random access networks can maintain reasonable QoS as long as traffic load is low to moderate. Hence, it is necessary to develop advanced resource allocation techniques to maintain QoS in M2M networks.  
In the last two decades, a number of short range wireless networking standards have emerged to support communications in local and personal area networking environments. Out of the many standards, several wireless networking standards have further progressed and become the potential candidates for M2M communication networks. In this section, we review the IEEE 802.15.4/6 LoWPAN and IEEE 802.11 standards for M2M network infrastructure design in the device and gateway domain. There are other short range wireless standards, such as Bluetooth/IEEE 802.15.1, Wireless HART, LoRa, ECMA 368, and ISA 100.11a which could have roles in M2M network design, but considering the paper size we will limit our discussions to two main IEEE short-range wireless standards.