# A Cooperative MAC Protocol for a M2M Heterogeneous Area Network
## 5 M2M Heterogeneous Area Network  
In this section, we present a heterogeneous M2M access network architecture based on IEEE 802.11 and IEEE 802.15.4 standards. The proposed heterogeneous network can support a large geographical area with deterministic QoS for M2M applications. One of the main problems of such a heterogeneous architecture is the overlap of the transmission spectrum in the 2.4 GHz band; particularly when operating in a dense networking environment. Spectrum overlap is referred to as the coexistence problem. The coexistence problem can generally arise when two or more co-located dissimilar networks share the same transmission spectrum with bands which are either partially or fully overlapped [22]. To mitigate the coexistence problem, advanced channel management algorithms need to be developed. The IEEE 802.15.4 standard currently supports a number of operating spectrum bands; the main spectrum bands are the 2.4 GHz ISM (Industrial, Scientific, and Medical) band and the sub-GHz band between 700-900 MHz. The IEEE 802.11x currently uses the 2.4 GHz and 5 GHz bands for all current versions of the standard. The emerging IEEE 802.11 ah standard will operate in the 900 MHz band. Hence, these network devices could generate inter-domain collisions (between 802.15.4/802.11 devices) since both standards are using the CSMA/CA protocol for packet transmissions. This work proposes a heterogeneous network architecture using 802.11 b/g and 802.15.4 standards to operate in the 2.4 GHz band that avoids the coexistence problem using a cooperative MAC protocol. For the smart grid and IoT applications, a lower transmission frequency will be more useful due to lower path loss which, in turn, could reduce transmission power and prolong the battery life. Figure 5 shows the operating spectra of the IEEE 802.15.4 and the IEEE 802.11b/g standards in the 2.4 GHz band. It can be seen from the figure that three IEEE 802.11b/g channels can generate interference for 12 out of 16 channels of the IEEE 802.15.4 standard. Additionally, the IEEE 802.11x channels are wider compared to the IEEE 802.15.4 channels. This overlap of transmission channels introduces a serious coexistence problem [23; 24]. The IEEE 802.15.4 standard has the option of using a frequency hopping technique to avoid interference among network devices. Transceivers can select one of the sixteen transmission channels to avoid interference from co-located transmitters. However, such a solution may not be effective for a dense heterogeneous network environment where the number of co-located transceivers will be high, particularly considering the larger transmission range of 802.11 devices. Figure 5: Spectrum allocation of IEEE 802.15.4 and IEEE 802.11b networking standards. The proposed heterogeneous network architecture is shown in Figure 6. The network is made up of \(M\) sensor nodes divided into \(N\) clusters where \(P\) (\(P\) = \(M\)/\(N\)) is the number of nodes supported by each of the 6 LoWPAN routers acting as the head of each cluster. The 6 LoWPAN routers interface to a multi-frequency dual-radio router (MFDRR) gateway using 6 LoWPAN interfaces. The MFDRR accepts data from the 6 LoWPAN routers, aggregates data blocks, and transmits them to a data sink using an 802.11 link. This unique MFDRR architecture has been developed to avoid the coexistence problem. As the name suggests, the MFDRR can support multiple channels to accept data from different 802.15.4 clusters. In this network, different channels are used by neighboring clusters to avoid mutual cluster interferences. Hence, to accommodate dense networking environments, multiple 6 LoWPAN radio interfaces are used at the MFDRR tuned to different carrier frequencies. In Figure 6, red clusters use one carrier frequency while the green clusters use a non-overlapping different carrier frequency. As shown in the figure, the neighboring clusters will avoid interference due to use of alternating carrier frequencies.