# A Cooperative MAC Protocol for a M2M Heterogeneous Area Network
## 4 M2M Area Network Design
### IEEE 802.15.4 Standard  
In recent times, the IEEE 802.15.4 network standard has received significant attention as a low power wireless sensor network and is the basis of Zigbee [20]. The standard defines the physical and medium access control (MAC) layers for low-rate wireless personal area networks (LR-WPAN). The main advantages of the standard are reliable data transfer over short transmission distance, low energy requirements, as well as lower hardware and installation costs. The physical layer supports three different frequency bands: 2.45 GHz with 16 channels, 915 MHz with 10 channels, and 868 MHz with one channel. The data rate supported by the standard varies between 20 kbits/s to 250 kbits/s. A new extension of the standard known as 802.15.4g supports a data rate of 1 Mbits/s. The new physical layer also supports advanced functions such as channel selection, link quality estimation, channel energy detection and clear channel assessment (CCA). In the following paragraphs, the 802.15.4 MAC layer design will be discussed. The MAC layer of the standard defines two types of node; reduced functionality device (RFD) and Full Functional Device (FFD). An FFD is enabled with a full set of MAC layer functionalities which allow this type of device to act as a network coordinator whereas an RFD device can only act as an end device, such as a sensor/actuator node. The standard can support both star/cluster and peer-to-peer topologies. The standard supports both beacon-based and non-beacon-based packet transmission techniques. This work only concentrates on the beacon-based packet transmission techniques. Figure 3 shows the superframe structure of the IEEE 802.15.4 standard. The frame starts with a beacon indicating the start of a transmission cycle. The beacons are separated by the beacon interval duration. The transmission interval is divided into two portions represented by active and inactive periods. The active period can be further divided into 16 time slots, which can be used by end devices to access the channel to transmit packets. The active period can be further categorized  
Figure 3: IEEE 802.15.4 superframe structure. into the contention access period (CAP) and the contention free period (CFP). In the CAP period, end devices can transmit packets using the CSMA/CA mechanism. In the CFP part of the frame, fixed time slots are used which are referred as guaranteed time slots (GTSs). The GTSs are allocated in advance where devices can transmit their packets in a contention free mode. The CAP and CFP durations are adjustable depending on the application needs. The active period is followed by an inactive period when all end devices can remain in the sleep mode to conserve energy. The super frame structure is controlled by two main parameters. Beacon Order (\(BO\)) controls the duration between two beacon frames known as the Beacon Interval (\(BI\)). Another parameter known as the Superframe Order (\(SO\)) regulates the length of the CAP period. The beacon interval (\(BI\)), superframe duration (\(SD\)) and duty cycle (DC) are represented by Equations (1)-(3). \[BI=aBaseSuperframeDuration\times 2^{BO}\ \left(symbols\right) \tag{1}\]  
\[SD=aBaseSuperframeDuration\times 2^{SO}\ \left(symbols\right) \tag{2}\]  
\[DC=\
icefrac{{SD}}{{BI}} \tag{3}\]  
For the above equations, the following relation must hold \(0\leq\text{SO}\leq\text{BO}\leq 14\). The IEEE 802.15.4 standard regulates the minimum superframe duration, which is equal to 960 symbols (15.36 ms). One time slot occupies 960/16 = 60 symbols (0.96 ms). Beacons contain control management information, such as the start and end of a superframe, address information, and the number of time slots allocated to the GTS service. If the CFP is disabled, then the entire active part of a super frame becomes the CAP duration. The CFP, in contrast, can allocate up to seven GTS slots that result in the minimum CAP length of 440 symbols, which is equal to eight time slots. This ensures sufficient time in which packets can be transmitted.