# A Cooperative MAC Protocol for a M2M Heterogeneous Area Network
## 4 M2M Area Network Design
### IEEE 802.11 MAC Standard  
The IEEE 802.11 wireless local area network (WLAN) standard has been evolving over the last two decades. The 802.11 standard has been developed mainly to act as an access network using unlicensed spectrum in the 2.4 GHz and 5 GHz bands. The standard has evolved from a low data rate network to a gigabit network, supporting broadband connectivity. Table 2 lists some of the key 802.11 standards with selected features. The table shows that revised versions of the standard have resulted in increased data rates over time. Very recently, the 802.11 ah standard has been developed to support M2M and IoT communications using narrow band channels in a spectrum band below 1 GHz [19]. This standard can support a large number of devices in outdoor environments,  
Figure 2: Cluster tree network architecture. Network operation showing data exchange between a server and end devices. supporting sensor networking requirements. The IEEE 802.11p is another variant of the standard that has been developed for vehicular networks to support vehicular M2M communication systems. For M2M communications, the 802.11 standard and in particular the 802.11 ah variant, could play a key role, offering flexibility in designing and implementing M2M area networks. The standard operates based on the carrier sense multiple access with collision avoidance (CSMA/CA) protocol. The standard does not limit the number of stations that can associate in a network and, hence, is suitable for M2M communications. However, the actual number of stations in a network will be limited by the QoS requirements, since it is a contention/random access-based network. For M2M applications, it is necessary to use appropriate techniques to minimize the effect of contention in dense networking environments. Most of the 802.11 devices use physical carrier sensing techniques; however, virtual carrier sensing techniques could be more energy efficient. The IEEE 802.11 standard specifies two different virtual carrier sensing mechanisms. The original virtual carrier sensing technique uses a network allocation vector (NAV) whereby network devices can overhear current transmissions to schedule their next channel sensing time to avoid collisions. A new virtual carrier sensing technique known as response indication deferral (RID) has been proposed for the IEEE 802.11 ah standard. The difference between NAV and RID is that, in the case of RID, a collision-sensing flag is set immediately after the PHY layer header reception instead of after a full packet reception. In the case of RID, devices can resolve the collision status quickly to schedule transmission and sleeping periods in an efficient manner to reduce energy consumption, as well as to minimize contention levels in a network. It is worthwhile to mention that virtual carrier sensing is more energy efficient than physical carrier sensing techniques because, in the case of the former, a device does not need to sense the channel multiple times. As mentioned earlier, one of the key concerns of the CSMA/CA protocol is the lack of QoS support. The IEEE 802.11e standard introduced the enhanced distributed channel access (EDCA) technique to offer priorities to traffic classes as well as to offer QoS guarantees. However, the EDCA technique is not suitable for transmitting infrequent short bursts of traffic as seen in M2M and IoT applications. The emerging 802.11 ah standard tries to address the contention problem in a dense networking environment by introducing a new technique known as restricted access window (RAW) that attempts to restrict contention levels by reducing channel access attempts over a longer period. The RAW technique divides network devices into groups and splits the transmission frame into slots. Contention levels within each time slot can be contained by restricting the number of terminals. The RAW technique can be seen as a combined TDM (Time Division Multiplex)/CSMA/CA channel structure. The technique needs to be further