Several studies have forecasted an annual growth rate over 20% in the number of M2M (Machine-to-Machine) connections globally, with more than 10 billion mobile-connected devices, exceeding the world’s population, in 2017.
The communication technologies currently used for M2M applications can be classified in two categories: Wireless Sensor Networks (WSNs), for interconnecting multiple sensor nodes spread over a particular area; and regular mobile (cellular) networks, for isolated/scattered nodes or to allow the gateway of a particular WSN to reach the Internet.
With respect to WSNs, different systems (Zigbee, 802.15.4, 6LoWPAN, Bluetooth or even proprietary radio solutions) have been considered for transmitting data in common M2M scenarios. However, none of those systems has prevailed because of the current diversity and complexity of the applications and en- vironments. As for mobile networks, M2M communications are currently mainly supported by GPRS/EDGE networks because of the growing but still reduced number of devices and light traffic requirements.
Simultaneously, the 3rd Generation Partnership Project (3GPP) is working towards supporting M2M applications on 4G broadband mobile networks, such as UMTS and LTE, with the goal of natively embedding M2M communications in the upcoming 5G systems.
The IEEE 802.11ah Task Group (TGah), created in 2010, addresses the need for an M2M wireless standard to cover the existing gap between traditional mobile networks and the growing demand for wireless sensor networks. TGah deals with the specification of an unlicensed sub-1GHz worldwide wireless local area network (WLAN) standard for future M2M communications supporting a wide set of scenarios based on a large number of devices, a long range and energy constraints.
This paper introduces the IEEE 802.11ah amendment, which allows WLANs to manage hundreds or even thousands of low-capability M2M devices with sporadic traffic needs. Although IEEE 802.11 has become the dominant standard for WLANs and one of the most commonly deployed technologies, we expect the new release of IEEE 802.11ah to be vastly adopted as a legacy solution to deploy WSNs in most markets and applications.
IEEE 802.11ah offers a simple, robust and efficient solution in the ISM band compared with existing WSNs. Moreover, the IEEE 802.11ah specification points out a comparable high quality of service to the one provisioned by current mobile networks, building a completely scalable and cost-effective operation.
The goal of this paper, apart from introducing the main features of the IEEE 802.11ah amendment, is to analyze its feasibility and evaluate its performance. The limitations that currently do not allow Wi-Fi to play an important role in M2M communications are solved with the adoption of the new IEEE 802.11ah amendment.
Its new energy-saving mechanisms ensure an efficient use of the limited energy resources available in sensor nodes. Moreover, its operation in a 1GHz band achieves larger coverage areas than the original IEEE 802.11. The number of simultaneously operable stations has also been increased, up to 8,191, where all the devices can be managed by a single AP using a new hierarchical organization.
Because of the infrequent data exchange in M2M applications, a large number of stations can share a single IEEE 802.11ah AP, as long as their activity periods are properly distributed over time. In addition to the communication needs, energy efficiency becomes a critical issue in applications based on battery-powered nodes. On average, the results obtained show that stations remained in sleeping mode more than 99% of the time, demonstrating a higher energy efficiency of IEEE 802.11ah.
In the future, due to the heterogeneous requirements of M2M applications, the definition of new QoS differentiation mechanisms will be needed to allow the coexistence of different M2M applications within the same AP coverage.
In addition, there are still open challenges regarding the performance of Non-TIM and unscheduled stations, as well as their integration with TIM stations in a single WLAN. Moreover, there is also a lack of comparative performance studies between the different existing and future technologies for M2M communications. Such comparative studies would provide a set of guidelines to select the best technology to use in a given scenario.
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