M2M Communications with Wi-Fi in the 5 GHz Band
Networks based on 2.4 GHz ISM band standards suffer from the effects of network congestion – data drop-outs, latencies, disconnections and the lesser-known but increasingly important problem of battery life reduction.By N.Venkatesh, Redpine Signals
Machine to machine (M2M) communication is rarely just a direct link between two machines. It usually involves a network of varying complexity comprising machines, sensors, equipment, controllers, storage devices and servers. The inclusion of all of these in the network is what enables increased levels of automation, operational efficiency and new process flows in all the diverse application areas of M2M.
Among the various methods of communication available, Wi-Fi is fast gaining popularity due, in part, to the ease with which Wi-Fi enabled devices can get on to a network with wired and wireless segments. Wi-Fi, in some of its modes, shares the 2.4 GHz ISM band with some other popular wireless protocol standards in M2M – Bluetooth, ZigBee, BT-LE and some proprietary ones. With the proliferation of communicating nodes, networks based on any of these standards suffer from the effects of network congestion – data drop-outs, latencies, disconnections and the lesser-known but increasingly important problem of battery life reduction.
Most of the communication in M2M takes place in short bursts of varying periodicity – from a few tens of milliseconds to several hours. This characteristic of transmission in bursts arises not only from the nature of the communicating device but also from the time-sliced medium sharing method used in these protocols. In order to save energy, battery-operated devices often are put in a low power standby state between the bursts. Wi-Fi sends data in high data-rate bursts that provide a further degree of energy efficiency. In all of these cases, excess energy over the optimum is used when the devices, in a congested environment, wait for the medium to be free, or re-transmit data due to loss of packets because of collisions.
To get a perspective on the effect of this – we would see battery life of a device going down from nine months to five months if, on average, it waits after waking up for a typical duration of one packet before getting the chance to transmit its packet 40 percent of the time. Moving to the 5 GHz band, with up to 24 available channels compared to the three at 2.4 GHz, would provide immediate mitigation of the problem to a large extent.
Another deleterious effect of congestion in the medium is loss of performance in critical applications. Loss of packets at the physical layer is often not a major problem since packet retransmission methods are available at different layers of the networking stack, with exceptions like streamed media where loss of quality of audio or video would result because packets would not be retransmitted. Delayed packets, and packets with variable jitter, would reduce performance in time-critical applications. In some applications such as real-time locationing (RTLS), interference in the medium would affect performance of the system – the accuracy of location estimates would potentially drop when signals are received with interference.
Enterprise networks therefore are designed to an optimum configuration considering the parameters of physical layout, number of active nodes, data throughput required, types of applications to be supported and quality of service required. Network administrators attempt to figure out the right number of access points to be deployed, and the channels they must operate in. With limited number channels, the network would support only a limited number of nodes with the required performance or quality of service.
With the M2M communications market growing exponentially, there is a clear need to plan for deployment utilizing the 5 GHz band. The challenges of the 5 GHz band would have to be considered though – these include reduction in operational range and increased device cost and size (because 5GHz devices are usually dual-band devices). These are addressed by some Wi-Fi module vendors today, enabling easier adoption of 5 GHz operation in all environments and utilizing advanced Wi-Fi standards such as 802.11n and 802.11ac, paving the way for the full potential of M2M communication to be realized.
N.Venkatesh is vice president of advanced technologies at Redpine Signals, and has over 25 years of engineering and management experience in wireless system design, semiconductor design, telecommunications, optical networking and avionics. With Redpine, Mr. Venkatesh is a key wireless technologist and champions the universal integration of wireless into embedded systems. His responsibilities include leading the development of wireless systems at Redpine's India center, and their application into diverse industry areas. Mr. Venkatesh holds a master’s degree in electrical engineering from the Indian Institute of Technology, Madras, India.