Wireless IoT connectivity: making sense of the protocols

The number of choices is still growing as both cellular and WiFi-based technologies adapt to the needs of the IoT. However, amid the wide range of options, the capabilities of each, such as coverage, range and data rate will help determine the right choice for a given application and situation.

One of the most important considerations is the range required. Many devices will use a gateway with a broadband connection to transfer data to and from the cloud. The availability of a gateway allows the use of short range protocols that operate at relatively low transmit power levels. In many cases, gateways will be too difficult or uneconomic to deploy. For example, a set of sensors deployed around a large open water reservoir or along a train track will generally be too widely dispersed to make access through a gateway.

Today, such highly dispersed sensors and meters are likely to use either cellular connections or proprietary protocols in the unlicensed sub-gigahertz bands such as 868MHz. This is an area that, propelled by the need for standards to successfully navigate the IoT and by strategic changes by cellular operators, is seeing major changes. As they seek to maximise their licensed spectrum allocation, operators are expected to switch existing sub-gigahertz GSM networks to support LTE because of its higher spectral efficiency.

To support the growing requirement for machine-to-machine (M2M) and IoT communications, several LTE-based cellular protocols have been proposed. In the short term, Cat-M provides a less complex form of LTE that is suitable for IoT devices, offering data rates of up to 1Mbit/s and access to the already widespread cellular network. However, within two or three years, the 3GPP-defined Narrowband-IoT is likely to provide support for IoT networks that will reduce both device and operational costs – using lower data rates on the order of 200kbit/s – and improve access to difficult locations such as underground meters.

Operating primarily in the unlicensed spectrum, LoRa and SIGFOX provide alternatives to cellular connections not just in terms of transceiver design but also in pricing models. Cellular operators today charge on a data usage basis, whereas flat rate models are available with commercial operators who support the unlicensed down to the level of $1.00 per year per device.

Developed by Semtech, LoRa provides IoT users with the option of accessing the internet through their own network of base stations to provide greater control, and potentially lower operating costs or through a burgeoning selection of commercial operators. Among them is The Things Network, a LoRa network set-up in Amsterdam in the space of just six weeks that is crowd sourcing efforts to repeat the exercise in cities around the world.

Supported by devices made by Microchip and STMicroelectronics, as well as Semtech, LoRa has the benefit compared to traditional radio systems of offering access to devices buried below ground such as parking and water meters and a transmit range on the order of ten kilometres. Resilience to interference from other unlicensed band users is helped by the use of a spread spectrum modulation scheme, with data rates ranging from 300bit/s to 50kbit/s, similar to that of existing GPRS connections.

SIGFOX, in contrast, employs an ultra-narrow band transmission technique to limit power and allow for ranges of up to 50km in urban environments and data rates between 10bit/s and 1kbit/s. Unlike most other protocols, SIGFOX is a unidirectional link. This can be used to minimise the power consumption of the IoT node, as there is no need to have the RF transceiver wake up and listen for long periods of time for incoming communications, but has the limitation that there is no way to update software in the node remotely using a conventional SIGFOX transceiver - another radio will be needed. However, a number of RF transceivers designed for the unlicensed ISM can handle the relatively simple requirements of SIGFOX transmissions. Vendors that have announced they are employing SIGFOX for IoT connectivity include ON Semiconductor with its AX-SIGFOX single chip solution, and Arrow Electronics, which launched its SmartEverything development board, an Arduino form factor prototyping platform for IoT and M2M applications.

For shorter range communications supported by a gateway, the 2.4GHz unlicensed band has become the primary choice for wireless networking. Used by both Bluetooth and WiFi, 2.4GHz also provides the spectrum for protocols designed specifically for the IoT, such as 6LoWPAN and Thread. The key underlying standard for both 6LowPAN and Thread, as well as ZigBee and Wireless HART, is IEEE 802.15.4.

The core IEEE 802.15.4 supports a transmit range of up to ten metres with a transfer rate of 250kbit/s and can operate on sub-gigahertz unlicensed bands as well as the 2.4GHz band. 6LowPAN adds a set of upper layer protocols to the core IEEE 802.15.4 standard that provide compatibility with the Internet Protocol (IP) and allow standard IP-based communications from the cloud to reach the IoT edge devices using standards such as TCP, HTTP, COAP and MQTT. To minimise protocol overhead on IoT data transfers, 6LowPAN employs header compression and other data encapsulation techniques.

To increase the maximum distance between nodes and a gateway, 6LowPAN supports mesh networking. With a mesh networking topology, nodes that are too far away from the gateway to reach it can relay packets through devices that are physically closer until one can reach the gateway directly. Because they analyse the routability of the network dynamically, mesh networks can automatically configure new devices so that they leverage usage patterns that the system has already learned.

Launched in 2014, Thread builds on top of 6LowPAN to provide further functions for the IoT such as authentication and encryption, which improve overall security. However, in most cases, a simple software upgrade will allow Thread to run on devices that support IEEE 802.15.4 including, for example, those from Silicon Labs, a founding board member of the Thread Group.

Although IEEE 802.15.4 has become the core infrastructure for a variety of IoT-oriented network protocols, Bluetooth has also become a key standard to consider. Helped by its ubiquitous support in smartphones, tablets and laptop computers, Bluetooth already provides an important mechanism for controlling and configuring IoT devices through apps. Thanks to enhancements to the standard, Bluetooth is also becoming a key protocol for IoT applications that do not rely on smartphone connectivity. The introduction of Bluetooth Smart made the short range network protocol more suitable to IoT applications by supporting modes with significantly reduced power consumption for transfers of small amounts of data.

A further change expected to be added to the specification this year will make it possible to extend the normal transmit range by a factor of four - trading off improved range against bit rate. The adaptive protocol will allow nodes that are closer together to employ higher bit rates, as much as twice Bluetooth Smart’s standard bandwidth of 1Mbit/s.

Mesh networking is also expected to improve the ability of Bluetooth devices to communicate over longer distances. The mesh networking planned for Bluetooth expands on the Scatternet concept introduced with the release of version 4.1 in 2013. Scatternet provides the ability for each node to switch between master and slave modes so it can pass a packet along to other slaves or master in order to reach the destination transceiver. Recent changes to the Bluetooth protocol also help extend the coverage of a network by allowing interaction with devices that use the 6LowPAN wireless protocol.

Finally, WiFi is a further option for IoT applications. The existing WiFi versions support higher data rates than most of the IoT protocols but these can be harnessed for video transmissions in applications such as premises security. The IEEE working group responsible for WiFi is now working on a version – IEEE 802.11ah – that will significantly reduce power consumption and extend the transmission range in exchange for lower data rates, around 100kbit/s per channel over a distance of one kilometre.

The result of the activity in wireless IoT networks has resulted in a rich selection of protocols that will suit different target markets. There is one that is just right for your application.

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