The Wireless Technologies a Sensor Network Can Use

Because it defines only two layers, other standards supply the rest:

• Zigbee adds a network layer and an application framework. Its network layer supports star, tree and mesh topologies; the device that starts and controls the network is the Zigbee coordinator, which the specification defines as "an IEEE 802.15.4 PAN coordinator"; a Zigbee router is "capable of routing messages between devices"; a Zigbee end device is neither, and only sends and receives its own traffic. Zigbee's routing is taught in [Built on 802.15.4: Zigbee Routing, Security and the Later Amendments].
• 6LoWPAN carries IPv6 over 802.15.4 frames, so that sensor nodes can be ordinary Internet hosts. It is the subject of [Sensor Networks in the Internet of Things: 6LoWPAN, RPL and CoAP].

Strengths: designed for battery-powered nodes; supports multi-hop mesh networks through the layers above it; worldwide 2.4 GHz band; cheap radios. Limits: tens of metres per hop, so large areas need many hops; the 2.4 GHz band is shared with Wi-Fi and Bluetooth.

Bluetooth Low Energy

Bluetooth Low Energy (Bluetooth LE, also branded Bluetooth Smart) was introduced in Bluetooth 4.0 and extended in 4.1 and later versions. RFC 7668 describes its target plainly: devices "that operate with very low-capacity (e.g., coin cell) batteries or minimalistic power sources". A device in the central role (typically a phone or a router) manages connections to several peripherals (the sensors), giving a star topology. Its packets are small: the default MTU of its link-layer channels is 27 octets, 23 of them available to the upper layers.

Strengths: in almost every phone, so a person's phone can be the gateway; very low energy per connection; good for wearable and personal sensors, such as the heart-rate monitor RFC 7668 gives as its example. Limits: short range; the star topology means a node must be within reach of a central device; not designed for large multi-hop fields.

Wi-Fi

IEEE 802.11 (Wi-Fi) gives tens of megabits per second, which the text book's table puts at 11 to 54 for the 802.11b and g generations. It is designed for laptops and phones with large batteries or mains power, and its receivers are built to be on whenever the device is in use. Strengths: existing infrastructure everywhere, high data rates, IP built in. Limits: far more power than a sensor node can sustain on small batteries; access points needed. It suits sensors with mains power (cameras, gateways) and the gateway's own link to the Internet.

The low-power wide-area networks

A newer family aims at a different corner of the trade-off. RFC 8376, the IETF's overview, puts their shared goal as "supporting large numbers of very low-cost, low-throughput devices with very low power consumption, so that even battery-powered devices can be deployed for years", and says they accept "severe bandwidth and duty cycle constraints" in return for "multiple-kilometer radio links". LoRaWAN, Sigfox and NB-IoT are star networks: every device talks directly to a base station or gateway, with no relaying.

LoRaWAN uses ISM bands, for example 433 MHz and 868 MHz in the European Union and 915 MHz in the Americas. In the EU 868 MHz band its three default channels are at 868.1, 868.2 and 868.3 MHz, and its data rate runs from 250 bit/s with frames of 59 octets up to 50,000 bit/s with frames of 250 octets. A device's transmission may be received by several gateways at once, and the network adjusts each device's data rate and power by an adaptive data rate scheme. After each transmission a basic (Class A) device opens two short receive windows, and only then may it transmit again.

Sigfox uses ultra narrow band transmission: channels of 100 or 600 hertz, 100 or 600 baud, modulated by DBPSK. Its devices send "only a few bytes per day, week, or month", which RFC 8376 says allows them to last "up to 10-15 years" on one battery.

NB-IoT (Narrowband IoT) runs in licensed mobile-operator spectrum, using a narrow band of 180 kHz inside, beside or outside an LTE carrier. Its peak rates are 60 kbit/s up and 30 kbit/s down, and its design targets include a module cost of less than 5 US dollars, coverage of 164 dB maximum coupling loss, battery life of over 10 years and about 55,000 devices per cell.

Wi-SUN FAN (Field Area Network) is the exception to the star pattern: RFC 8376 describes it as "an IPv6 wireless mesh network" using frequency hopping, typically 2 to 3 km in line of sight and up to 300 kbit/s.

Worked example: what a 1 per cent duty cycle allows

Some ISM bands limit how much of the time a device may transmit, a limit RFC 8376 says is "usually expressed as a percentage of time per hour", and its LoRaWAN table gives 1 per cent as the tightest such limit. Take a LoRaWAN device in the EU 868 MHz band at its slowest and longest-reaching setting, 250 bit/s, sending a full 59-octet frame.

• Time on air for one frame: 59 × 8 / 250 = 1.888 seconds.
• Transmitting time allowed in an hour: 3,600 × 0.01 = 36 seconds.
• Frames allowed in an hour: 36 / 1.888, about 19.

So even this long-range network can carry about nineteen small messages an hour from each device at its longest range, which is plenty for a meter and hopeless for anything that streams.

Cellular data

A node can also use the mobile phone network directly, through GPRS or later data services, taught in [New Data Services: HSCSD, GPRS and EDGE]. It needs a SIM and a subscription for each node and draws considerable power, so it is usually used by the gateway rather than by every node: the sensor field talks to the gateway over 802.15.4, and the gateway talks to the task manager over the cellular network.

Worked example: choosing for three applications

The same reasoning chooses for any application: first the range and topology (can every node reach a gateway directly, or must nodes relay?), then the data rate and duty cycle (does the traffic fit?), then energy and cost.

A last classification: how constrained is the node?

RFC 7228 names three classes of constrained device, a useful vocabulary when choosing a stack.

The MSP430F1611 of [Inside a Sensor Node: The Five Units], with 10 kB of RAM and 48 kB of flash, sits between classes 0 and 1: too small for a full Internet protocol stack, which is why sensor networks needed stacks of their own.

Distinctions

What it does not mean

Zigbee and 802.15.4 are not the same thing. 802.15.4 is the radio and MAC; Zigbee is one of several stacks that run on top of it.

A long-range LPWAN is not a replacement for a mesh in every case. It trades data rate and duty cycle for range, and a node that must report every few seconds does not fit.

Wi-Fi is not wrong for sensors; it is wrong for battery sensors. A mains-powered camera or a gateway uses it well.

"Low power" is not a property of a radio alone. Bluetooth LE or 802.15.4 left listening all the time will empty a coin cell quickly. The MAC protocol's duty cycle decides the energy as much as the radio does.

Quick revision

• Text book Table 1.3: GPRS 0.064 to 0.128+ Mbps, 3000+ ft; Wi-Fi 11 to 54 Mbps, 1 to 300+ ft; Bluetooth 0.7 Mbps, 1 to 30+ ft; 802.15.4/ZigBee 0.020 to 0.25 Mbps, 1 to 300+ ft, designed for reliability, power and cost.
• 802.15.4: PHY and MAC only, three bands, 20/40/250 kb/s, 127-byte frames; Zigbee (star, tree, mesh; coordinator = 802.15.4 PAN coordinator) and 6LoWPAN above it.
• Bluetooth LE: from Bluetooth 4.0, coin cells, central and peripherals, star, 27-octet MTU.
• LPWAN: star (Wi-SUN mesh), kilometres, very low rate and duty cycle. LoRaWAN EU 868: channels 868.1/868.2/868.3 MHz, 250 bit/s to 50,000 bit/s; Sigfox ultra narrow band, 100 or 600 baud, DBPSK; NB-IoT licensed, 180 kHz, 60/30 kbit/s peak, 164 dB, over 10 years, about 55,000 devices a cell; Wi-SUN 2 to 3 km, 300 kbit/s.
• 1 per cent duty cycle at 250 bit/s: 1.888 s a frame, 36 s an hour, about 19 frames an hour.
• RFC 7228: Class 0 (much less than 10 KiB RAM), Class 1 (about 10 KiB), Class 2 (about 50 KiB).

Test yourself

1. Compare 802.15.4, Bluetooth, Wi-Fi and GPRS as technologies for a sensor network. From the text book's table: GPRS reaches kilometres at 0.064 to 0.128+ Mbps for wide-area voice and data; Wi-Fi gives 11 to 54 Mbps over up to 300+ feet for enterprise data; Bluetooth gives 0.7 Mbps over up to 30+ feet for cable replacement; 802.15.4 gives 0.020 to 0.25 Mbps over up to 300+ feet and is the only one designed around power, reliability and cost for monitoring and control.

2. What is the relationship between IEEE 802.15.4 and Zigbee? 802.15.4 defines the physical and MAC layers of a low-rate personal area network. Zigbee defines a network layer and application framework on top of it, supporting star, tree and mesh topologies; the Zigbee coordinator is an 802.15.4 PAN coordinator.

3. What do low-power wide-area networks trade, and for what? They accept very low data rates and strict duty cycles in exchange for multi-kilometre links and years of battery life, and they use a star topology with no relaying by the nodes.

4. A LoRaWAN device sends 59-octet frames at 250 bit/s under a 1 per cent duty cycle. How many frames can it send an hour? One frame takes 59 × 8 / 250 = 1.888 s; the device may transmit for 3,600 × 0.01 = 36 s an hour; 36 / 1.888 gives about 19 frames.

5. Which technology would you choose for a wearable heart-rate sensor, and why? Bluetooth Low Energy: it is designed for coin-cell devices, it is in every phone, so the phone serves as the gateway, and a star topology around the phone is all a single wearer needs.