Benchmarking consortium EEMBC has launched a standardised set of tests for WiFi power consumption in the Internet of Things (IoT). The IoTMark-Wi-Fi benchmark is designed specifically to measure the power consumption of low-power 802.11 WiFi chips under realistic load situations.
EEMBC worked with European companies including STMicroelectronics, Infineon and Dialog Semiconductor to develop a standardised behavioural model for WiFi links in the way they are used across the IoT.
Recent improvements in chip design have made the more familiar 802.11 WiFi protocol viable for IoT, by drastically reducing power usage while keeping its advantages of greater range and reliability, and broader industry support. EEMBC is using a behavioural profile that measures power usage under a carefully selected workload, reflecting the sometimes unexpected demands experienced by IoT devices.
"We had to be very clear about what we're measuring, and why," said Peter Torelli, president of EEMBC. "Long battery life is crucial when you've got an office or production floor full of IoT sensors, but actual power consumption depends on a wide range of variables."
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WiFi routers can vary widely, for example, in how frequently they address devices on the network, with "noisier" routers causing them to rapidly use up battery life, especially if there are several routers within range.
The 2.4 GHz band where 802.11 operates is crowded with non-IoT devices as well, which can further deplete battery life with their own communications. However, this variability makes a single metric, like standby power consumption, unrealistic for most IoT devices.
IoT devices spend the vast majority of their time in standby, ultimately consuming more power in this state than in active communication—even when using low-power protocols like MQTT. For this reason, IoTMark-Wi-Fi's behavioural profile was designed to include both a "connected standby" and an application layer component. The official score provided by the benchmark reflects a device's average power consumption over these two components; its parameters can also be adjusted to give a more accurate result for specific, known situations.
"For customers to make informed decisions, the industry needs a way to make objective measurements of battery life for IoT products," said Omer Cheema, the Senior Director of IoT WiFi Business Unit at Dialog Semiconductor. "That's why Dialog Semiconductor has been so excited to collaborate with EEMBC in developing a standardized low power Wi-Fi benchmark."
Perhaps the most user-friendly aspect of the benchmark is the way its output is scaled: running it provides a number that roughly indicates the number of days the device will last on a pair of standard AA alkaline batteries. "Benchmarks can be very technical, abstract measurements, with good reason," said Torelli at EEMBC, "so we thought it'd be nice to tie the output to something everyone can understand."
The benchmark can illustrate this sensitivity even more dramatically when used in conjunction with an RF isolation chamber, such as those made by octoScope, a subsidiary of UK test house Spirent.
The execution of the profile must be tightly controlled and completely repeatable for the benchmark to be meaningful. It must also be clearly defined so that measurements are comparible across different vendor's devices. The behavioural model is split into three phases, two of which are used for scoring while the third is used for analysis.
Low-power devices spend a significant portion of their time in a mode called connected idle. This is a mode where the DUT (station) indicates to the Access Point (AP) that it is still connected, but will be entering a low-power state and will not be alert for every 802.11 beacon. Instead, the AP and the DUT agree on a sleep interval ahead of time, indicating precisely when the DUT will check to see if it needs to fully power-on. By using what is known as a Delivery Traffic Indication Map, or DTIM, the AP can tell the DUT to wake up and receive its queued data. The DUT DTIM check occurs every sleep interval of beacons. If there is data waiting, then the DUT powers-up to receive and process this data, re-entering connecterd standby after it has completed.
In order to minimize energy, several low-power communication protocols have been developed to operate over TCP/IP on 802.11. MQTT is one such protocol: it is a publication/subscribe model, where a central server is responsible for displatching incoming messages to subscribers. The benchmark uses an MQTT server installed on the AP to communicate with the DUT in a tightly-controlled manner. During this phase of the benchmark, the host system instructs the the DUT to perform both an Rx-Tx and an Rx-only transaction using MQTTS. MQTTS is a TLS enabled mode of MQTT, which means the data packets transmitted are encrypted with AES.
The Active Connect MOde portion of the benchmark exposes the energy costs of high-bandwidth communication, and is not part of benchmark score. However, it does allow the user to investigate the real-world power of their device when sending large amounts of data via TCP or UDP. It uses an iperf server running on the AP.
The key to the benchmark is the Framework. This is a combination of hardware, software, and firmware that controls when the device connects to the Access Point (AP), the connection parameters, and when the device sends and receives data using the MQTT communication protocol. The hardware bill of materials is available as a list from distributors Digi-Key and Farnell, and the software is available from EEMBC.
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