Can Proprietary Protocols Cut the Cost of Simple Sensor Networking?

Industry standards such as ZigBee® and Bluetooth® Smart are among the best known protocols currently being designed into smart living and home automation equipment such as lighting and heating controls, metering, and security and safety systems. However, other options are available to designers including some proprietary protocols that can offer advantages such as a shorter learning curve, lower costs, and low power consumption suitable for battery-operated appliances.

Among proprietary wireless protocols, Texas Instruments' SimpliciTI™ is a simple protocol designed for small RF networks that have low data rate requirements and demand low power consumption. It is designed to support peer-to-peer topologies, use an Access Point (AP) in a one-to-one connection, or as the center of a star topology. As such, SimpliciTI is a strong contender for systems that are designed to share sensor data such as occupancy, light level and/or temperature information for controlling smart lighting. It can also support communications for equipment such as RF remote controls including garage door openers, as well as smart meters, safety equipment like gas sensors and smoke detectors, security sensors, and various other devices.

SimpliciTI has some important attractions for embedded designers. For one, the network protocol is provided as source code under a free license without royalties. It also imposes minimal demand for system resources, requiring less than 8KB of Flash and 1KB of RAM depending on configuration. At the same time, however, it allows versatile network topologies suitable for connecting various types of devices such as sensors, actuators and RF tags, thereby enabling designers to fulfill numerous use cases. By supporting direct device-to-device communication, as well as simple star topologies using an AP capable of storing and forwarding messages, it has capabilities similar to those of Bluetooth Smart. In addition, by also allowing range extenders, SimpliciTI provides some of the attractive features of mesh networking protocols such as ZigBee or the forthcoming standardized Bluetooth mesh. The range is allowed to be extended up to four hops using these extenders. Figure 1 shows how SimpliciTI peers, APs and range extenders are able to support a wide range of smart-building or home-automation tasks as part of a wireless sensor network.

Figure 1: SimpliciTI network topology for wireless sensing.

Simple by name...

SimpliciTI is designed to be extremely energy-efficient and cost-effective, supporting many of the features often required of a network protocol and placing minimal load on system resources. SimpliciTI APs are required to be always on, but the protocol is able to support sleeping end devices. These may operate either by polling the central access point for waiting messages, or by looking for messages upon detecting activity.

The SimpliciTI protocol is designed to simplify linking together of arbitrary peer applications. Its layer architecture, as seen in Figure 2, comprises a small number of network applications at the top level which handle associations, connections and information exchange between peers, as well as frequency management and general network management. The radio takes care of data framing which allows a Minimal RF Interface (MRFI) that eliminates any need for formal physical or data-link layers. There is a Board Support Package (BSP) which is concerned only with the radio interface. Although the BSP supports LEDs and pushbuttons, it provides minimal hardware abstraction thereby allowing developers to implement services such as UART or LCD drivers in the customer application only as needed.

Figure 2: The SimpliciTI hierarchy comprises a small number of network applications, and requires no formal PHY/link layer.

Aside from the small amount of Flash required for storage of code and constants, and the very low RAM requirement, SimpliciTI requires no other resources of the host microcontroller. As it is not designed to run as an independent task that requires its own threading context, it can coexist with the customer application without requiring a scheduler or other support from the operating system.

The Application Program Interface (API) allows the network to be configured and managed using only a handful of calls. The SMPL_Init( ) call, for example, handles not only network initialization but also enables an initialized end device to join the network, thereby eliminating any need for an explicit network-join call. The API is completed by SMPL_Link( ) and SMPL_LinkListen( ) bidirectional linking calls, SMPL_Send( ) and SMPL_Receive( ) peer-to-peer messaging calls, and a configuration call, SMPL_Ioctl( ). Figure 3 shows how an ED can be initialized and joined to a network to take regular temperature measurements from an on-board sensor.

void main()

{

linkID_t linkID;

uint32_t temp;

// Initialize the board’s HW

BSP_Init();

SMPL_Init(0);

// link.

SMPL_Link(&linkID);

while (TRUE)

{

// sleep until timer. read temp sensor

MCU_Sleep();

HW_ReadTempSensor(&temp);

if (temp > TOO_HIGH)

{

SMPL_Send(linkID, “Hot!”, 4);

}

if (temp < TOO_LOW)

{

SMPL_Send(linkID, “Cold!”, 5);

Figure 3: Sample code for configuring a SimpliciTI End Device (ED) to share sensor data with the wireless network.

Any downside?

SimpliciTI’s lightweight nature makes it possible for engineers to learn the protocol quickly and start designing functioning projects. The host microcontroller can be selected without requiring significant extra provision to support the needs of the protocol, and the impact on power consumption is minimal. On the other hand, the intentionally lean feature requires the customer application to take responsibility for some aspects of managing communications between devices. If for any reason access to a channel is unavailable, the application must take responsibility for handling retries based on the importance of the message. This effectively leaves the developer to ensure the right balance between low power consumption and communication reliability.

The security features built into SimpliciTI enable developers to create products with robust end-user protection against attacks that threaten networks containing wirelessly connected intelligent devices. Any device seeking to connect to a SimpliciTI network must have a link token which is access controlled. In addition, encryption support allows network access control to be further strengthened.

SimpliciTI in practice

SimpliciTI was designed for easy implementation and deployment on several TI RF platforms, such as MSP430 ultra-low-power microcontrollers and IEEE 8902.15.4 RF transceivers such as the CC2510, as well as the later CC2530 and CC430. SimpliciTI examples are available online and are easily ported to SmartRF target boards like the CC2510EM evaluation module. Figure 4 shows how a SimpliciTI network can be built using a variety of currently available wireless development boards.

Figure 4: SimpliciTI can run on a number of development boards, in star or peer-to-peer connection.

There are also two easy ways to build wireless sensor networks to demonstrate the properties of the SimpliciTI protocol. TI has published a battery-operated wireless sensor-monitoring reference design that runs on eZ430-2500T wireless target boards. In addition, the eZ430-2500 development kit includes two of these target boards that are pre-programmed with firmware to function as an AP/ED pair in a SimpliciTI sensor monitoring network. Additional eZ430-2500T wireless target boards can be purchased individually to build larger networks.

Other valuable development tools include a guide to estimating and measuring current consumption, and the SmartRF Packet Sniffer. The latter can be downloaded free of charge and will run on a variety of capture tools to analyze radio messages in real-time and verify correct functioning of the AP and EDs.

Conclusion

Engineers looking for a fast and easy start to building simple, low-power wireless networks for home-automation, remote control, and other smart living devices can look to the SimpliciTI protocol as a potential low-cost solution.  Its support for star or peer-to-peer topologies as well as multi-hop range extension provides some of the functionality of more complex standards such as ZigBee and Bluetooth Smart mesh. There is built-in security, although the general lightness of the protocol means developers must balance advantages such as lower power consumption and greater ease of use against reduced flexibility and the complexity of an application.