Wireless Link

This post is a continuation of The Talk² Protocol post, which explained some basics about the Talk² protocol as well basic details about the Wired link.

One more time is important to highlight that the Talk² protocol is independent of the hardware it runs on. For our own boards designs we’ve made some decision, defining which technologies and hardware to use based on our experience. Remember that you’re free to implement the same protocol rules on top of any other media.

RF: Not so simple

A wireless link sounds much simpler than a wired connection. There’s no cables laying around, no mess and all works. But in reality, from an engineering point of view, it’s exactly the opposite. Wireless or radio-frequency (RF) links are much more susceptible to interference, reliability, privacy issues and even local government regulations. In an attempt to simplify our decision to which hardware should be used, we evaluate the following requirements: frequency, cost and features.

Frequency: ISM Band

In other words the operating frequency for the equipment needs to be open or free for everyone to use. Be aware that not all ISM frequencies are available in all countries. Another important point to consider is that even using a frequency from the ISM Band you still need to follow some rules, for example, limiting the transmission power or using frequency hopping as well respecting exclusion zones according to the legislation.

Here in Australia the Radiocommunications (Low Interference Potential Devices) Class Licence defines how the open frequencies should be used and its restrictions. For US there is the FCC part 15, which describe use rules for radio frequency devices.

The most common open bands are:

Band Where it’s used
40MHz Old garage remote control and kids toys like RC cars and “walkie-talkie”
315MHz Used on some garage remote, alarm systems and door bells
433MHz Garage door and power outlet remote controls as well telemetry and restaurant pagers
868/915MHz Telemetry and some older cordless phones. The 868MHz is the European version of 915MHz.
2.4GHz Everybody’s home WiFi, Bluetooth, ZigBee, cordless phone, even your microwave
5.8GHz Some WiFi routers and cordless phone

References related to ISM Bands:

  1. https://en.wikipedia.org/wiki/ISM_band
  2. http://www.radiometrix.com/files/additional/choice-of-frequency-band-can-really-make-a-difference.pdf

Knowing that we need to use an ISM Band we’ve narrowed down our decision to the following bands: 433MHz, 868MHz/915MHz and 2.4GHz. We limited our choices to this list because of multiple factors, including hardware availability, cost and antenna size – the lower the frequency the bigger the antenna you’re going to need.

No to 2.4GHz!

After not much thinking we simply discarded 2.4GHz. Even this frequency being very popular and offering great bandwidth, low cost and only a tiny antenna, it has some major problems:

  • It’s crowded. Every single WiFi device use it: Your and everybody’s home wireless network, Bluetooth, cordless phone, and so on.
  • Poor penetration. The 2.4GHz frequency is quite high, having troubles to penetrate solid materials like the walls of your house or rain.
  • Range. We all have good experience and is easy to tell that 2.4GHz devices don’t offer good range, well below our expectation.

At the end with two frequency to work with: 433MHz and 868/915MHz.

Cost

Another important factor when deciding which technology to use was the cost. You might have already heard about ZigBee and 6LoWPAN, and even they working on 2.4GHz spectrum they offer great features, including mesh networking, encryption and low power out-of-the-box. ZigBee is even starting to run on sub-GHz frequencies, but those RF modules are very expensive compared with others manufacturers’ proprietary solutions.

Again, if you would like you still able to build a ZigBee Gateway or simply transmit Talk² messages over a ZigBee mesh network.

Features

The most important features listed by us were: transmitter power, receiver sensibility, power consumption and built-in encryption. That in the real life translates into a radio that offers a good range, can be powered by battery and does not add overhead to the MCU when securing the transmitted data.

If you look around you’ll find many products from different manufactures like TI, Microchip, Silicon Labs and others, all on sub-GHz frequencies. Some are even integrated with a 8bit or 32bit ARM MCU.

Chosen Hardware

chip_sw1231Having considered many options the Wireless link will be based on the silicon SX1231 from Semtech  – http://www.semtech.com/wireless-rf/rf-transceivers/sx1231/.

This transceiver offers many features really like:

  • Wide range of frequencies (290MHz to 1000MHz)
  • Build in CRC and AES-128 bit encryption
  • 66 bytes FIFO and RSSI
  • Low RX power comsumption of 16mA

The SX1231 also has a big brother, the SX1231H capable of 20dBm (or 100mW power output).

This same chip is being used by popular RF Modules like the RFM69 by HopeRF, which means that they are 100% compatible.

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