Bluetooth is one of those technologies that just keeps slowly creeping into our industrial automation applications. A lot of that is that it’s being forced on us. As PC manufacturers continue their crusade to make personal computers less expensive than a cup of coffee, they’ve eliminated all the serial connections we’ve used for the last thirty years. And now Bluetooth is one of the ways we can connect devices that in the past were connected with good old RS232 serial cables.
I don’t know this for certain, but it’s believed that Bluetooth technology, developed in Denmark, was named for Harald “Blåtand” Gormsson, the King of Denmark and Norway in the latter part of the 10th century. Harald was the son of “King Gorm the Old” – though I doubt that any of his subjects called him that to his face. Harald was also known as King Harald Bluetooth (“Blåtand” in Danish). The common explanation for that moniker being that Harald must have had a conspicuous bad tooth that appeared “blue” (i.e. black, as “blue” meant dark).
Given all the cell phone applications for Bluetooth, we all use it daily; but few of us really understand the technology. Here is a list of the most important concepts you should know about a technology that you not only use every day but is becoming more critical to factory floor applications:
- Bluetooth was conceived from a desire to replace serial cables with wireless communications. There are multiple versions of Bluetooth. Bluetooth Low Energy (BLE), also called Bluetooth 4.0, is the standard designed for devices where power is the major concern.
- Other versions of Bluetooth include 1.0 (original Bluetooth) and 3.0. Version 3.0, which vastly increased the possible data rate, is also known as Enhanced Data Rate (BT/EDR) Bluetooth. These higher throughput versions require much more power from the end devices the BLE does.
Bluetooth doesn’t really compete with Wi Fi (802.11). WiFi is faster and more powerful with higher throughput and covers longer distances.
- Bluetooth operates in the 2.4 GHz band which is the same band used by other wireless communication technologies. Devices like microwave ovens put out a lot of noise in this area. Just yesterday my Bluetooth speaker stopped working when I turned on the microwave to warm my coffee.
- Bluetooth is covered under the 802.15 standard (802.15.1). Other wireless communication technologies are also covered under the same standard (e.g. Zigbee is 802.15.4).
- Bluetooth avoids disruption from other 2.4 GHz devices by using Frequency Hopping Spread Spectrum (FHSS) techniques to constantly change frequencies. An interesting side note is that FHSS was an invention of American actress and WWII pin up girl, Heddy Lamarr. You’ll want to read that story. Apparently her mind was as incredibly well developed as her physique.
- There are 79 different frequencies used by BLE. Three of the frequencies are used exclusively for advertising the presence of a device.
- The distance that can be covered by BLE varies with the power of the transmitter and the obstructions in the path. Given that BLE devices are by nature low power, the transmission distance is generally pretty short.
- Classic Bluetooth (not BLE) uses piconets – networks of up to eight devices where one device is the “master” device. Multiple piconets can connect to one another forming networks called “scatternets.”
- BLE devices can operate in two basic modes. In Advertising mode a BLE device sends an advertising packet at some rate (usually not faster than 100msec intervals). The advertising packet can contain data, so that a listener can receive data from a BLE device without forming a connection. In the other operating mode, a Master device creates a connection with a BLE device and uses that connection to send commands to that BLE device.
- There is no peer to peer communication. A device is either a BLE broadcaster or a BLE listener.
- Every BLE device can be identified by its unique 48 bit ID. Just like Ethernet devices, this ID is composed of 24 bits identifying the device manufacturer and 24 bits of unique identification within that manufacturer’s products.
There are two components to every Bluetooth device: the GAP and the GATT:
- The GAP (Generic Access Protocol) defines the general topology of the BLE network stack. GAP defines how BLE-enabled devices can make themselves available and how two devices can communicate directly with each other.
- The GATT (Generic Attributes Profile) describes in detail how attributes (data) are transferred once devices have a dedicated connection. GATT specifically focuses on how data is formatted, packaged, and sent according to its described rules.
There is a lot more to BLE than I’ve covered in this short article. For more information a good book on the subject is Getting Started with Bluetooth Low Energy: Tools and Techniques for Low-Power Networking 1st Edition by Kevin Townsend.