1.2. Design Goals
When reviewing any technology, the first question to be asked is how did the designers optimize this technology? Most technologies have one or two things that they are very good at, and many things that they are not. By determining what these one or two things are, a greater understanding of that technology can be achieved.
With Bluetooth low energy, this is very simple. It was designed for ultra-low power consumption. The unique structure of the Bluetooth SIG is that the organization creates and controls everything from the Physical Layer up to the application. The SIG does this in a cooperative and open but commercially driven standards model, and over more than ten years, it has optimized the process of creating wireless specifications that not only work at the point of release but are also interoperable, robust, and of extremely high quality.
When the low energy work started, the goal was to create the lowest-power short-range wireless technology possible. To do this, each layer of the architecture has been optimized to reduce the power consumption required to perform a given task. For example, the Physical Layer’s relaxation of the radio parameters, when compared with a Bluetooth classic radio, means that the radio can use less power when transmitting or receiving data. The link layer is optimized for very rapid reconnections and the efficient broadcast of data so that connections may not even be needed. The protocols in the host are optimized to reduce the time required once a link layer connection has been made until the application data can be sent. All of this is possible only when all parts of the system are designed at the same time by the same group of people.
The design goals for the original Bluetooth radio have not been forgotten. These include the following:
- Worldwide operation
- Low cost
- Short range
- Low power
For global operation, a wireless band that is available worldwide is required. There is only one available band that can be implemented using low-cost and high-volume manufacturing technology today: the 2.45GHz band. This is available because it is of no interest to astronomers, cell phone operators, or other commercial interests. Unfortunately, just like everything that is free, everybody wants to be part of it, causing congestion. Other wireless bands are available, for example, the 60GHz ISM band, but this is not practical from a low-cost point of view, or the 800/900MHz bands that have different frequencies and rules depending on where you are on the planet.
The design goal of low cost is interesting because it implies that the system should be kept as small and efficient as possible. Although it could be possible, for example, to add scatter net support or full-mesh networking into Bluetooth low energy, this would increase the cost because more memory and processing power would be required to maintain this network. The system has therefore been optimized for low cost above interesting research-based networking topologies.
The 2.45GHz band that Bluetooth low energy uses is already very crowded. Just taking into account standards-based technologies, it includes Bluetooth classic, Bluetooth low energy, IEEE 802.11, IEEE 802.11b, IEEE 802.11g, IEEE 802.11n, and IEEE 802.15.4. In addition, a number of proprietary radios are also using the band, including X10 video repeaters, wireless alarms, keyboards, and mice. A number of devices also emit noise in the band, such as street lights and microwave ovens.
It is therefore almost impossible to design a radio that will work at all times with all possible interferers, unless it uses adaptive frequency hopping, as pioneered by Bluetooth classic. Adaptive frequency hopping helps by not only detecting sources of interference quickly but also by adaptively avoiding them in the future. It also quickly recovers from the inevitable dropped packets caused by interference from other radios. It is this robustness that is absolutely key to the success of any wireless technology in the most congested radio spectrum available.
Robustness also covers the ability to detect and recover from bit errors caused by background noise. Most short-range wireless standards compromise by using a short cyclic redundancy check (CRC), although there are some that use very long checks. A good design will see compromise between the strength of the checks and the time taken to send this information.
Short range is actually a slight problem. If you want a low-power system, you must keep the transmitted power as low as possible to reduce the energy used to transmit the signal. Similarly, you must keep the receiver sensitivity fairly high to reduce the power required to pick up the radio signals of other devices from amongst the noise. What short range means in this context is really that it is not centered around a cellular base station system. Short range means that Bluetooth low energy should be a personal area network.
The original Bluetooth design goal of low power hasn’t changed that much, except that the design goals for power consumption have been reduced by one or two orders of magnitude. Bluetooth classic had a design goal of a few days standby and a few hours talk time for a headset, whereas Bluetooth low energy has a design goal of a few years for a sensor measuring the temperature or measuring how far you’ve walked.