Viewpoint: Reliability, and Bluetooth overcoming interference with AFH

Wireless communication systems usually employ radio as the underlying, physical basis for getting data from one device to another.

Bluetooth itself is a radio communications technology. But there’s a problem. Radio is undeniably, unambiguously, uncontestably unreliable. And that’s a fact. So logically, if Bluetooth technology uses radio and radio is unreliable, it may appear problematic to describe Bluetooth technology as reliable.

There are a number of techniques Bluetooth technology uses to overcome interference and increase reliability. This article will pay close attention to one in particular, concerning adaptive frequency hopping (AFH). But before exploring this mitigation technique, what do we mean by reliability?

Defining reliability

A plain English definition of reliability which is suited to our purposes follows:

Communication can be regarded as reliable if the data sent is the data received and any intended action relating to that data happens as expected.

But sometimes requirements for reliability in a product or solution are a little more nuanced. The basic requirements that data should arrive in the correct state and that actions should occur as expected may be supplemented or refined with other requirements such as:

A tolerance for failure: if the expected result is observed in 99.9999% of cases, we may still regard the operation of the system as reliable. Sometimes this type of reliability requirement is expressed with respect to a time period such as a tolerance for no more than 1 failed operation every 24 hours and we may even talk about mean time between failures.

Latency: the system may be regarded as operating reliably only if the lights always switch on no more than 500ms after the light switch was activated.

Resilience: operations are still carried out correctly even when certain system, product or component failures have occurred. If one of the specified failures occurs and interrupts the intended service or functionality, then the system is said to be unreliable. If the system can experience such failures and continue to deliver its intended service or function, it is said to be reliable.

Reliability needs to be fit for purpose and is not an absolute. Like security, this is a concept best understood in the context of a set of requirements. And talking of security, it should be noted that changes to data that take place somewhere between the transmitter and receiver could be caused by naturally occurring phenomena or be deliberately brought about, with malicious intent.

In some scenarios, there will be a great deal of tolerance for some transmitted data not being received. If the heart rate monitoring application I use on my smartphone while cycling does not receive a certain proportion of the values that are transmitted by the heart rate monitor strapped around my chest, that is probably still OK. The overall functionality of the system would be unaffected and I would almost certainly be unaware that this was happening. The reliability of the system in this context is fit for purpose. On the other hand, there may be situations which demand a much higher level of reliability, with very little tolerance for failures. The same heart rate monitor in a medical rather than sports and fitness context is likely to need to hit far more stringent reliability targets.

Overcoming challenges

There are many types of issue and circumstances that can cause reliability challenges in wireless communications systems.

One of the major challenges in radio communications concerns collisions, which are particularly problematic in busy radio environments. A collision occurs when two or more devices transmit data on the same radio channel in overlapping time periods, and different radio technologies such as Bluetooth and Wi-Fi can interfere with each other if their use of the radio spectrum overlaps.

Bluetooth technology uses the 2.4GHz ISM radio band, which is also used by Wi-Fi, cordless phones and even microwave ovens. Bluetooth devices are pretty much everywhere and in ever increasing numbers too, so sharing the ISM band with large numbers of devices and multiple wireless technologies is one of the main challenges.

Adaptive frequency hopping (AFH)

One of the techniques Bluetooth technology uses to mitigate the risk of collisions is its use of spread spectrum techniques. When two devices are connected, this involves a specific technique known as adaptive frequency hopping (AFH).

When two Bluetooth LE devices connect, they agree a number of parameters which then govern how they subsequently communicate. Key among these parameters is the connection interval which controls how often a connection may use the radio. Every time the connection interval starts for a connection, we say that there has been a connection event.

At each connection event, a pair of connected devices have the opportunity to use their radios to exchange packets at precisely timed intervals. But in addition to this, at the start of each connection event, frequency hopping occurs, with a radio channel being deterministically selected from the set of available channels using a channel selection algorithm. Each device in the connection will then switch to the selected channel and over time and a series of connection events, communication will take place using a frequently changing series of different channels, distributed across the 2.4 GHz band, thereby significantly reducing the probability of collisions occurring.

Of the 40 channels defined for use by Bluetooth LE, 37 of these channels (known as the general-purpose channels) are available for use during connected communication. Frequency hopping makes a great contribution to reliability in communication between connected devices but Bluetooth goes one step further.

In a given environment, some Bluetooth radio channels might not be functioning well, perhaps because interference is impacting them, whereas other channels are working reliably. Over time, the list of reliable channels and unreliable channels may change, as other wireless communication devices in the environment come and go.

The primary device in a connection maintains a channel map which classifies each channel that is working well as used or otherwise as unused. The channel map is shared with the second device so that they each have the same information about which channels will be used and which will be avoided.

Devices use implementation-specific techniques to monitor how well each channel is functioning. If it is determined that one or more previously working channels are no longer working well enough, the channel map is updated. Conversely, if a previously bad channel is found to be working well now, its status will also be updated in the channel map. Channel map updates are then shared with the second device. In this way, Bluetooth ensures that it uses only known good channels, avoids problematic channels and keeps the channel map up to date so that it is always the optimal subset of channels that are being used. This is the adaptive aspect of the Bluetooth adaptive frequency hopping system.

In this way, the frequency hopping algorithm adapts and avoids parts of the ISM band that are currently experiencing interference. The combination of frequency hopping to spread communication across the entire ISM band and the way in which AFH adapts to the current conditions, makes Bluetooth communication reliable, even in quite hostile radio environments.

Reliable by design

Bluetooth technology is capable of achieving highly reliable communication in even the most challenging circumstances. Much of the system was designed with reliability in mind, from features like adaptive frequency hopping through to flow control and transactions in the enhanced attribute protocol. Designers and developers can optimise the reliability of their products and applications in a number of ways, too. Reliability in Bluetooth technology is more than just the sum of the reliability of its parts. Bluetooth technology is reliable by design.

To learn more about reliability in Bluetooth connection-oriented and connectionless communication systems as well as how to achieve reliability in Bluetooth mesh networks, check out my in-depth paper on Understanding Reliability in Bluetooth Technology.

Martin Woolley, Senior Developer Relations Manager, EMEA, at Bluetooth SIG