How Bluetooth Technology Uses
Adaptive Frequency Hopping to
Overcome Packet Interference
Interference is one of the biggest challenges for any wireless technology in providing reliable data communication. Since wireless technologies like Bluetooth ® , Wi-Fi, and 802.15.4 devices share a transmission medium, it’s possible for a data packet that is being transmitted to be corrupted or lost if it collides with another packet being transmitted at the exact same time and on the same frequency channel.
One of the techniques Bluetooth technology uses to overcome interference and find a clear transmission path that avoids packet collision is the application of a form of frequency-hopping spread spectrum (FHSS) called adaptive frequency hopping (AFH). Bluetooth divides the frequency band into smaller channels (e.g. 40 channels in the case of Bluetooth Low Energy) and rapidly hops between those channels when transmitting packets. To further reduce the chance of interference, Bluetooth adapts its hopping sequence. Channels that are noisy and busy are dynamically tracked and avoided when sending packets.
Recently, Martin Woolley, developer relations manager with the Bluetooth Special Interest Group (SIG), released a paper on Understanding Reliability in Bluetooth Technology. In this deep dive into Bluetooth reliability, Woolley explains how Bluetooth technology uses adaptive frequency hopping to lower the probability of collisions and offset inevitable packet loss.
Below is an excerpt from that section of Woolley’s paper.
Adaptive Frequency Hopping
One of the major challenges in radio communications concerns collisions, which are particularly problematic in busy radio environments. Earlier we explored collisions and explained that a collision occurs when two or more devices transmit data on the same radio channel in overlapping time periods, and that different radio technologies such as Bluetooth ® technology and Wi-Fi can interfere with each other if their use of the radio spectrum overlaps.
Bluetooth technology mitigates the risk of collisions through its use of spread spectrum techniques. When two devices are connected, this involves a specific technique known as adaptive frequency hopping.
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 Low Energy (LE), 37 of these channels (known as the general purpose channels) are available for use during connected communication.
Bluetooth Wireless Technology
Bluetooth is a proprietary wireless technology intended for short-range communication that is overseen by the Bluetooth Special Interest Group. It provides a secure, robust, low power, and low cost method for exchanging information between devices. Because it is a global standard, any Bluetooth-enabled device can communicate, or «pair», with any other. A Bluetooth device can also connect with up to seven other devices in an ad-hoc personal-area network called a «piconet», and be a member of several piconets at the same time. Bluetooth uses a technology called frequency-hopping to ensure that it is resilient against interference. [1]
History
The Bluetooth Special Interest Group (SIG) was created in 1998 by five companies (Ericsson, Nokia, IBM, Toshiba, and Intel) «to establish a de facto standard for the air interface and the software that controls it.» [1]
By the end of their first year, SIG had 400 member companies, and the first Bluetooth Specification was released in 1999. Despite numerous problems with the initial specifications, mobile phones with Bluetooth capabilities started being released in 2000. In 2012, Bluetooth SIG membership exceeded 16,000 companies. The most recent specification is Bluetooth Core Specification version 4.0, which contains many significant improvements over the initial specification and is the Bluetooth we know today.
How Bluetooth Works
Bluetooth is based on frequency-hopping spread spectrum radio technology, making use of a packet-based structure in a master-slave arrangement. That is, information is transmitted in discrete chunks known as packets, and in each piconet, there exists a master device that dictates which of the other (slave) devices it is communicating with. Devices can also switch roles from master to slave and vice versa, and they belong to multiple piconets, where they are master in one network and slave in another. These two connected piconets are then referred to as a «scatternet». [2]
Information is passed between the master of a piconet and one of its slaves at any given time. All communication is done on radio frequencies in the Industrial, Scientific and Medical (ISM) 2.4 GHz range, and so though it does not require a direct visual line of sight in order to operate, it does require radio line of sight, which can pass through most non-metallic objects. Bluetooth uses a low-powered signal; there are three classes of radios used in Bluetooth devices, with the shortest range being Class 3 radios (whose maximum power output is 1mW, producing a range of up to about one meter) and the longest-range being Class 1 radios (with a maximum output power of 100 mW and a range of 100 meters). [3]
Frequency-Hopping
Bluetooth utilizes frequency-hopping spread spectrum technology to avoid interference problems. The ISM 2.4 GHz band is 2400 to 2483.5 MHz, and Bluetooth uses 79 radio frequency channels in this band, starting at 2402 MHz and continuing every 1 MHz. It is these frequency channels that Bluetooth technology is «hopping» over. The signal switches carrier channels rapidly, at a rate of 1600 hops per second, over a determined pattern of channels. There are six defined types of hopping sequences.
Information is conveyed by modulating the carrier channel frequency, using one of several modulation schemes. Gaussian frequency-shift keying (GFSK) modulation was initially the only type available, but recently other varieties have been enabled. GFSK is simply a type of frequency-shift keying (FSK), which is a modulation scheme where the bits of the transfered information correspond to discrete frequency changes in the carrier signal. The carrier signal is whichever band the device happens to be using at that moment (before it hops to another), and the modified signal is broadcast out. [4]
Because Bluetooth uses this frequency-hopping scheme, it is very unlikely that there will be much interference from other devices, be they Bluetooth or not. Given that the hopping patterns are pseudo-random, the chances that another Bluetooth device would use the same pattern and disrupt a large amount of data-flow is very low. Additionally, other devices that simply broadcast at a fixed frequency can only have a minimal impact on the data transferred using Bluetooth.
Security
While Bluetooth is intended to be a safe protocol for wireless transfer of data, and is often touted for its security, it does have its flaws. It is beyond the scope of this article to delve into the numerous vulnerabilities of a system like Bluetooth, but it suffices to say that Bluetooth devices are susceptible to a number of attacks, including eavesdropping, man-in-the-middle, and denial of service attacks. The National Institute of Standards and Technology has published a Guide to Bluetooth Security detailing the vulnerabilities of Bluetooth and recommendations for dealing with them. [5]
© Graham Roth. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
References
[1] J. Haartsen, «Bluetooth — The Universal Radio Interface for Ad Hoc Wireless Connectivity.» Ericsson Review 3, 110 (1998)
[2] J. Haartsen et al., «Bluetooth: Vision, Goals, and Architecture,» ACM Mobile Comp. Commun. Rev. 2, No. 4, 38 (1998).
[3] J. Bray and C. F. Sturman, Bluetooth 1.1: Connect Without Cables, 2nd Ed. (Prentice Hall, 2001).
[4] R. K. Morrow, Bluetooth Operation and Use, (McGraw-Hill, 2002).
[5] K. Scarfone J. and Padgette, «Guide to Bluetooth Security,» U.S. National Institute of Standards and Technology, NIST SP-800-121, September 2008.
What is frequency hopping in bluetooth
Bluetooth specification 1.2 introduced adaptive frequency hopping — AFH that can reduce the effects of interference between Bluetooth and other types of devices. AFH adapts the access channel sharing method so that the transmission does not occur on channels that have significant interference. By using interference avoidance, devices that operate within the same frequency band and within the same physical area can detect the presence of each other and adjust their communication systems to reduce the amount of overlap (interference) caused by each other.
The adaptive frequency hopping process reassigns the transmission of packets on frequency channels that have interference to other channels that have lesser interference levels. This reduced level of interference increases the amount of successful transmissions therefore increasing the overall efficiency and increased overall data transmission rates for the Bluetooth device and reduces the effects of interference from the Bluetooth transmitter to other devices.
This figure shows how the Bluetooth specification 1.2 allows the Bluetooth device to change its hopping pattern to avoid interference to and from other devices that operate within its frequency band. This example shows a video camera that is using multiple frequency channels. After the Bluetooth device detects the presence of a continuous signal being transmitted by the video camera in the 2.4 GHz frequency band, it automatically changes its frequency hopping patter to avoid transmitting on the frequency band that is used by the video camera signal transmission. This results in more packets being successfully sent by the Bluetooth device and reduced interference from the Bluetooth device to the transmitted video signal.
This book explains how Bluetooth devices can locate, authenticate, discover the capabilities, and setup connections with other Bluetooth devices. Radio packet structures, modulation types, and protocols are described and explained along with Bluetooth security pairing (authentication) and privacy (encryption) processes.
Copyright � 2009 All rights reserved