NIST SP 800-121 REV. 2 GUIDE TO BLUETOOTH SECURITY
This publication is available free of charge from: https://doi.org/10.6028/NIST.SP.800-121r2
2.1 Bluetooth Wireless Technology Characteristics
Bluetooth operates in the unlicensed 2.4000 gigahertz (GHz) to 2.4835 GHz Industrial, Scientific,
and Medical (ISM) frequency band. Numerous technologies operate in this band, including the IEEE
802.11b/g/n wireless local area network (WLAN) standard, making it somewhat crowded from the
standpoint of the volume of wireless transmissions. Bluetooth employs frequency hopping spread
spectrum (FHSS) technology for transmissions. FHSS reduces interference and transmission errors
but provides minimal transmission security.
With FHSS technology, communications between Bluetooth Basic Rate (BR)/EDR devices use 79
different 1 megahertz (MHz) radio channels by hopping (i.e., changing) frequencies about 1600 times
per second for data/voice links and 3200 times per second during page and inquiry scanning. A
channel is used for a very short period (e.g., 625 μs for data/voice links), followed by a hop to another
channel designated by a pre-determined pseudo-random sequence; this process is repeated
continuously in the frequency hopping sequence.
Bluetooth low energy communication uses the same frequency range as BR/EDR devices but splits it
instead into 40 channels of 2 MHz width. Three of these channels are used for advertising
(broadcasting data and for connection setup) and the other 37 are data channels. These 40 channels,
combined with a time division multiple access (TDMA) scheme, provide the two multiple access
schemes for the low energy feature of Bluetooth. A polling scheme is used in which the first device
sends a packet at a predetermined time and a corresponding device responds after a predetermined
interval. These exchanges of data are known as either Advertising or Connection Events.
Bluetooth also provides for radio link power control, which allows devices to negotiate and adjust
their radio power according to signal strength measurements. Each device in a Bluetooth network can
determine its received signal strength indication (RSSI) and request that the other network device
adjust its relative radio power level (i.e., incrementally increase or decrease the transmission power).
This is performed to conserve power and/or to keep the received signal characteristics within a
preferred range.
The combination of a frequency hopping scheme and radio link power control provides Bluetooth
with some additional, albeit limited, protection from eavesdropping and malicious access. The
frequency-hopping scheme, primarily a technique to avoid interference, makes it slightly more
difficult for an adversary to locate and capture Bluetooth transmissions than to capture transmissions
from fixed-frequency technologies, like those used in IEEE 802.11b/g. Research has shown that the
Bluetooth frequency hopping sequence for an active piconet can be determined using relatively
inexpensive hardware and free open source software.
5
The range of Bluetooth BR/EDR devices is characterized by three classes that define power
management. Table 2-1 summarizes the classes, including their power levels in milliwatts (mW) and
decibels referenced to one milliwatt (dBm), and their operating ranges in meters (m).
6
Most small,
battery-powered devices are Class 2, while Class 1 devices are typically universal serial bus (USB)
5
Spill, Dominic and Bittau, Andrea, BlueSniff: Eve meets Alice and Bluetooth, 2007.
6
The ranges listed in Table 2-1 are the designed operating ranges. Attackers may be able to intercept communications at
significantly larger distances, especially if they use high-gain antennas and high-sensitivity receivers.