Bluetooth Technologies
Bluetooth technology is based on a short-range radio specification defining transmission protocols
between computers and other devices like cell phones and printers [462]. It was initially invented in
1994 by the Swedish L. M. Ericsson Company (who named it after the tenth-century Danish King
Harald Blaatand “Bluetooth” II). In 1998 the Bluetooth Special Interest Group, Inc. (SIG) was founded
by Ericsson, IBM, Intel, Nokia, and Toshiba. Their specifications for wireless connectivity were
published in 1999. The Bluetooth SIG now consists of nearly 2000 companies, including Microsoft,
Lucent, Motorola, and 3COM [463].
Bluetooth and 802.11b Wi-Fi can be thought of in some sense competitors, but in the real world
they are complements of one another, or cousins. Bluetooth-compliant devices are easily upgradable to
work with 802.15 WPAN-compliant devices [463]. Bluetooth’s basic function is to provide a standard
wireless technology to replace the multitude of propriety cables currently linking computing devices
[462]. “The technology is designed to be low cost and low power to preserve the pocketbook and
conserve battery life” [463].
These are the features of the Bluetooth technology:
• It separates the frequency band into hops. This spread spectrum is used to hop from one channel
to another, which adds a strong layer of security.
234 WIRELESS DATA NETWORKS
• Up to eight devices can be networked in a piconet (the Bluetooth and 802.15 designation for
a special personal area network (PAN)).
• Signals can be transmitted through walls and briefcases, thus eliminating the need for line-ofsight.
• Devices do not need to be pointed at each other, as signals are omni-directional.
• Both synchronous and asynchronous applications are supported, making it easy to implement
on a variety of devices and for a variety of services, such as voice and Internet.
• Governments worldwide regulate it, so it is possible to utilize the same standard wherever one
travels [462].
The way Bluetooth devices communicate is similar in concept to the IEEE 802.11b ad hoc
mode. A Bluetooth device automatically and spontaneously forms informal PANs, called piconets,
with other Bluetooth devices. The connection and disconnection of these devices is almost without
any user command or interaction – a capability called unconscious connectivity. A particular Bluetooth
device can be a member of any number of piconets at any moment in time. Each piconet has
one master, usually the device that first initiates the connection. Other participants in a piconet are
called slaves [453]. When only data is being communicated, a master can handle up to seven slaves
in asynchronous connections When only voice is being communicated, a master can handle up to
three slaves in synchronous connections. When both data and voice are being communicated, the
piconet can contain only two devices; the voice connection is synchronous, and the data connection
is asynchronous – taken together, the connection is isochronous. Voice transmission is accomplished
via a 64-kbps Synchronous Connection-Oriented (SCO) link. Data transmission is via 1-Mbps Asynchronous
Connectionless links (ACLs) (in actuality the transmission rate is lower than 1 Mbps) [453].
4.8.1 Bluetooth Protocol Stack
One of the distinct features of Bluetooth is that it provides a complete protocol stack that allows
different applications to communicate over a variety of devices [454]. The Bluetooth protocol stack,
from bottom (physical) to top (applications) includes a radio frequency layer (RF), a baseband layer, a
link management protocol (LMP) layer, a LLC and adaptation protocol layer (L2CAP), a layer of three
side-by-side protocols: the telephony control protocol (TCP) (represented, inexplicably, by “TCS” and
“TSC” in the figures), the RFCOMM protocol, and the service discovery protocol (SDP), and the
applications layer. The RF layer uses the 2.4-GHz ISM band for communications between devices
within 10 m of one another. Bluetooth uses a special FHSS protocol to fully utilize the bandwidth
and reduce interference (each piconet is assigned its own frequency hopping pattern [454]) [453].
The baseband layer specifies coding for the frequency hopping and packet assembly [454]. It also
manages the RF channels, performs error correction and authentication, regulates the SCO and ACL
links, and watches for inquiries from other Bluetooth devices in the vicinity [453]. The LMP layer
keeps track of the status of the devices in the piconet and schedules traffic [454]. L2CAP allows
applications to demand quality of service in terms of bandwidth, latency, and delay variation [453].
The TCP protocol works with cordless telephones and can interface with legacy telecommunication
devices. The RFCOMM protocol provides wireless emulation of RS-232 signal control technology
(“cable replacement”). The SDP protocol determines the characteristics of piconet devices to support
printing, faxing, and teleconferencing [454]. The bottom three layers (RF, baseband, and LMP) are
usually implemented in hardware or firmware. Software is used for the other layers [453].
The LMP and L2CAP layers take care of link setup, authentication, and configuration. In an 802.11
network the terminals can be master (M) or slave (S) terminals. In a Bluetooth network terminals can also be in standby (SB) mode or parked (P), and S terminals can join more than one piconet. An important
issue in a truly ad hoc network is how to establish and maintain all the connections in a network
whose elements appear and disappear in an ad hoc manner, and there is no central unit transmitting
signals to coordinate these terminals. The Bluetooth specification achieves initiation of the network
through a unique inquiry and page algorithm [454]. Initially, all terminals are in SB mode. The first
device to initiate an inquiry becomes the M terminal. The inquiry process registers the SB terminals
as S terminals. The M terminal sends timing information to the S terminals in a page message. The S
terminals can return to power-saving modes: SB, hold, park, or sniff, depending on the device type.
Parked devices do not send transmissions, they only listen to M messages to resynchronize. Sniffing
devices monitor the piconet’s traffic at intervals. Devices on hold status can join other piconets [454].
There can be some interference between Bluetooth’s fast FHSS and 802.11’s slow FHSS and
DSSS. Students at Worcester Polytechnic Institute tested for interference on voice and data channels.
They found that the closer a Bluetooth device is to an 802.11b device, the greater the packet loss
rate for both networks.
4.8.2 Bluetooth Security
Wireless LANs running either 802.11b or Bluetooth have the advantage of being able to work from
virtually anywhere; however, both these technologies depend on open communication from point to
point. Unless access point devices are configured with some level of security, virtually anyone can
connect [458]. Bluetooth uses two secret keys, a 128-bit key for authentication and an 8 to 128-bit
key for encryption, along with a 128-bit random number and 48-bit MAC addresses. The encryption
key length can be selected so that Bluetooth technology can be used in countries where encryption
strength is regulated. A session key (called a link key) is also used between communicating devices,
based on an initialization key, MAC addresses, and a PIN number. This protocol has been shown
to have several vulnerabilities by which a malicious entity could obtain the PIN numbers and keys
depending on how the session initialization of the communication protocol is performed [454].
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