The BlueZ towards a wireless world of penguins

The BlueZ towards a wireless world of penguins
Marcel Holtmann
BlueZ Project
Piconet 1
Piconet 2
The Bluetooth wireless technology is getting
more and more attention. There are a lot of
devices available and most of them are working perfect with Linux, because Linux has the
BlueZ. This is the codename of the official
Bluetooth protocol stack for Linux. It is possible to use Bluetooth for simple cable free serial connections, dialup networks, TCP/IP networks, ISDN networks, human interface devices, printing, imaging, file transfers, contact
and calendar synchronization etc. All these services are designed to integrate seamlessly into
existing and established parts of Linux, like the
kernel TTY layer, the network subsystem, the
CUPS printing architecture, the OpenOBEX library and so on.
The Bluetooth technology was announced in
May 1998 with the goal to create an easy usable cable replacement. Therefor it uses radio transmission within the 2.4 GHz ISM band
to connect mobile devices like mobile phones,
handhelds, notebooks, printer etc. from different manufactures without any cables. But
Bluetooth is more than a simple cable replacement technology and with more and more devices using Bluetooth we see scenarios that are
Figure 1: Bluetooth topology
now making perfect sense. Examples for this
are the communication of mobile phones with
handhelds for exchanging contact and calendar
information. Also the wireless printing of pictures without the interaction of a desktop computer.
Many of these applications are also possible with other technologies like IrDA or
IEEE 802.11 (WiFi), but Bluetooth make the
use a lot easier and defines clear application
The first steps into supporting Bluetooth with
Linux are done by Axis Communications and
they released their OpenBT Bluetooth Stack in
April 1999. Also IBM released its BlueDrekar
which was only available as binary modules.
The problem of both stacks was that they are
character device driven, but the Bluetooth technology is for connecting devices. So it is bet-
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240 • The BlueZ towards a wireless world of penguins
ter to intergrate it into the Linux network layer
and to use the socket interface as primary API.
On May 3, 2001, the Bluetooth protocol stack
called BlueZ which was written by Qualcomm
was released under GPL. This new stack followed the socket based approach. One month
later it was picked up by Linus Torvalds and integrated into the Linux 2.4.6-pre2 kernel. Another Bluetooth stack for Linux was released
by Nokia Research Center in Helsinki and it is
called Affix. The open source community already decided to support BlueZ as official Bluetooth protocol stack Linux and it became one of
the best implementations of the Bluetooth specification.
Bluetooth architecture
The Bluetooth architecture separates between
three different core layers; hardware, host stack
and applications. The hardware consists of radio, baseband and the link manager and this
will be found in Bluetooth chips, dongles and
notebooks. The control of the hardware is
done via the host controller interface (HCI) and
for the communication between the host stack
and the Bluetooth hardware a hardware specific
host transport driver is used. For the USB and
UART transports it is possible to use general
drivers, because these host transport are part of
the Bluetooth specification. For PCMCIA or
SDIO vendor specific driver are needed.
BlueZ implements the host stack and also the
applications. The lowest layer is the logical
link control and adaptation protocol (L2CAP).
This protocol uses segmentation and reassembly (SAR) and protocol and service multiplexing (PSM) to abstract from the Bluetooth
packet types and low-level connection links.
Starting with Bluetooth 1.2 this protocol layers was extended with retransmission and flow
control (RFC).
Logical Link Control and Adaptation Protocol
Host Controller Interface
Link Manager
Figure 2: Bluetooth architecture
All protocol layers above L2CAP are presenting the interaction with the applications. In
general it is possible to use L2CAP directly
(at least with Linux), but this is not specified
within the Bluetooth specification. The real
Bluetooth parts of the host stack are only the
L2CAP layer and the adaption layer which integrates it into other subsystems or protocol
suites, like TCP/IP and OBEX.
Design of BlueZ
The main components of BlueZ are integrated
into the Linux kernel as part of the network
subsystem. It provides its own protocol family
and uses the socket interface. This basic design
makes it easy for application to adapt the Bluetooth technology and the integration is simple
and straight forward. The use of different Bluetooth hardware is handled by the hardware abstraction inside the kernel. The BlueZ core supports the usage of 16 Bluetooth adapters at the
same time. The list of supported devices is
growing every day and currently over 300 different working adapters are known.
Besides the BlueZ core and the hardware abstraction also the L2CAP layer is running inside the kernel. It provides a socket interface
2005 Linux Symposium • 241
Bluetooth Core Layer
USB Subsystem
PCMCIA Subsystem
Serial Subsystem
ery protocol (SDP) or the object exchange protocol (OBEX). Some of them are also directly
integrated into applications. For example the
hardcopy cable replacement protocol (HCRP)
and the audio video distribution transport protocol (AVDTP).
Almost every Linux distribution contains a
Bluetooth enabled kernel and a decent version
of the BlueZ library and the BlueZ utilities.
Figure 3: BlueZ core
with sequential packet characteristics and in future the RFC feature will add the stream interface.
With RFCOMM it is possible to emulate terminal devices. It is called cable replacement for
legacy applications. With BlueZ it is possible
to access this protocol layer from two levels.
One is again a socket interface and this time
with stream characteristics and the second is
through the Linux TTY layer. RFCOMM emulates a full serial port and it is possible to use
the point-to-point protocol (PPP) over it to create dialup or network connections.
The Bluetooth network encapsulation protocol
(BNEP), the CAPI message transport protocol
(CMTP) and the human interface device protocol (HIDP) are transport protocols for the network layer, the CAPI subsystem and the HID
driver. Their main job is to shrink the protocol
overhead and keep the latency low.
All of these protocols are implemented inside
the kernel. Other Bluetooth protocols are implemented as libraries, like the service discovSerial
HID Driver
L2CAP Layer
Bluetooth Core Layer
Figure 4: BlueZ protocols
SCO Audio
Bluetooth configuration
After plugging in a Bluetooth USB dongle or
inserting a Bluetooth PCMCIA card a call to
hciconfig will show the new device.
This device is still unconfigured and like a network card it needs to be activated first. This can
be done via hciconfig hci0 up or in the
background by hcid.
The detailed output shows the Bluetooth device
address (BD_ADDR) and additional information like name, class of device and manufacturer specific details. With hciconfig all of
these settings can be changed.
Now it is possible to scan for other Bluetooth
devices in range. For this and some other actions hcitool is used.
# hciconfig -a
hci0: Type: USB
BD Address: 00:02:5B:01:66:F5 ACL MTU: 384:8 SCO MTU: 64:8
RX bytes:3217853 acl:79756 sco:0 events:199989 errors:0
TX bytes:77188889 acl:294284 sco:0 commands:206 errors:0
Features: 0xff 0xff 0x8f 0xfe 0x9b 0xf9 0x00 0x80
Packet type: DM1 DM3 DM5 DH1 DH3 DH5 HV1 HV2 HV3
Name: ’Casira BlueCore4 module’
Class: 0x3e0100
Service Classes: Networking, Rendering, Capturing
Device Class: Computer, Laptop
HCI Ver: 2.0 (0x3) HCI Rev: 0x77b LMP Ver: 2.0 (0x3)
LMP Subver: 0x77b
Manufacturer: Cambridge Silicon Radio (10)
Figure 5: Local Bluetooth adapter
242 • The BlueZ towards a wireless world of penguins
# hcitool scan
Scanning ...
Pico Plug
Nokia 6210
Bluetooth Printer
Anycom LAP 00:06:C6:C4:08:27
Bluetooth ISDN Access Point
Apple Wireless Keyboard
ELSA Vianect Blue ISDN
Ericsson T39m
Figure 6: Scanning for Bluetooth devices
After the scan the program sdptool can be
used to retrieve the available services of a remote Bluetooth device. The service list identifies the supported Bluetooth profiles and reveals protocol specific information that are
used by other tools. The example shows a
dialup networking service and a fax service
which both are using the RFCOMM channel 1.
# sdptool browse 00:E0:03:04:6D:36
Browsing 00:E0:03:04:6D:36 ...
Service Name: Dial-up networking
Service RecHandle: 0x10000
Service Class ID List:
"Dialup Networking" (0x1103)
"Generic Networking" (0x1201)
Protocol Descriptor List:
"L2CAP" (0x0100)
"RFCOMM" (0x0003)
Channel: 1
Profile Descriptor List:
"Dialup Networking" (0x1103)
Version: 0x0100
Service Name: Fax
Service RecHandle: 0x10001
Service Class ID List:
"Fax" (0x1111)
"Generic Telephony" (0x1204)
Protocol Descriptor List:
"L2CAP" (0x0100)
"RFCOMM" (0x0003)
Channel: 1
Profile Descriptor List:
"Fax" (0x1111)
Version: 0x0100
Figure 7: Requesting service information
Some tools have integrated SDP browsing support and will determine the needed service information by themself. Others don’t have this
capability, because it is not always useful. Every SDP request involves the creation of a piconet and this can fail or timeout. So for all
Bluetooth tools running at boot time this is not
a desired behavior.
With the Bluetooth device address and the
channel number it is possible to setup a RFCOMM TTY terminal connection for using
AT commands or PPP for Internet access.
The command rfcomm bind 0 00:E0:
03:04:6D:36 1 creates the device /dev/
rfcomm0 which is connected to the RFCOMM channel 1 on the mobile phone with
the Bluetooth address 00:E0:03:04:6D:
36. The connection itself is not created by this
command. It will first established when an application, like pppd, opens this device node
and terminated when the last user closes it.
Bluetooth networks
For creating network connection over Bluetooth the preferable method is using a personal area network (PAN) with BNEP. The old
method was called LAN access using PPP and
it used PPP over RFCOMM. This was a bad decision for the performance and now this profile
is deprecated. A PAN connection can be created with the command pand --connect
00:06:C6:C4:08:27 and after a successful
connect ifconfig will show a bnep0 device
with the same MAC address as the BD_ADDR
of the remote device.
This network device is a virtual network card,
# ifconfig -a
bnep0 Link encap:Ethernet HWaddr 00:06:C6:C4:08:27
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:100
RX bytes:4 (4.0 b) TX bytes:0 (0.0 b)
Figure 8: Bluetooth network device
2005 Linux Symposium • 243
but not limited in its functionality. It is possible
to use all Ethernet related commands on it and
besides IPv4 and IPv6 it can also used inside
an IPX network. Even methods like bridging or
network address translation (NAT) are working
without any problems.
Input Subsystem
HIDP Module
Another possibility to create network connections is using ISDN and the CAPI subsystem.
The Bluetooth part is called common ISDN
profile (CIP) and it uses CMTP. Once a connection to an ISDN access point with ciptool
connect 00:04:0E:21:06:FD has been
created, a virtual ISDN card will be presented
by the CAPI subsystem and the standard tools
can be used.
HID Protocol
L2CAP Layer
Figure 9: HID architecture
Printing over Bluetooth
For accessing printers over Bluetooth it is
possible to do this via RFCOMM or HCRP.
The Bluetooth CUPS backend supports both
methods and is able to choose the best
one by itself. The setup of a Bluetooth
printer is very easy. The only thing that
it is needed is an URI and this is created from its BD_ADDR by removing the
colons. For accessing a printer with the Bluetooth device address 00:90:02:63:E0:83
the URI bluetooth://00900263E083/
should be given to CUPS.
Bluetooth input devices
With the human interface device specification
for Bluetooth it is also possible to use wireless mice and keyboards. Since HID devices
can disconnect and reconnect at any time it is
necessary to run hidd --server to handle
such events. To bind a mouse or keyboard to
your system it is only needed to contact it once.
This initial connection can be done with the
command hidd --connect 00:0A:95:
98:37:18. All further connects are initiated
by the device.
Bluetooth audio
The Bluetooth technology can be used for data
communication, but it also support audio connections. For example headsets for voice connection and headphones for high-quality stereo
transmission. For both device types an integration into the ALSA sound system is planned.
Like all other subsystem or library integrations
done by BlueZ so far, this will be almost invisible for the end user. First beta versions of the
ALSA plugins exists by now and a final version
is expected very soon.
244 • The BlueZ towards a wireless world of penguins
Other applications
Many more applications started to integrate native Bluetooth support. The popular examples are the contact and calendar synchronization program MultiSync and the Gnokii tool
for accessing Nokia mobile phones. Both programs can also use IrDA or cable connections
as transport and Bluetooth is only another access method. In most programs the lines of
Bluetooth specific code are very small, but the
mobility increases a lot.
Since 2001 a lot of things have been improved
and the current Bluetooth subsystem is ready
for every day usage. But the development is not
finished and the end user experience can be still
be improved. The GNOME Bluetooth subsystem and the KDE Bluetooth framework are two
projects to integrate Bluetooth into the desktop.
With Bluetooth the need of cables is decreasing
and BlueZ tries to paint the Linux world blue.
[1] Special Interest Group Bluetooth:
Bluetooth Core Specification v1.2, 2003.
[2] Special Interest Group Bluetooth:
Bluetooth Network Encapsulation
Protocol Specification, 2003.
[3] Special Interest Group Bluetooth:
Common ISDN Access Profile, 2002.
[4] Special Interest Group Bluetooth:
Human Interface Device Profile, 2003.
[5] Infrared Data Association (IrDA): Object
Exchange Protocol OBEX, Version 1.3,
Proceedings of the
Linux Symposium
Volume One
July 20nd–23th, 2005
Ottawa, Ontario
Conference Organizers
Andrew J. Hutton, Steamballoon, Inc.
C. Craig Ross, Linux Symposium
Stephanie Donovan, Linux Symposium
Review Committee
Gerrit Huizenga, IBM
Matthew Wilcox, HP
Dirk Hohndel, Intel
Val Henson, Sun Microsystems
Jamal Hadi Salimi, Znyx
Matt Domsch, Dell
Andrew Hutton, Steamballoon, Inc.
Proceedings Formatting Team
John W. Lockhart, Red Hat, Inc.
Authors retain copyright to all submitted papers, but have granted unlimited redistribution rights
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