The Bluetooth Protocol

Bluetooth connects devices wirelessly in ranges of 1 to 100 meters, depending on power. There are several versions of Bluetooth: 1.0, 1.1, 1.2, and 2.0. (The Java API for accessing a Bluetooth device is the same regardless of version.) Bluetooth 1.0, 1.1, and 1.2 devices can reach speeds of 723 kilobits per second. Version 2.0 accelerates this to 2.1 megabits per second. In other words, Bluetooth is fast enough for network devices, but not fast enough for disk drives and monitors.

Because Bluetooth is wireless, it is less secure than wired alternatives such as USB connections (which themselves aren as secure as is often thought). With the right equipment, its possible to sniff Bluetooth communications one isn meant to access. To prevent this, pairs of devices may require a shared secret passkey and may encrypt data passed back and forth between them. However, many devices don bother to do this.

Each Bluetooth controller can talk to up to seven different Bluetooth devices in a "master/slave" configuration. Among the eight devices, each device takes a turn at being the master while the other seven are slaves, in rotating order. The master is responsible for picking frequencies on which to communicate and deciding when to change them. The group of devices, one master and up to seven slaves, is called a piconet . In the future, it may be possible for piconets to be joined in larger scatternets . However, this is not yet possible.

Each Bluetooth device has a fixed 6-byte address, such as 00-13-c2-00-0d-23 or 00-0a-95-09-5a-59. Theoretically this address is unique, but conflicting addresses have shipped in real products. Each device also has a more human-friendly name, such as "elharos mouse" or "WACOM pen tablet." Users and manufacturers can change the names, and name conflicts are possible.

Every device also has a 3-byte class identifier that is divided into four parts, as shown in Figure 25-2:

  • The first two bits are always 0.
  • The next six bits are a little-endian int for the minor device class.
  • The next five bits are a little-endian int for the major device class.
  • The final 11 bits are flags identifying the type of the device. For instance, if bit 22 is 1, its a telephony device. If its 0, it isn .

Figure 25-2. Bluetooth class identifier layout, in big-endian form as seen by Java


The currently defined major device class codes are shown in Table 25-1. Note that the numbers given are as Java reports them. Bits 812 define the major class. However, rather than interpreting this as a 5-bit number, Java represents it as a 13-bit number in which the low-order eight bits are always 0. (If you prefer, you can think of this as a 32-bit number in which all bits except 812 are 0.)

Other Bluetooth APIs in other languages can and do provide different representations of the bit patterns in the class identifier shown in Figure 25-2. For instance, the Apple System Profiler presents major device classes as numbers from 0 to 31 (5-bit unsigned ints) instead of numbers from 0 to 7936. The following tables are valid for the Java Bluetooth API. They may not be valid in other environments.


Table 25-1. Bluetooth major device classes

Decimal

Hexadecimal

Device type

0

0x0000

Miscellaneous devices

256

0x0100

Computers and PDAs

512

0x0200

Phones, including modems and faxes

768

0x0300

LAN adapters, routers, and network access points

1024

0x0400

Audio/video devices (headsets, speakers, televisions, DVRs, etc.)

1280

0x0500

Input peripherals (mice, joysticks, keyboards, graphics tablets, etc.)

1536

0x0600

Imaging devices (printers, scanners, cameras, monitors, etc.)

1792

0x0700

Wearable devices

2048

0x0800

Toys

7936

0x1F00

Uncategorized: anything for which the Bluetooth Special Interest Group (SIG) has not yet defined a standardized code (e.g., GPS locators or laboratory probes)


Minor device class codes depend on the major code. For example, the peripherals class has the 10 minor device classes shown in Table 25-2. Although the minor device class code is logically a 6-bit number, Java represents it as a 1-byte number, the first two bits of which are always 0.

Table 25-2. Bluetooth peripheral minor device classes

Decimal

Hexadecimal

Type

0

0x00

Uncategorized

4

0x04

Joystick

8

0x08

Gamepad

12

0x0C

Remote control

16

0x10

Sensing device

20

0x14

Digitizer tablet

24

0x18

Card reader

64

0x40

Keyboard

128

0x80

Pointing device (mouse, trackball, etc.)

192

0xC0

Keyboard/mouse combination (0x80 | 0x40)


While each device has exactly one major class and exactly one minor class, a device may support multiple services. For instance, a combination cell phone/PDA might be both a telephony device and an object transfer device. Table 25-3 lists the service classes and their associated bit fields. Each service a device supports is indicated a by a single bit in the class identifier. Java reports this as a 24-bit number in which the first 13 bits are always 0.

Table 25-3. Bluetooth service classes

Bit

Decimal

Hexadecimal

Service class

13

8192

0x2000

Reserved

14

16384

0x4000

Reserved

15

32768

0x8000

Reserved

16

65536

0x10000

Positioning

17

131072

0x20000

Networking

18

262144

0x40000

Rendering

19

524288

0x80000

Capturing

20

1048576

0x100000

Object Transfer

21

2097152

0x200000

Audio

22

4194304

0x400000

Telephony

23

8388608

0x800000

Information


For example, a GPS-enabled cell phone would have the major class 0x0200 and the minor class 0x04 and would support the positioning (0x10000) and telephony (0x400000) services. Therefore, its class ID would be 0x0200 | 0x04 | 0x10000 | 0x400000, or 0x410204.

Many devices support one or more standard profiles that offer particular types of services. These include the Serial Port Profile for streaming connections, the Basic Printing, Video Conferencing, File Transfer, Cordless Telephony, Fax, and Personal Area Network Profiles, and several dozen more.

Each profile communicates using a specified protocol. The lowest-level protocol is a packet-based protocol called the Logical Link Control and Adaptation Protocol (L2CAP). This is analogous to IP in the TCP stack. That is, other higher-level protocols are built on top of L2CAP and provide additional services. For instance, the RFCOMM protocol assembles L2CAP packets into input and output streams. If L2CAP is like IP, RFCOMM is like TCP. The Object Exchange (OBEX) protocol is a still higher-level protocol for exchanging binary data over RFCOMM. Stretching the analogy to the breaking point, if L2CAP is IP and RFCOMM is TCP, OBEX is HTTP. Different profiles and devices can plug into this stack wherever they find convenient, as shown in Figure 25-3. The Java Bluetooth API supports all three. There are also several additional protocols the Java Bluetooth API does not support, such as Bluetooth Network Encapsulation (BNEP) and the A/V Control Protocol.

Figure 25-3. Bluetooth protocol stack



Basic I/O

Introducing I/O

Output Streams

Input Streams

Data Sources

File Streams

Network Streams

Filter Streams

Filter Streams

Print Streams

Data Streams

Streams in Memory

Compressing Streams

JAR Archives

Cryptographic Streams

Object Serialization

New I/O

Buffers

Channels

Nonblocking I/O

The File System

Working with Files

File Dialogs and Choosers

Text

Character Sets and Unicode

Readers and Writers

Formatted I/O with java.text

Devices

The Java Communications API

USB

The J2ME Generic Connection Framework

Bluetooth

Character Sets





Java I/O
Java I/O
ISBN: 0596527500
EAN: 2147483647
Year: 2004
Pages: 244
Similar book on Amazon

Flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net