Communication and connection
A master Bluetooth device can communicate with up to seven devices. This
network group of up to eight devices is called a piconet.
A piconet is an ad-hoc computer network, using Bluetooth technology protocols
to allow one master device to interconnect with up to seven active devices. Up
to 255 further devices can be inactive, or parked, which the master device can
bring into active status at any time.
At any given time, data can be transferred between the master and one other
device, however, the devices can switch roles and the slave can become the
master at any time. The master switches rapidly from one device to another in a
round-robin fashion. (Simultaneous transmission from the master to multiple
other devices is possible, but not used much.)
Bluetooth specification allows connecting two or more piconets together to
form a scatternet, with some devices acting as a bridge by simultaneously
playing the master role and the slave role in one piconet. These devices are
planned for 2007.
Many USB Bluetooth adapters are available, some of which also include an IrDA
adapter. Older (pre-2003) Bluetooth adapters, however, have limited services,
offering only the Bluetooth Enumerator and a less-powerful Bluetooth Radio
incarnation. Such devices can link computers with Bluetooth, but they do not
offer much in the way of services that modern adapters do.
Setting up connections
Any Bluetooth device will transmit the following sets of information on
demand:
- Device name.
- Device class.
- List of services.
- Technical information, for example, device features, manufacturer,
Bluetooth specification, clock offset.
Any device may perform an inquiry to find other devices to which to connect,
and any device can be configured to respond to such inquiries. However, if the
device trying to connect knows the address of the device, it always responds to
direct connection requests and transmits the information shown in the list above
if requested. Use of device services may require pairing or acceptance by its
owner, but the connection itself can be started by any device and held until it
goes out of range. Some devices can be connected to only one device at a time,
and connecting to them prevents them from connecting to other devices and
appearing in inquiries until they disconnect from the other device.
Every device has a unique 48-bit address. However these addresses are
generally not shown in inquiries. Instead, friendly Bluetooth names are used,
which can be set by the user. This name appears when another user scans for
devices and in lists of paired devices.
Most phones have the Bluetooth name set to the manufacturer and model of the
phone by default. Most phones and laptops show only the Bluetooth names and
special programs that are required to get additional information about remote
devices. This can be confusing as, for example, there could be several phones in
range named T610 (see Bluejacking).
Pairing
Pairs of devices may establish a trusted relationship by learning (by user
input) a shared secret known as a passkey. A device that wants to
communicate only with a trusted device can cryptographically authenticate the
identity of the other device. Trusted devices may also encrypt the data that
they exchange over the air so that no one can listen in. The encryption can,
however, be turned off, and passkeys are stored on the device file system, not
on the Bluetooth chip itself. Since the Bluetooth address is permanent, a
pairing is preserved, even if the Bluetooth name is changed. Pairs can be
deleted at any time by either device. Devices generally require pairing or
prompt the owner before they allow a remote device to use any or most of their
services. Some devices, such as Sony Ericsson phones, usually accept OBEX
business cards and notes without any pairing or prompts.
Certain printers and access points allow any device to use its services by
default, much like unsecured Wi-Fi networks. Pairing algorithms are sometimes
manufacturer-specific for transmitters and receivers used in applications such
as music and entertainment.
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