Bluetooth; How networks are formed and controlled

Information from
Johnson Consulting

Bluetooth - An Overview

How networks are formed and controlled

The Various States of Bluetooth Units

Bluetooth supports both point-to-point and point-to- multi-point connections. Several piconets can be established and linked together ad hoc, where each piconet is identified by a different frequency hopping sequence. All users participating on the same piconet are synchronized to this hopping sequence.

Before any connections in a piconet are created, all devices are in STANDBY mode. In this mode, an unconnected unit periodically "listens" for messages every 1.28 seconds. Each time a device wakes up, it listens on a set of 32 hop frequencies defined for that unit. The number of hop frequencies varies in different geographic regions; 32 is the number for most countries.

The connection procedure for a non-existent piconet is initiated by any of the devices, which then becomes master of the piconet thus created. A connection is made by a PAGE message being sent if the address is already known, or by an INQUIRY message followed by a subsequent PAGE message if the address is unknown.

In the initial PAGE state, the master unit will send a train of 16 identical page messages on 16 different hop frequencies defined for the device to be paged (slave unit). If there is no response, the master transmits a train on the remaining 16 hop frequencies in the wake-up sequence. The maximum delay before the master reaches the slave is twice the wakeup period (2.56 seconds) while the average delay is half the wakeup period (0.64 seconds).

The INQUIRY message is typically used for finding Bluetooth devices, including public printers, fax machines and similar devices with an unknown address. The INQUIRY message is very similar to the page message, but may require one additional train period to collect all the responses.

A power saving mode can be used for connected units in a piconet if no data needs to be transmitted.

The master unit can put slave units into HOLD mode, where only an internal timer is running. Slave units can also demand to be put into HOLD mode. Data transfer restarts instantly when units transition out of HOLD mode. The HOLD is used when connecting several piconets or managing a low power device such as a temperature sensor.

In the SNIFF mode, a slave device listens to the piconet at reduced rate, thus reducing its duty cycle. The SNIFF interval is programmable and depends on the application.

In the PARK mode, a device is still synchronized to the piconet but does not participate in the traffic. Parked devices have given up their MAC address and occasionally listen to the traffic of the master to re-synchronize and check on broadcast messages.

Bluetooth Operational States

The illustration at rigt shows the 4 states that a Bluetooth unit can exist in. The "Connecting State" is transitory, but the other three have no limit to their duration. The "sniffing" corresponds to the Inquiry exchange, where the Master looks for a unit to which it does not have the address. The search criteria in such a case would be certain attributes, such as "looking for a laser writer that can handle post-script" or the like.

Letting Master and Slave trade places

In principle, the unit that creates the piconet becomes the Master. However, a Master-Slave switch can take place when a Slave wants to become a Master. These 2 units then have to reverse their TX and RX timing; a so-called TDD switch.

However, since the piconet parameters are derived from the device addresses and clock of the Master, a Master-Slave switch involves a redefinition of the piconet as well; a piconet switch.

The new piconet´s parameters are derived from the former Slave´s device address and clock.

As a consequence of this piconet switch, other slaves in the piconet, not involved in the switch, have to be logically moved to the new piconet. This means changing their timing and their hopping scheme. A rather complex procedure.