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Bluetooth networks are ad hoc networks—no network infrastructure (e.g., an access point) is
needed. Instead, Bluetooth devices must organize themselves into a piconet of up to eight active
devices, as shown in Figure 7.16. One of these devices

Figure 19: A Bluetooth piconet

is designated as the master, with the remaining devices acting as clients. The master node truly
rules the piconet—its clock determines time in the piconet (e.g., determines TDM slot
boundaries), it determine the slot-to-slot frequency hopping sequence, it controls entry of client
devices into the piconet, it controls the power (100 mW, 2.5mW, or 1 mW) at which client
devices transmit; and uses polling to grant clients permission to transmit once admitted to the
network.

In addition to the active devices, there can also be up to 255 “parked” devices in the piconet.
These parked devices are often in some form of “sleep mode” to conserve energy (as we saw
with 802.11 power management) and will awaken periodically, according to the master’s
schedule, to receive beacon messages from the master.

A parked device cannot communicate until its status has been changed from parked to active by
the master node. Because Bluetooth ad hoc networks must be self-organizing, it’s worth looking
into how they bootstrap their network structure. When a master node wants to form a Bluetooth
network, it must first determine which other Bluetooth devices are within range; this is the
neighbor discovery problem.
The master does this by broadcasting a series of 32 inquiry messages, each on a different
frequency channel, and repeats the transmission sequence for up to 128 times.

A client device listens on its chosen frequency, hoping to hear one of the master’s inquiry
messages on this frequency. When it hears an inquiry message, it backs off a random amount of
time between 0 and 0.3 seconds (to avoid collisions with other responding nodes, reminiscent
of Ethernet’s binary backoff) and then responds to the master with a message containing its
device ID. Once the Bluetooth master has discovered all of the potential clients within range, it
then invites those clients that it wishes to join the piconet. This second phase is known as
Bluetooth paging, and is reminiscent of 802.11 clients associating with a base station. Through
the paging process, the master will inform the client of the frequency-hopping pattern to be
used, and the sender’s clock.
The master begins the paging process by again sending 32 identical paging invitation messages,
each now addressed to a specific client, but again using different frequencies, since that client
has yet to learn the frequency-hopping pattern.

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