Bluetooth network topology

Bluetooth is a short-range wireless standard in the 2.4 GHz ISM band. Devices form ad hoc networks with two topologies: the piconet and the scatternet.

Piconet

A piconet is a network of one master and up to seven active slaves (8 active devices, with up to 255 more devices parked). The master sets the frequency-hopping sequence and the clock, and all slaves synchronise to it. Communication is master-slave and time division duplex (625 us slots): the master transmits in even slots, and a slave answers in the following odd slot only when addressed. Slaves cannot talk directly to each other; all data goes through the master.

            S1
            |
     S3 --- M --- S2          M = master
            |                 S = active slave
            S4

Each piconet uses its own hopping sequence (derived from the master's address), so several piconets can coexist in the same area.

Scatternet

A scatternet is formed when two or more piconets overlap and are connected by a bridge device (a device that is a member of more than one piconet). The bridge may be a slave in both piconets, or a master in one and a slave in the other. It shares its time between the piconets, since it can be active in only one at a time. This extends the range and the number of devices beyond 8.

   S1   S2        S5   S6
    \   /          \   /
     M1 ---- B ---- M2
    /   \          /   \
   S3   S4        S7   S8

   piconet 1        piconet 2        B = bridge (slave in both)

A device cannot be the master of two piconets at once. Scatternets allow flexible, larger networks such as sensor and logger networks.