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I'm considering using Bluetooth as a wireless solution for a power saving system. Today I was told that there may be a 8 device limit for Bluetooth.

I cannot find any information about this for Bluetooth in general. Does this statement hold any truth and if it does what type of limitation is it e.g. specification, software, hardware?

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2 Answers 2

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The number of devices that can be paired is practically unlimited – as long as you have enough storage for every device's MAC address and link key.

In a piconet, one master can actively communicate with seven other devices (limited by a 3-bit address), and up to 255 devices can be part of the piconet but inactive ("parked").

Bluetooth communications are not limited to building piconets, however. More often the connection is between exactly two devices (for example, file transfers between two phones). In this case, since the full 48-bit MAC address is used, the master could juggle many such connections at once, within hardware limitations.

Another possibility is a scatternet, where multiple piconets are linked by a shared device. Wikipedia says, however, that "currently there are very few actual implementations of scatternets due to limitations of Bluetooth and the MAC address protocol."

If you choose your own hardware, also consider ZigBee, which is another wireless networking specification with less restrictive limits and more efficient power use:

Because ZigBee nodes can go from sleep to active mode in 30 msec or less, the latency can be low and devices can be responsive, particularly compared to Bluetooth wake-up delays, which are typically around three seconds. Because ZigBee nodes can sleep most of the time, average power consumption can be low, resulting in long battery life.

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ZigBee is very interesting and at face-value appears to be a better fit for my purpose. I will definitely take it into consideration! Thank you. –  Simon Campbell Sep 8 '11 at 7:56

A single Bluetooth device can connect to seven other devices simultaneously. See this excerpt from the Wikipedia article on Bluetooth, under Communication and connection:

A master Bluetooth device can communicate with a maximum of seven devices in a piconet (an ad-hoc computer network using Bluetooth technology), though not all devices support this limit.

This limit lies in the specification and, as above, some devices do not support this many connections.

Note, however, that a device may have an arbitrary number of other devices registered for Bluetooth communication; the foregoing limitation is with regard to the number of devices connected at one time.

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Seven devices at the same time. The number of pairings is only limited by the memory available for storing link keys. –  grawity Sep 7 '11 at 7:03
    
@grawity That's what I was hoping to hear. This is for a software design (maybe I should have mentioned that?). I have control over the master device and to some extent the slave devices (through standards proposal). If the device limit can be lifted easily via software on the master device then that's fine! –  Simon Campbell Sep 7 '11 at 7:30

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