Question

Consider a Bluetooth Piconet with a Master node and six active slaves. Given that the six slaves
follow the same frequency hopping pattern specified by the Master, how do these slaves
communicate with the Master? i.e. send to the Master without interference and also receive,
different information, from the Master.

Answer 1

Bluetooth works using Spread Spectrum Frequency Hopping which is one of the most important reasons why the interference is reduced. Herein, the transmitter continuously changes the transmitting frequency, and selects any one from the available 79 alternate frequencies to transmt data at.

These alternate frequencies fall under the same ISM Bluetooth operation band of 2.4-2.48 GHz (separated by 1MHz each). Each transmitter changes its frequency while communicating at a rate of 1600 hops per second which is 1600 times every second, leading to very less chances that two of the devices would be transmitting at the same frequency, and even if so, will last for less than 0.001 seconds at a time.
The hopping process is automatic and highly randomized, leading to better noiseless transfers.
If however interference even happens for a longer period, the data is held back by the particular transmitter and shared only when the frequencies have changed and no longer match wth the transmitting frequency of any other device.

On top of the above mentioned scenarios, the Adaptive Hop Selection Facility was also introduced, using which noisy channels can be completely ignored by transmitters so as to keep communcation seamless within the 79 bands.

In piconets, another way of ensuring interfernce reduction apart from the above mentioned practices involve giving each slave a unique 3 bit address, and all commincation packets have that unique address associated with them, allowing the packet to reach at the destination intended and not getting lost midway. This address is mentioned in both master to slave and slave to master communication. The 3 bit address is temporary, as long as a slave is active. When disconnected or parked these are given up and a new device can acquire the same once it becomes active or joins piconet.

In real terms, a master connects to a sngle slave at a time, while delivering/receiving data. However, the rate at which it switches to address the other slaves is so high that it appears as if it is communiacating with 6 other devices simultaneously. This is another reason why interfernces are reduced since at a very short snapshot of time, the master-slave communication is almost one to one.

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