content: entries on bluetooth

zk/Bluetooth.md

@@ -2,6 +2,7 @@
 tags:
   - networks
   - procedural
+  - bluetooth
   - Linux
 ---
 

zk/Bluetooth_frequency_hopping.md

@@ -0,0 +1,20 @@
+---
+tags:
+  - bluetooth
+  - networks
+  - radio
+---
+
+# Bluetooth frequency hopping
+
+Bluetooth transmits on the 2.5GHz frequency band. As this is an unlicenced band
+there can be interference to the signal and interception.
+
+To get around this, bluetooth engages in **frequency hopping**. It rapidly
+switches its channel of transmission between 79 distinct frequences within the
+2.5GHz band where each channel occupies a bandwidth of 1MHz.
+
+Each available band is divided into time slots. This allows multiple devices to
+communicate simultaneously without interfering with each other (that much). The
+[master device](./Bluetooth_piconet.md) controls the sequencing and
+synchronisation of the time slots.

zk/Bluetooth_piconet.md

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+---
+tags:
+  - bluetooth
+  - networks
+---
+
+# Bluetooth piconet
+
+Bluetooth devices connect via a micro-network known as a "piconet". Devices
+connect to each other in a master/slave relationship (since renamed
+"central/peripheral").
+
+Prior to the establishment of a piconet, devices remain in discoverable/scanning
+mode.
+
+At least one master and one slave is required to make a piconet however a master
+can have multiple slaves simultaneously (e.g. a computer with keyboard, mouse,
+speakers etc).
+
+The master controls the connection timing and frequency modulation (necessary
+for the Bluetooth protocol) and the slave synchronises its clock with the
+master's. A slave can have its own controls (e.g. a set of headphones with
+playback controls) but it is the master that orchestrates the action.
+
+What does this mean?
+
+Take a mouse as a slave that is sending the click data to the master it is
+paired with. The mouse's microcontroller controls the click and what it "means"
+(although this is handled at a level up, in the application layer) but _when_
+the data is processed is managed by the master. The master controls:
+
+- when devices can transmit
+- what frequency they use
+- the timing and synchronisation of the data exchanged
+
+So in other words, the mouse waits for the master to give it a timeslot once the
+click has occurred. Then it's data is handed off to the application responsible
+on the master device.