Bluetooth 5 communication process and topologies

Bluetooth wireless is comprised of two wireless technology systems: Basic Rate (BR) and Low Energy (LE or BLE). Nodes can either be advertisers or scanners by this definition:

• Advertiser: Devices transmitting advertiser packets
• Scanner: Devices receiving advertiser packets without the intention to connect
• Initiator: Devices attempting to form a connection

There are several Bluetooth events that transpire in a Bluetooth WPAN:

• Advertising: Initiated by a device to broadcast to scanning devices to alert them of the presence of a device wishing to either pair or simply relay a message in the advertisement packet.
• Connecting: This event is the process of pairing a device and a host.
• Periodic advertising: (for Bluetooth 5) allows an advertising device to periodically advertise over the 37 non-primary channels by channel hopping at an interval of 7.5ms to 81.91875s.
• Extended advertising: (for Bluetooth 5) allows for extended PDUs to support advertisement chaining and large PDU payloads, possibly as well as new use cases involving audio or other multimedia (covered in the Beaconing section of this chapter).

In LE mode, a device may complete an entire communication by simply using the advertising channel. Alternatively, communication may require pair-wise bi-directional communication and force the devices to formally connect. Devices that must form this type of connection will start the process by listening to advertising packets. The listener is called an initiator in this case. If the advertiser issues a connectable advertising event, the initiator can make a connection request using the same PHY channel it received the connectable advertising packet on.

The advertiser can then determine if it wishes to form a connection. If a connection is formed, the advertising event ends and the initiator is now called the master and the advertiser is called the slave. This connection is termed a piconet in Bluetooth jargon and connection events transpire. The connection events all take place on the same starting channel between the master and slave. After data has been exchanged and the connection event ends, a new channel can be chosen for the pair using frequency hopping. 

Piconets form in two different fashions depending on BR/EDR mode or BLE mode. In BR/EDR, the piconet uses three-bit addressing and can only reference seven slaves on one piconet. Multiple piconets can form a union and then be called a scatternet, but there must be a second master to connect to and manage the secondary network. The slave/master node takes on the responsibility of bridging two piconets together. In BR/EDR mode, the network uses the same frequency hopping schedule and all the nodes will be guaranteed to be on the same channel at a given time. In BLE mode, that system uses 24-bit addressing so the number of slaves associated with a master is in the millions. Each master-slave relationship is itself a piconet and can be on a unique channel. In a piconet, nodes may be a master (M), slaves (S), standby (SB), or parked (P). Standby mode is the default state for a device. In this state, it has the option to be in a low-power mode. Up to 255 other devices can be in an SB or P mode on a single piconet.

Note that Bluetooth 5.0 has deprecated and removed parked states in piconets; only Bluetooth devices up to version 4.2 will support a parked state. Standby state is still supported by Bluetooth 5.0.

A piconet topology is illustrated in the following diagram:

The difference between classic (BR/EDR) Bluetooth and BLE piconets. In BR/EDR mode up to seven slaves can be associated on a single piconet due to 3-bit addressing. They all share a common channel between the seven slaves. Other piconets can join the network and form a scatternet only if an associated master on the secondary network is present. In BLE mode millions of slaves can join in multiple piconets with a single master due to 24-bit addressing. Each piconet can be on a different channel but only one slave can associate with the master in each piconet. Practically speaking, BLE piconets tend to be much smaller.