Product Specs

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A central is a device that initiates connections to peripherals. As such, it becomes a

master when connected. Just like a peripheral, a central needs to have both a

transmitter and a receiver. We explained before that since the E-RL 2 has adopted the

role of peripheral device the smartphone will need to accept the role of central in this

system. The role that is already normal for smartphone’s since most other BLE

peripherals connected to the smartphone are peripheral’s, for example smartwatches or

other body sensors that measure vital signs while cycling. A device can support

multiple GAP roles at the same time. For example, a device can be a broadcaster and a

peripheral at the same time.

1.1.6 Pairing and Bonding

Just a quick write up on the difference between pairing and bonding, since these terms

get used interchangeably. This has to do with the usage of ‘pairing’ in Bluetooth

Classic, or BR/EDR.

As far as Bluetooth LE is concerned, pairing and bonding are two very distinct things.

The short explanations are that pairing is the exchange of security features each device

has, and creating temporary encryption for the live cycle of the connection. Bonding is

the exchange of long term keys after pairing has occurred, and storing those keys

for later use. Pairing is not the creation of permanent security between devices, which

is called bonding. Pairing is the mechanism that allows bonding to occur.

Pairing

Pairing is the exchange of security features. This includes things like i/o capabilities,

requirement for man-in-the-middle protection, etc. The client side begins this

exchange. The client essentially says ‘hey, i’d like it if you had these features’. The

server replies, ‘yeah, well, this is what I can do’. Once this exchange is made, the

security that will be used has been determed. For example, if a server supports just

noInput/noOutput for i/o capabilities, the Just Works pairing mechanism is going to be

used. Once the pairing feature exchange is complete, a temporary security key is

exchanged and the connection is encrypted, but only using the temporary key. In this

encrypted connection, long term keys are exchanged. These keys are things like the

(long term) encryption key to encrypt a connection, and also things like a digital

signature key. The exact keys exchanged are determined by the security features of

each device.

Bonding

This really just means that after the pairing features exchange and the connection has

been encrypted (these two together are called ‘pairing’), and keys have been

exchanged, the devices store and use those keys the next time they connect. Keys can