GAP (Advertising and Connections) - Getting Started with Bluetooth Low Energy - page 37

asymmetric , which means that the computing requirements of the Link Layer mas‐
ter are larger than the ones of a Link Layer slave. The central role is usually played
by a smartphone or tablet in the network, because it has access to powerful CPUs
and memory resources. This allows it to maintain connections to multiple devices.
The central starts by listening for other devices' advertising packets and then ini‐
tiates a connection with a selected device. This process can be repeated to include
multiple devices in a single network.

Peripheral
The peripheral role corresponds to the Link Layer slave. This role uses advertising
packets to allow centrals to find it and, subsequently, to establish a connection with
it. The BLE protocol is optimized to require few resources for peripheral imple‐
mentation, at least in terms of processing power and memory. This paves the way
to a large market of inexpensive BLE peripherals.
Each particular device can operate in one or more roles at a time, and the specification
imposes no restrictions on this regard.
Many developers mistakenly try to associate the BLE GATT client and server roles with
GAP roles. There is no connection between those at all, and any device can be a GATT
client, server, or both, depending on the application and situation.
Consider, for example, a fitness tracker paired with a smartphone. The fitness tracker's
GAP role is peripheral , and it acts as a GATT server when the phone requests data from
its sensors. It can also sometimes act as a GATT client when it requests accurate time
data from the smartphone to update its internal clock for data timestamping. The GATT
client/server roles depend exclusively on the direction in which the data requests and
responses transactions flow, whereas GAP roles stay constant as peripheral for the fitness
tracker and central for the smartphone.

Modes and Procedures
Table 3-1 shows GAP modes and their applicable procedures (modes that do not have
a natural counterpart procedure are marked with “N/A”).

Table 3-1. Modes and their applicable procedures

Mode Applicable Role(s) Applicable Peer Procedure(s)

Broadcast Broadcaster Observation

Non-discoverable Peripheral N/A

Limited discoverable Peripheral Limited and General discovery

General discoverable Peripheral General discovery

Non-connectable Peripheral, broadcaster, observer N/A

Any connectable Peripheral Any connection establishment