Hardware Reference
In-Depth Information
Connection interval
The time between the beginning of two consecutive connection events. This value
ranges from 7.5 ms (high throughput) to 4 s (lowest possible throughput but also
least power hungry).
Slave latency
The number of connection events that a slave can choose to skip without risking a
disconnection.
Connection supervision timeout
The maximum time between two received valid data packets before a connection
is considered lost.
Because many BLE devices might exist in a given area, or even just for security reasons
(in which the master or the slave might be interested in only a small set of preknown
devices), the Link Layer implements a
white list
feature, which specifies device addresses
of interest to the advertiser or the scanner. Any advertising (if a scanner) or connection
request (if an advertiser) packets received from devices whose Bluetooth Address is not
present in the white list will simply be dropped.
White Lists
An important feature available in BLE controllers,
white lists
allow hosts to filter devices
when advertising, scanning, and establishing connections on both sides. White lists are
simply arrays of Bluetooth device addresses that are populated by the host and stored
and used in the controller.
A device scanning or initiating a connection can use a white list to limit the number of
devices that will be detected or with which it can connect, and the advertising device
can use a white list to specify which peers it will accept an incoming connection from.
The setting that defines whether a white list is to be used or not is called a
filter policy
.
This essentially acts as a switch to turn white list filtering on and off.
Data packets are the workhorse of the protocol and are used to transport user data
bidirectionally between the master and slave. These packets have a usable data payload
of 27 bytes, but additional procotols further up the stack typically limit the actual
amount of user data to 20 bytes per packet, although that logically depends on the
protocol being used.
It is important to note that the Link Layer acts as a reliable data bearer. All packets
received are checked against a 24-bit CRC and retransmissions are requested when the
error checking detects a transmission failure. There is no upper limit for retransmis‐
sions; the Link Layer will resend the packet until it is finally acknowledged by the re‐
ceiver.
