Page 514 - From GMS to LTE

500 From GSM to LTE-Advanced Pro and 5G

reach a compromise between data throughput, power consumption and latency
depending on the application:
Interval: The time between two connection events. In Figure 7.30 the interval is set to
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81 milliseconds.
Window size: This is the period during a connection event in which master and slave
●
can exchange data packets. In the example the window size is set to 3.75 ms. This
means that during a period of 81 milliseconds, the radio part of the BLE device only
needs to be active for approximately 5% of the time. If no data needs to be exchanged
during a connection interval this is reduced even further. Selecting such a short win-
dow size compared to the relatively long connection interval means that the resulting
overall datarate will also be very low.

In addition the Connect Request message contains a number of other parameters that
control the subsequent periodic data exchange such as:
Timeout: The interval after which a connection is considered broken if no packets
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have been received.
Latency: The number of connection events a slave is allowed to skip.
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Channel map: If a master device detects that there is considerable interference on
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some of the 2 MHz channels it can exclude them from the hopping pattern that is
used to distribute consecutive connection events over the ISM band.
Hop: Defines the hopping pattern.
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If connection requirements concerning latency, data throughput and power
consumption change during the lifetime of a connection it is possible for the master
to change these parameters later on with an ‘LL_Connection_Update_Request’
message.
After the Connection Request message, master and slave enter bidirectional com-
munication during each connection event. During the communication window, master
and slave alternatively send data packets with a short gap in between until neither side
has further data to transmit during the window. The next connection event will then
use a different 2 MHz channel as defined by the hopping pattern.
It should be noted at this point that an advertiser can accept connection requests
from any master device or only from a particular device, should the device only want to
be contacted by a previously bonded device. In this case the advertising packet contains
details about which device is allowed to connect.

7.7.4 BLE Authentication, Security and Privacy
While there are many BLE applications in which any device should be able to use ser-
vices and to retrieve information from a device, there are also scenarios in which it is
required that only certain devices are allowed to connect. In such scenarios it is also
required that data is encrypted before being sent over the air. This is implemented in
Bluetooth Low Energy as follows.
When two devices establish a connection they are paired temporarily. To control
future connectivity a bonding procedure is required during which devices authenticate
themselves to each other. Afterward, security keys are generated and stored perma-
nently in flash memory or other non‐volatile memory on the devices. As in ‘classic’