Page 480 - From GMS to LTE

466 From GSM to LTE-Advanced Pro and 5G

the ID fame was received to higher layers. Thus, it is possible to sort the list of detected
devices by the signal strength and to present devices that are closer to the user at the top
of the list. This is especially useful if many devices are in close proximity such as during
an exhibition. In this environment, it can become quite difficult to send an electronic
business card to a nearby device, as the result of the scan often reveals the presence of
several dozen devices and it is necessary to scroll through a long list. If the list is ordered
on the signal strength, however, it is very likely that the response of the device that
should receive the electronic business card is received with a high signal level because
of its closer proximity to the sender and that device is thus presented at the top of
the list.
If a device receives an ID packet, it returns an FHS packet, which includes its address,
frequency hopping and synchronization information.
After receiving an FHS packet, the searching device can continue its search.
Alternatively, the inquiry procedure can also be terminated to establish an ACL con-
nection with the detected device by performing a paging procedure.
To be detectable, master devices can also enter the inquiry scan state from time to
time. Thus, it is possible to detect and connect to them even if they are already engaged
in a connection with another device. It has to be noted, however, that some devices like
mobile phones do not support this optional functionality.
If a user wants its device to remain invisible, it is possible to deactivate the inquiry
scan functionality. Thus, a device can only initiate a paging procedure and thus a con-
nection with the user’s device if it already knows the device’s hardware address. It is
useful to activate this setting once a user has paired all devices (see Section 7.5.1) that
are frequently used together. In this way, the devices of the user remain invisible to the
rest of the world but are still able to establish connections with each other. This drasti-
cally reduces the opportunity for malicious attacks on Bluetooth devices, which may try
to take advantage of security holes of some Bluetooth implementations [4].
To establish an ACL connection by initiating a paging procedure, a device must be
aware of the hardware address of the device to be connected to either from a previous
connection or as a result of an inquiry procedure. The paging procedure works in a
similar way to the inquiry procedure, that is, ID packets are sent in a rapid sequence on
different frequencies. Instead of a generic address, the hardware address of the target
device is included. The target device in turn replies with an ID packet and thus enables
the requesting device to return an FHS packet that contains its hopping sequence.
Figure 7.8 shows how the paging procedure is performed and how the devices enter the
connected state upon success.
The power consumption of a device that is not engaged in any connection and thus
only performs inquiry and page scans at regular intervals is very low. Typically, the
power consumption in this state is less than 1 mW. As mobile phones have a battery
capacity of typically 4000–5000 mWh, the Bluetooth functionality has only a small
effect on the standby time of a mobile device.
After successful paging, both devices enter the connection‐active state and data
transfer can start over the established ACL connection.
During connection establishment, it can happen that the slave device is master of
another connection at the same time. In such cases, the Bluetooth protocol stack
enables the device to indicate during connection establishment that a connection is
possible only if a master–slave role change is performed after establishment of the