Page 479 - From GMS to LTE

Bluetooth and Bluetooth Low Energy 465

DQPSK modulation is a mandatory feature of the Bluetooth 2.0 standard, 8DPSK has
been declared an implementation option. Thus, it is not possible to derive the maximum
possible speed of a device by merely looking at the Bluetooth 2.0 + EDR compliancy.
Today, most Bluetooth 2.0 devices are capable of sending 3‐DH5 packets.
Apart from ACL, SCO and eSCO packets for transferring user data, there are a number
of additional packet types that are used for the establishment or maintenance of a
connection.
ID packets are sent by a device before the actual connection establishment to find
other devices in the area. As the timing and the hopping sequence of the other device
are not known at this time, the packet is very short and contains only the access code.
Frequency‐hopping synchronization (FHS) packets are used for the establishment of
connection during the inquiry and paging phases, which are further described below.
An FHS packet contains the 48‐bit device address of the sending device and timing
information to enable a remote device to predict its hopping sequence and thus to allow
connection establishment.
NULL packets are used for the acknowledgment of a received packet if no user data is
waiting in the output buffer of a device that could be used in the acknowledgment
packet. NULL packets do not have to be acknowledged and thus interrupt the mutual
acknowledgment cycle if no further data is to be sent.
An additional packet type is the POLL packet. It is used to verify if a slave device is
still available in the piconet after a prolonged time of inactivity due to lack of user data
to be sent. Similar to the NULL packet it does not contain any user data.

7.4.2 The Link Controller
The link control layer is located on top of the baseband layer previously discussed. As
the name suggests, this protocol layer is responsible for the establishment, maintenance
and correct release of connections. To administrate connections, a state model is used
on this layer. The following states are defined for a device that wants to establish a con-
nection to a remote device.
If a device wants to scan the vicinity for other devices, the link controller is instructed
by higher‐layer protocols to change into the inquiry state. In this state, the device starts
to send two ID packets per slot on two different frequencies to request for listening
devices with unknown frequency‐hopping patterns to reply to the inquiry.
If a device is set by the user to be detectable by other devices, it has to change to the
inquiry scan state periodically and scan for ID packets on alternating frequencies. The
frequency that a device listens to is changed every 1.28 seconds. To save power, or to
be able to maintain already ongoing connections, it is not necessary to remain in the
inquiry scan state continuously. The Bluetooth standard suggests a scan time of 11.25
milliseconds per 1.28‐second interval. The combination of fast frequency change of the
searching device on the one hand and a slow frequency change of the detectable device
on the other hand results in a 90% probability that a device can be found within a scan
period of 10 seconds.
To improve the time it takes to find devices, version 1.2 of the standard introduces the
interlaced inquiry scan. Instead of listening only on one frequency per interval, the
device has to search for ID packets on two frequencies. Furthermore, this version of
the standard introduces the possibility to report the signal strength (RSSI) with which