Page 473 - From GMS to LTE

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Slave 1, 1-slot packet
Slave 3, 3-slot packet
Slave 1, 1-slot packet
Master, 1-slot packet Master, 5-slot packet Slave 2, 3-slot packet Master, 5-slot packet Master, 1-slot packet

Slave 1 Slave 2 Slave 1 Slave 3
625 microseconds

Figure 7.3 Data exchange between a master and three slave devices.
slaves answer on the following hopping frequency of the hopping sequence. The Bluetooth
1.2 specification slightly changes this behavior and thus Bluetooth 1.2‐compliant devices
answer on the same frequency that the master has previously used. The slave sends an
answer packet regardless of whether data is waiting in the buffer to be sent to the master.
If no data is waiting in the slave’s buffer, an empty packet is sent to acknowledge to the
master that the device is still active and accessible. After a maximum number of five
slots, the right to use the channel expires and is automatically returned to the master
even if there is still data waiting in the slave’s output buffer to be sent. Afterward, the
master device can decide whether the channel has to be granted to the same or a differ-
ent slave device. If the master did not receive any user data from the slave and the master’s
output buffer for the particular slave is also empty, it can pause the data transmission for
up to 800 slots to save power. As the duration of a slot is 625 microseconds, 800 slots
equal a transmission pause of 0.5 seconds (see Figure 7.3).
As a slave cannot anticipate when a new packet from a master will arrive, it is not able
to establish a connection to additional devices. Therefore, in some cases it is necessary
that master and slave change their roles during the lifetime of the piconet. This is neces-
sary, for example, if a smartphone has established a connection to a PC to synchronize
data. As the smartphone is the initiator of the connection, it is the master of the piconet.
While the connection is still established, the user wants to use the PC to access a picture
file on another device and thus has to include this device in the piconet. This is only
possible if the smartphone (master) and the PC (slave) change their roles in the piconet.
This procedure is called a ‘master–slave role switch’. After the role switch, the PC is the
master of the piconet between itself and the smartphone. Now, the PC is able to estab-
lish contact with a third device while the connection to the smartphone remains in
place. By contacting the third device and transferring the picture, however, the datarate
between the PC and the smartphone is reduced.

7.4 The Bluetooth Protocol Stack

Figure 7.4 shows the different layers of the Bluetooth protocol stack and will be used in
the following sections as a reference. The different Bluetooth protocol layers can be
only loosely coupled to the seven‐layer OSI model, as some Bluetooth layers perform
the tasks of several OSI layers.