Page 477 - From GMS to LTE

Bluetooth and Bluetooth Low Energy 463

independent ACL packets in slots that are not used by the SCO connection. Finally, a
third possibility for sending both ACL and SCO information between two devices is to
drop the SCO information of a slot and to send an ACL packet instead.
As CRC and FEC are not used for SCO packets, it is not possible to detect whether the
user data in the payload field was received correctly. Thus, defective data is forwarded
to higher layers if a transmission error occurs. This produces audible errors in the
reproduced voice signal. Furthermore, the bandwidth limit of 64 kbit/s of SCO connec-
tion prevents the use of this transmission mechanism for other types of interactive
application such as audio streaming in MP3 format, which usually requires a higher
datarate. Bluetooth 1.2 thus introduces a new packet type called eSCO, which improves
the SCO mechanism as follows.
The datarate of an eSCO channel can be chosen during channel establishment.
Therefore, a constant datarate of up to 288 kbit/s in full‐duplex mode (in both direc-
tions simultaneously) can be achieved.
The eSCO packets use a checksum for the payload part of the packet. If a transmis-
sion error occurs, the packet can be retransmitted if there is still enough time before
the next regular eSCO packet has to be transmitted. Figure 7.7 shows this scenario.
Retransmitting a bad packet and still maintaining a certain bandwidth is possible, as an
eSCO connection with a constant bandwidth of 64 kbit/s uses only a fraction of the
total bandwidth available in the piconet. Thus, there is still some time in which to
retransmit a bad packet in the transmission gap to the next packet. Despite transfer-
ring the packet several times, the datarate of the overall eSCO connection remains
constant. If a packet cannot be transmitted by the time another regular packet has to
be sent, it is simply discarded. Thus, it is ensured that the data stream is not slowed
down and the constant bandwidth and delay times required for audio transmissions
are maintained. Bluetooth 2.1 introduced an option to forward erroneous packets to
higher layers with an error indication. This might be useful if a codec can correct small
transmission errors by itself.
For some applications such as wireless printing or transmission of large pictures from
a camera to a PC, the maximum transmission rate of Bluetooth up to version 1.2 is not
sufficient. Thus, the Bluetooth standard was enhanced with a high‐speed data transfer
mode called Bluetooth 2.0 + EDRs. The core of EDR is the use of a new modulation
technique for the payload part of an ACL or eSCO packet. While the header and the
payload of the packet types described before are modulated using GFSK, the payload of
EDR ACL and eSCO packets are modulated using DQPSK or 8DPSK. These

eSCO transmission interval

Error t
Transmission
Transmission pause Renewed transmission
during a transmission pause
Reception
Figure 7.7 Retransmission of an eSCO packet caused by a transmission error.