Page 476 - From GMS to LTE

462 From GSM to LTE-Advanced Pro and 5G

The payload field follows the ACL header and is composed of the following fields. The
first bits of the payload header field again contain some administrative information.
The first field is called the logical channel (L_CH) field. It informs the receiver if the
payload field contains user data (Logical Link Control and Adaptation Protocol (L2CAP)
packets, see Section 7.4.5) or Link Manager Protocol (LMP) signaling messages (see
Section 7.4.3) for the administration of the piconet.
The flow bit is used to indicate to the L2CAP layer above that the receiver buffer is
full. Finally, the payload header includes a length field before the actual payload part is
transmitted. After the actual payload, an ACL packet ends with a 16‐bit CRC checksum.
As no bandwidth is guaranteed for an ACL connection, this type of data transmission
is not well suited to the transmission of bidirectional real‐time data such as a voice con-
versation. For this kind of application, the baseband layer offers a second transmission
mode, which uses synchronous connection‐oriented (SCO) packets. The difference
between this and ACL packets is the fact that SCO packets are exchanged between a
master and a slave device in fixed intervals. The interval is chosen in a way that results
in a total bandwidth of exactly 64 kbit/s.
When an SCO connection between a master and a slave device is established, the
slave device is allowed to send its SCO packets autonomously even if no SCO packet is
received from the master. This can be done very easily as the timing for the exchange of
SCO packets between two devices is fixed. Therefore, the slave does not depend on a
grant from the master, and thus it is implicitly ensured that only this slave sends in the
timeslot. This way, it is furthermore ensured that the slave device can send its packet
containing voice data even if it has not received the voice packet of the master device.
The header of an SCO packet is equal to the header of an ACL packet with the excep-
tion that the flow, ARQN and SEQN fields are not used. The length of the payload field
is always 30 bytes. Depending on the error correction mechanism used, this equals 10,
20 or 30 user data bytes. Table 7.3 gives an overview of the different SCO packet types.
The last line of the table shows a special packet type, which can contain both SCO and
ACL data. This packet type can be used to send both voice data and signaling messages
at the same time. As shown in Section 7.6.3, for the headset profile, an SCO connection
between a mobile phone and a headset requires not only a speech channel but also a
channel for signaling messages (e.g. to control the volume). The SCO voice data can
then be embedded in the first 10 bytes of a ‘DV’ packet, which are followed by up to 9
bytes for the ACL channel. The FEC and the checksum are applied only to the ACL part
of the payload.
It has to be noted that it is not mandatory to use DV packets if voice and data have to
be transmitted simultaneously between two devices. Another possibility is to use

Table 7.3 SCO packet types.

Packet type Number of slots Link type Payload (bytes) FEC CRC

0101 1 HV1 10 1/3 No
0110 1 HV2 20 2/3 No
0111 1 HV3 30 None No
1000 1 DV 10 (+0–9) 2/3 Yes