Page 486 - From GMS to LTE

472 From GSM to LTE-Advanced Pro and 5G

a single ACL connection. Thus it is possible, for example, to open a second L_CH
between a PC and a mobile phone to exchange an address book entry, while a Bluetooth
dial‐up connection is already established which connects the PC to the Internet via the
mobile phone. If further ACL connections exist to other devices at the same time,
L2CAP is also able to multiplex data to and from different devices. Such a scenario is
shown in Figure 7.11. While a dial‐up connection is established to slave 1, a file is transmitted
over the same connection, and an MP‐3 data stream is simultaneously received from
slave 2.
An L2CAP connection is established from the host device by sending an L2CAP_
Connection_Request message to the Bluetooth controller. The most important param-
eter of the message is the protocol service multiplexer (PSM). This parameter decides
which higher layer the user data packets are to be sent to once the L2CAP layer is
established. For most Bluetooth applications, PSM 0x0003 is used to establish a connec-
tion to the RFCOMM layer. This layer offers virtual serial connections to other devices
for application layer programs and is described in more detail in Section 7.4.7.
Furthermore, the L2CAP_Connection_Request message contains a connection identity
(CID) which is used to identify all packets of a particular L2CAP connection. The CID
is necessary, as the RFCOMM layer can be used by several applications at the same
time, and thus the PSM is only unique during the connection establishment phase. If
the remote device accepts the connection, it returns an L2CAP_Connection_Response
message and also assigns a CID, which is used to identify the L2CAP packets in
the reverse direction. Later, the connection is fully established and can be used by the

Dial-up File MP-3 stream
connection transmission

Bluetooth master

Air interface

Bluetooth slave 1 ACL Bluetooth slave 2
connections
(time multiplex)

L2CAP connections

Dial-up File MP-3 stream
connection transmission

Figure 7.11 Multiplexing of several data streams.