Section 8.2.3 provides additional details about MTUs.
Figure 8.2 shows the ACL link between the two devices. The baseband
layer of the protocol stack establishes the ACL link. There is exactly one ACL
link between two Bluetooth devices communicating with each other. The
ACL link and the baseband functions provide the infrastructure needed to
support the high-level, ??????logical??™??™ abstractions of L2CAP channels and L2CAP
packets that L2CAP presents to higher-level protocols and applications.
L2CAP packets have to be converted into one or more baseband
packets for transmission over the ACL link. The receiving device then
reassembles these baseband packets into L2CAP packets. There are various
sizes of baseband packets, but the largest payload is 339 bytes. This
is much smaller than the largest payload possible for an L2CAP packet,
65,535 bytes. Because baseband packets are much smaller than packet
sizes used by higher-level protocols and applications, the segmentation
and reassembly process hides the details about Bluetooth baseband packets
from the higher levels of the stack and from applications. By presenting
abstractions such as L2CAP channels and L2CAP packets to higher
levels, L2CAP makes it easier for higher-level protocols and applications
to use Bluetooth communications. This adaptation function is one of
the important contributions of L2CAP.
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