The logical link control (LLC) is the highest layer of the IEEE 802
Reference Model. The purpose of the LLC is to exchange data between end users
across a LAN that uses 802-based MAC protocols. The LLC provides identification
of the upper-layer protocol (ULP), data link control functions, and connection
services. It is independent of the topology, transmission medium, and medium
access control techniques used at the MAC and PHY layers. Higher layers, such
as the network layer, pass user data down to the LLC, expecting error-free
transmissions across the network.
The LLC provides the following three
connection services for the ULP:
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Unacknowledged connectionless services – This is the usual
best-effort service of a LAN. Network entities can exchange link service data
units (LSDUs) without the establishment of a data link level connection. The
data transfer can be point-to-point, multicast, or broadcast.
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Acknowledged connection-oriented services – This set of services
provides the means for establishing, using, resetting, and terminating data
link layer connections. This service also provides data link layer sequencing,
flow control, and error recovery, for reliably exchanging LSDUs on the
established connection. Connections are point-to-point.
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Acknowledged connectionless services – The acknowledged
connectionless services provide the means by which network layer entities can
reliably exchange LSDUs, but without the establishment of a data link
connection. The data unit transfer is point-to-point.
These services apply to the communication between peer LLC layers.
Figure
shows the format
of the 802.2 LLC header. The destination and source service access points (DSAP
and SSAP) identify the ULP used, typically a network layer protocol. The
Control field indicates whether the LSDU contains control information or user
data. For user data, sequence numbers are also kept here. When acknowledged,
connection-oriented services are used.