2.1 802.11 Standards  
  2.1.3 IEEE 802.2 LLC review  
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:

  • 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.
  • 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.
  • 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.