The data unit in the LLC level is called the protocol data unit
(PDU). The PDU contains of four fields a destination service access point
(DSAP), a source service access point (SSAP), a control field and an
information field. DSAP, SSAP are addresses used by the LLC to identify the
protocol stacks on the receiving and sending machines that are generating and
using the data. The control field specifies whether the PDU frame is a
information frame (I - frame) or a supervisory frame (S - frame) or a unnumbered
frame (U - frame).
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