The architecture of a LAN can be considered as a set of layered protocols.
In OSI terms, the higher layer protocols are totally independent of the LAN architecture. Hence, only lower order layers are considered for the design of LAN architecture.
The datalink layer of LAN is split into two sub layers.
- Medium Access Control (MAC),
- Logical Link Control Layer (LLC)
LLC Frame Format
Destination Service Access Point (DSAP): IEEE 802.2 header begins with a 1 byte field, which identifies the receiving upper-layer process.
Source Service Access Point (SSAP): Following the DSAP address is the 1-byte address, which identifies the sending upper-layer process.
Control: The Control field employs three different formats, depending on the type of LLC frame used.
- Information (I) frame -- Carries upper-layer information and some control information.
- Supervisory (S) frame -- Provides control information. An S frame can request and suspend transmission, reports on status, and acknowledge receipt of I S frames do not have an Information field.
- Unnumbered (U) frame -- Used for control purposes and is not sequenced. A U frame can be used to initialize secondaries. Depending on the function of the U frame, its Control field is 1 or 2 Some U frames have an Information field.
Data: Variable-length field bounded by the MAC format implemented. Usually contains IEEE 802.2 Subnetwork Access Protocol (SNAP) header information, as well as application-specific data.
MAC Frame Format