The local area networks (LAN) can be structured in one of the following network topology:
- BUS topology;
- RING topology;
- STAR topology.
The data transfer on physical layer can be implemented with cable system or as wireless transmission. The cable system can be constructed with:
- twisted-pair cable,
- coaxial cable, or
- optical fiber.
In case of wireless transmission, radio or infrared signals are used. Every station that is connected on a LAN network has an NIC - Network Interface Card or LAN adapter card.
The LAN standards are defined under IEEE 802 standards. The LAN network layers configuration is based on the OSI reference model, as shown on Picture 1. The Data link layer from OSI reference model in LAN networks is divided into two sublayers:
> LLC - Logical Link Control sublayer;
> MAC - Medium Access Control sublayer;
Picture 1: The LAN Network layers based on OSI reference model
The data transferred via LLC sublayer are encapsulated into the LLC frames. The LLC frame configuration is shown on Picture 2. The first byte of the frame is reserved for Destination SAP address and the second byte is for Source SAP address, where SAP stands for Service Access Point. The next 1 or 2 bytes are reserved for control. These three fields are the header of the LLC frame. The rest of the bytes in the frame are reserved for storing data information.
Picture 2: LLC frame
On Picture 3 is represented the packing of the data from the Network layer (as an example on the figure is shown an IP packet) into the data packets from Data Link layer. First, the IP packet is encapsulated into the LLC packet data unit, or LLC frame, then the whole LLC frame is encapsulated into the MAC frame in the fields reserved for the data information. In front of these fields, the MAC header is added, and the packet ends with FCS field, where FCS stands for Frame Check Sequence. The resulting packet is MAC frame. The constitution of the MAC header is different according to the used IEEE 802 substandard, like for example, IEEE 820.3 (Ethernet) standard, IEEE 802.5 (Token-Ring) standard, etc.
Picture 3: MAC frame - Data encapsulation
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