C-2 Two sublayers of the data link layer
C-2-1 The data link layer is divided into two sublayers.
In IEEE802, the data link layer is divided into two sublayers (Logical Link Control (LLC) sublayer and Media Access Control (MAC) sublayer).
This allows to use the header standard in common in each standard because there are various standards (such as Ethernet, token ring and FDDI) for data links.
This section describes the two sublayers and the important items called MAC address.
C-2-2 Logical Link Control (LLC layer)
Logical Link Control sublayer which is the upper one of the sublayers of the data link layer is a protocol for the data transfer between terminals connected to a LAN.
The Logical Link Control considers the following when sending and receiving a frame between terminals: (1) whether there are not dropouts in all the frames received, and (2) whether it could be received as the correct information. And it asks the source to resend the frame when there were errors in the received frame.
In addition, When the destination is not ready to receive normally, the source temporarily postpones sending and extends the interval to resend.
C-2-3 Media Access Control (MAC layer)
Media Access Control sublayer is the lower one of the sublayers of the data link layer.
The processing of the Media Access Control sublayer is as follows.
C-2-4 MAC address (Media Access Control address)
MAC address (Media Access Control address) is an identification code to identify each device in the network at the data link layer.
The MAC address is built into each device connected to the LAN and is called "physical address", "hardware address" and "Ethernet address".
In the case of PCs, the MAC address is built into not the PC itself but the NIC (Network Interface Card) in the PC. Therefore, note that the address changes when the NIC is replaced.
The following figure shows the MAC address construction.