＝OSI Model and Transmission Path＝
　A．OSI 7-Layer Model
　B．Signals and Network Media
　C．Data Link Layer
　D．Bridges and Repeaters
　E．ARP and ARP tables
　F．Topologies and Access Methods
　 Related Items
　C-1　Roles of the data link layer
　C-2　Two sublayers of the data link layer
　C-3　Standards for the data link layer
　・　Exercise
　


　C-2　Two sublayers of the data link layer
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　C-2-1　The data link layer is divided into two sublayers.
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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.


Figure C-2-1 Two sublayers of the data link layer



　C-2-2　Logical Link Control (LLC layer)
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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.


Figure C-2-2 Fram construction example of LLC (Logical Link Control sublayer) (Unit: bit)




　　The meaning of each data
　　　≪SAP:Service Access Point≫


Service Access Point is a value to indicate which upper layer protocol data a frame data is.
LLC decides which protocol to pass the data next, based on the value of SAP.
　　　≪Data≫

The header of the upper layer, and the actual information to send and receive



　C-2-3　Media Access Control (MAC layer)
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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.

　　　　　≪Source≫
Adds the control information such as the destination MAC address and source MAC address to the information passed from the LLC, assembles it as a frame and passes to the physical layer as a bit string.
　　　　　≪Destination≫
Receives the bit string decoded from the physical layer, disassembles the frame, deletes the control information and passes only the information to the LLC layer.


Figure C-2-3 Frame construction of MAC (Media Access Control layer) (Unit: bit)



　C-2-4　MAC address (Media Access Control address)
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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.


Figure C-2-4　MAC address construction (Unit: bit)


　　　　　≪I/G (Individual/Group)≫

The individual address or group address can be identified here.

　　　　　≪U/L (Universal/Local) ≫

The global address or local address can be identified here.

　　　　　≪Maker code≫

This is the identification number assigned to each maker producing NICs.
It is also called the OUI (Organizationally Unique Identifier) code.
*NOTE*
Everyone can refer to OUI codes on the IEEE homepage. The address is as follows:
http://standards.ieee.org/regauth/oui/index.shtml 。


The following figure shows a part of OUI codes.
　　　　　≪Product number≫
This is the number assigned to each device by the maker.