Ethernet protocols

The Ethernet, as a communication technology that is mostly used in LANs, operates on the first two layers of the OSI reference model. Then, naturally, the protocols like Logical Link Control (LLC), Media Access Control (MAC), and physical (network medium) come into play, as shown in Figure 2.12. Let's talk a little bit about these protocols in the following paragraphs:

• The Logical Link Control (LLC) sublayer is the upper half of Layer 2 (that is, the Data Link layer). Being placed between Layer 3 (that is, the Network Layer) and the MAC sublayer, means that there are many LLC functions. Thus, the LLC sublayer is responsible for isolating the functions occurring in the MAC sublayer from those occurring on Layer 3. Then, LLC defines the additional error-recovery mechanism from that defined by the MAC sublayer. Furthermore, LLC provides flow control so that the computer on destination is not overwhelmed by the delivery of packets. Last but not least, LLC establishes logical connections between the sender and receiving computers.
• The Media Access Control (MAC) sublayer is the lower half of Layer 2. The same as LLC, the MAC sublayer has a considerable number of functions too. When it comes to accessing the network medium by the individual network devices, it is the MAC sublayer that manages it. Another important feature of the MAC sublayer is Layer 2 addressing, which has to do with the process of adding the source and destination MAC addresses to the data packets that are coming from the upper layers, including LLC. Although the Ethernet in its sole nature communicates in the half-duplex channel, it is the responsibility of the MAC sublayer to enable communication in the full duplex channel without causing data loss.
• The Ethernet Physical (Network Medium) layer actually represents Layer 1 of the OSI reference model. From the Ethernet's point of view, in this layer, we are dealing with electrical, optical, and wave properties of physical connections between computers and network devices or between network devices. More precisely, the Ethernet physical layer deals with the physical network mediums such as twisted pair, coaxial, and optical fiber. Also, this layer designates connectors, cable lengths, network devices, topologies, and the number of computers required to form a network: