Ethernet Network Layers

Physical Layer

The physical layer determines basic physical attributes of Ethernet, including data coding, time scale, and electrical frequency.

The physical layer is the lowest layer in the Open Systems Interconnection (OSI) reference model and is closest to the physical medium (communication channel) that transmits data. Data is transmitted on the physical layer in binary bits (0 or 1). Transmission of bits depends on transmission devices and physical media, but the physical layer does not refer to a specific physical device or a physical media. Actually, the physical layer is located above a physical medium and provides the data link layer with physical connections to transmit original bit streams.

Data Link Layer

The data link layer is the second layer in the OSI reference model, located between the physical layer and network layer. The data link layer obtains service from the physical layer and provides service for the network layer. The basic service that the data link layer provides is to reliably transmit data from the network layer of a source device to the network layer of an adjacent destination device.

The physical layer and data link layer depend on each other. Therefore, different working modes of the physical layer must be supported by corresponding data link layer modes. This hinders Ethernet design and application.

Some organizations and vendors propose to divide the data link layer into two sub-layers: the Media Access Control (MAC) sub-layer and the Logical Link Control (LLC) sub-layer. Then different physical layers correspond to different MAC sub-layers, and the LLC sub-layer becomes totally independent, as shown in Figure 1.

Figure 1 Hierarchy of Ethernet data link layer

The following sections describe concepts involved in the physical layer and data link layer.