The physical layer is the lowest layer. This layer provides mechanical, electrical, and other functional tools to enable or disable physical connections, maintain them, and transmit bits over them. This layer thus corresponds to the underlying hardware. Protocols of this layer specify the characteristics of the interfaces, such as port properties, allowable transmission rates, signal levels and electrical coding of the individual bits (e.g., modems modems). In addition, it must be resolved at this level in which way a single bit should be transmitted.
This means the following: In computer networks today information is usually transmitted in the form of bit or symbol sequences. In the copper cable and in radio transmission, however, modulated high-frequency electromagnetic waves are the information carriers, in the optical waveguide light waves of a specific or different wavelength. The information carriers do not know any bit sequences, but can accept far more different states than just 0 or 1. Therefore, an encoding must be defined for each transmission type. This is done by means of the specification of the physical layer of a network.
Hardware on this layer: repeaters, hubs, wires, connectors, and other.
Common representatives of these protocols are RS-232 (serial port) and X.21.
The Ethernet protocol describes both layer 1 and layer 2, on which CSMA / CD is used as access control.
Protocols and standards based on other Layer 2 protocols and standards: HDLC, SDLC, DDCMP, IEEE 802.2 (LLC), ARP, RARP, STP
Protocols and standards based directly on layer 1: IEEE 802.11 (WLAN), IEEE 802.4 (token bus), IEEE 802.5 (token ring), FDDI