Physical layer

Layer 1: Physical layer

The Physical layer defines all the electrical and physical specifications for devices. In particular, it defines the relationship between a device and a physical medium. This includes the layout of the pins,voltages,hubs,cable specifications, repeaters, network adapters, Host Bus Adapters(HBA) and more.

To understand the function of the physical layer in contrast to the functions of the data link layer, think of the physical layer as concerned primarily with the interaction of a single device with a medium, where the data link layer is concerned more with the interactions of multiple devices (i.e., at least two) with a shared medium. The physical layer will tell one device how to transmit to the medium, and another device how to receive from it (in most cases it does not tell the device how to connect to the medium). Obsolescent physical layer standards such as the RS-232 do use physical wires to control access to the medium.

The major functions and services performed by the physical layer are:

• Establishment and termination of a connection to a communication medium.
  
• Participation in the process whereby the communication resources are effectively shared among multiple users. For example, flow control and contention resolution.
• The Modulation, or conversion between the representation of the digital data in user equipment and the corresponding signals transmitted over a communications channel. These are signals operating over the physical cabling (such as copper and the optical fiber) or over a radio link.