The physical layer is level one in the seven level OSI model of computer networking. It performs services requested by the data link layer.

This level is referring to network hardware, physical cabling or a wireless electromagnetic connection. It also deals with electrical specifications, collision control and other low-level functions.

The physical layer is the very simplest, defining only exactly what a bit is: in other words how to transmit a one or a zero. For example, you would specify at this layer things like what shapes the electrical connectors are, what frequencies to broadcast at, and what frequencies are allowed and will not blow up the network cards. In a snail-mail network, that is a network made up of people posting letters to one another, the physical layer is all about how you write and read individual letters of the alphabet.

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

• establishment and termination of a connection to a communications medium;
• participation in the process whereby the communication resources are effectively shared among multiple users, e.g., contention resolution and flow control;
• conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel.

Encodings

Two-binary, one-quaternary

Two-binary, one-quaternary (2B1Q) is a physical layer encoding used for Integrated Services Digital Network basic rate interface. 2B1Q uses four signal levels (1Q), equivalent to two bits (2B). Other encoding techniques are B8ZS, AMI or HDB3.

Physical signaling sublayer

In a local area network (LAN) or a metropolitan area network (MAN) using open systems interconnection (OSI) architecture, the Physical signaling sublayer is the portion of the physical layer that:

• interfaces with the medium access control sublayer
• performs character encoding, transmission, reception, and decoding
• performs optional isolation functions.

Source: from Federal Standard 1037C

Examples

• EIA standards: RS-232, RS-422, RS-423 , RS-449 , RS-485
• ITU standards: V.24, V.35
• ISDN
• T1, E1
• 10BASE-T, 10BASE2, 10BASE5, 100BASE-TX, 100BASE-FX, 100BASE-T, 1000BASE-T, 1000BASE-SX are all physical layer transports for Ethernet
• electricity
• radio