Physical Layer [PHY]. Layer 1 of the OSI protocol stack

Physical Layer: OSI Layer 1; Defines the physical [hardware] implementation and the electrical [signal level] implementation of the bus; network cabling, connector type, pin-out, physical data rates, maximum transmission distances, and data transmission encoding. At this layer information is placed on the physical network medium. RS-232, and RS422 are examples of a physical layer specification. Protocol Data Unit [PDU] is called a Bit at this layer.

The Physical layer performs services requested by the Data Link Layer. The major functions and services performed by the physical layer are: (a) establishment and termination of a connection to a communications medium; (b) participation in the process whereby the communication resources are effectively shared among multiple users, e.g., contention resolution and flow control; and, (c) conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel.

Other physical layer specifications are listed on the Electronic Bus page.

All current interface buses are serial buses, parallel buses are in decline. Keep in mind that a serial bus, like PCIe, may have a number of redundant serial interfaces that carry more data. The PCIe bus may appear to be a parallel bus, but it's really a stacked serial bus. The PCIe interface is a serial bus [1x], or two parallel serial buses [2x], four serial buses [4x], or eight serial buses working in parallel to increase the over-all speed of the interface.