The physical layer is part of the lower layer in the OSI model. It is also the first layer of the OSI model. It can be considered as the hardware level. This is because it is concerned primarily with the physical makeup and aspects of the network and the media through which data is transmitted. The layer provides insight into such things as the interface and connectors. More over, the layer also defines the mediums (cables) to be used. For this reason, the transport layer describes the physical means through which data may be sent over devices in the network. It is used to define what electrical, optical and mechanical including procedural and functional terms may used for the purposes of setting in motion, sustaining and disengaging the physical links between the communicating network systems.
That is, the requirements necessary for conveying a flow of bits over a network. Consequently, it is used to identify such things as pinouts. Accordingly, in the physical layer the characteristics of the voltage levels, timing of the voltage changes, the rates of physical data, maximum transmission distances, and the physical connectors are defined. As such, the layer comprises of a series of components. These include, the network interface card (NIC), adapters that are used to connect the physical interfaces and the media, plus the components of the cabling system. Again, it is at this physical layer that converters, from one media to another, operate.
The physical layer applications are then grouped as either Wide Area Network (WAN) or Local Area Network (LAN) specifications. Hence, this physical layer may define the cable or the medium to be used as thicknet, unshielded twisted pairs or even thinnet. While all media that may be used are considered to be comparable as far as functionality is concerned, in the LAN scenario, what is referred to as the unshielded twisted pair (category 5e UTP) cable is used. This is considered as the best cable for use in the connection of individual devices. Still, fiber optic cables can be used.
However, they are often used in cases, where there are riser links or vertical backbone links. Different media may be used depending on their cost to install and maintain and their convenience. Nonetheless, it is worth noting that the physical layer is only one of the parts of a Local Area Network. That is, a LAN is made up of multiple parts working together.
