Layer 1: The Physical Layer

The bottom layer of the OSI Model is the Physical Layer. It addresses the physical characteristics of the network, such as the types of cables used to connect devices, the types of connectors used, how long the cables can be, and so on. For example, the Ethernet standard for 100BaseT cable specifies the electrical characteristics of the twisted-pair cables, the size and shape of the connectors, the maximum length of the cables, and so on.

Another aspect of the Physical Layer is that it specifies the electrical characteristics of the signals used to transmit data over cables from one network node to another. The Physical Layer doesn't define any particular meaning for those signals other than the basic binary values 0 and 1. The higher levels of the OSI model must assign meanings to the bits transmitted at the Physical Layer.

One type of Physical Layer device commonly used in networks is a repeater. A repeater is used to regenerate signals when you need to exceed the cable length allowed by the Physical Layer standard or when you need to redistribute a signal from one cable onto two or more cables.

An old-style 10BaseT hub is also a Physical Layer device. Technically, a hub is a multi-port repeater because its purpose is to regenerate every signal received on any port on all the hub's other ports. Repeaters and hubs don't examine the contents of the signals that they regenerate. If they did, they'd be working at the Data Link Layer, not at the Physical Layer. Which leads us to ….