Network Basics: OSI Physical Layer

The bottom layer of the Open Systems Interconnection (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 10BaseT 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. The star, bus, ring, and mesh network topologies apply to the Physical layer.

Another aspect of the Physical layer is the electrical characteristics of the signals used to transmit data over the cables from one network node to another. The Physical layer doesn’t define any meaning to those signals other than the basic binary values of zero and one. The higher levels of the OSI model must assign meanings to the bits that are transmitted at the Physical layer.

One type of Physical layer device commonly used in networks is a repeater. A repeater is used to regenerate the signal whenever you need to exceed the cable length allowed by the Physical layer standard. 10BaseT hubs are also Physical layer devices. Technically, they’re known as multiport repeaters because the purpose of a hub is to regenerate every packet received on any port on all of the hub’s other ports.

Repeaters and hubs don’t examine the contents of the packets that they regenerate. If they did, they would be working at the Data Link layer, and not at the Physical layer.

The network adapter (also called a network interface card or NIC) that’s installed in each computer on the network is a Physical layer device. You can display information about the network adapter (or adapters) installed in a Windows computer by displaying the adapter’s Properties dialog box.

To access this dialog box in Windows 7 or Vista, open the Control Panel, choose Network and Internet, choose View Network Status and Tasks, and choose Change Adapter Settings. Then, right-click the Local Area Connection icon and choose Properties from the menu that appears.