Cisco Networking: OSI Model Layer 1 - Physical

By Edward Tetz from Cisco Networking All-in-One For Dummies

The lowest level of the Open System Interconnection (OSI) model is the physical layer which represents the physical media, such as Category 5e cables and the Ethernet frame type that is used to move the binary data across the network. Because the data is binary, it is composed of two states that represent zeros and ones: Zero is electrically off, and one is electrically on.

With the OSI model, the application layer is not concerned with the physical media of the network; think of it this way, people are not typically concerned with the underlying technology used by their cellphone carriers, such as analog, digital, 1x, Edge, 3G, or 4G, as long as they can make calls. (If you are a geek, you probably want 4G or are already on it.)

So in the cellphone example, you could think of the call dialing and processing features of the cellphone as the application layer, the cellphone’s Electronic Serial Number (ESN) or its Subscriber Identity Module (SIM) as being on the datalink layer, and the service type (3G or 4G) as the physical layer.

The datalink layer cares about the underlying physical service type (a SIM-based phone does not work with ESN-based networks); while at the application layer, it only cares that information it sends down the network layers crosses the network and is able to communicate with the call processing application components at the cellphone carriers’ offices.

The physical networking layer could use any or all the variants (a/b/g/n) of 802.11 wireless technologies, fiber, twisted pairs of copper wire (Cat4, Cat5, Cat6), cable (also copper wire, but different), satellite, or microwave. Again, typically the user of the device does not care what the technology is as long as it moves the data bits at a rate that is sufficient for their needs; for many users, this does not need to be very fast.