Overview

In this chapter we present an overview of the first layer of the OSI model, namely, the physical layer. This is the "touch-and-feel" layer. It provides for the physical transmission of data. As part of our presentation we discuss various transmission media, including twisted pair, coaxial, and fiber-optic cables. We also examine various forms of wireless and satellite communications.

1. What is the physical layer?

   The physical layer is the lowest layer (layer 1) of the OSI Reference Model. The OSI model was discussed in Chapter 2.

2. What does the physical layer do?

   Before sending data on the network, the physical layer on the local node must process the data stream, translating frames received from the data link layer (layer 2) into electrical, optical, or electromagnetic signals representing 0 and 1 values, or bits. A frame is a specially formatted sequence of bits that incorporates both data and control information. (Frames are discussed in more detail in Chapter 5.) The local physical layer is responsible for transmitting these sequences of bits through the network medium to the physical layer of the remote node, where frames are reconstructed and passed to the data link layer there.

3. If you were to use one word to describe the physical layer what would it be?

   Wire. Or, if you are from the state of Texas (U.S.A.), it's called "wharr".

4. Wire?

   Yup. Actually, it's a little more than just wire, but wire gives you an idea of what we are dealing with...