Have you ever troubleshot a network issue for hours, racking your brain, only to find out that someone pulled a cable slightly out of the port? This chapter focuses on problems at layer 1—the physical layer—and how this layer is overlooked when network problems are experienced. A common example is a cable with a loose connection when troubleshooting another issue. I once left a network down for two days before actually looking at the port to determine the issue, which was a cable with a loose connection. It is very easy for you to blame your commercial carrier, but before you do so, you should exhaust all fault possibilities. This chapter discusses the importance of the physical medium in network design. Topics begin with the physical medium, including transmission media such as copper, coaxial cable, fiber optic cable, and the standards associated with each. Next, the Ethernet, duplex communication systems, autonegotation, Unidirectional Link Detection (UDLD), and common issues associated with layer 1 are covered.