Encoding, Modulation, and the Physical Layer

The Physical layer doesn't get any respect. This negligence results partly from the fact that there are few end-user interventions once a network's cabling or wireless infrastructure is successfully put into place. The actual behavior of electromagnetic signals and raw digital sequences is generally built into hardware interfaces and is beyond any ordinary fiddling or tuning. Nevertheless, a bit of education about the Physical layer can help prepare you for a deeper understanding of new wireless, optical, and local-loop technologies that will have important future roles in every network.

The Physical layer is responsible for turning some medium into a bit pipe. Copper cabling is the most common data-networking medium of LANs and in the local loop, while fiber optic cabling is prevalent in most wide area networks and is perhaps the local-loop and the LAN medium of the future. Radio frequency bands also play a role in some data networks, including signals that are relayed via satellites. Some local networks and point-to-point links employ infrared signals propagated through space, and visible light lasers are sometimes used to transmit data without the aid of a fiber optic cable.

Each medium has its strengths, vulnerabilities, and quirks. Cost is obviously an important factor, which explains the prevalence of copper cable in today's infrastructures. Performance, in the form of reliable throughput, is closely related to cost. The rapidly increasing price/performance of fiber optic cabling accounts for the growing hegemony of that medium as it migrates from interstate...