chapter 8

OSI Physical Layer

      Encoding of the binary digits to represent Data Link layer frames into signals and to transmit and receive these signals across the physical media (copper wires, optical fiber, and wireless) that connect network devices are the roles of OSI Physical layer.  It provides the means to carry across the network media the bits that make up a Data Link layer frame. This layer accepts a complete frame from the Data Link layer and encodes an of series of signals that are transmitted into the local media.

      The purpose of the Physical layer is to create the electrical, optical, or microwave signal that represents the bits in each frame. The media does not carry the frame as a single entity. The media carries signals, one at a time, to represent the bits that make up the frame. In identifying a frame, the devices on the media would not recognize when a frame has been fully received. In that case, the destination device would only receive a string of signals and would not be able to properly reconstruct the frame.

      Physical layer contains the three fundamental functions, the physical components, data encoding and signaling. Encoding is a method of converting a stream of data bits into a predefined code. Manchester Encoding, bit values are represented as voltage transitions. While Signaling  it  must generate the electrical, optical, or wireless signals that represent the "1" and "0" on the media. NRZ Signaling a simple method only suited for slow speed data links. NRZ signaling uses bandwidth inefficiently and is susceptible to electromagnetic interference.

      Symbols is a technique of encoding that use bit patterns. Code groups represent encoded data or control information. It is a consecutive sequence of code bits that are interpreted and mapped as data bit patterns. The symbols encoded onto the media are all unique. The symbols representing the data being sent through the network have different bit patterns than the symbols used for control. And these differences allow the Physical layer in the receiving node to immediately distinguish data from control information. The code groups have three types of symbols, Data symbols that represent the data of the frame as it is passed down to the Physical layer. Control symbols it is a special codes injected by the Physical layer used to control transmission. These include end-of-frame and idle media symbols. And the Invalid symbols that have patterns not allowed on the media. The receipt of an invalid symbol indicates a frame error.

Different physical media support the transfer of bits at different speeds. It can be measured in three ways: Bandwidth it is a capacity of a medium to carry data is described as the raw data bandwidth of the media. Throughput is the measure of the transfer of bits across the media over a given period of time. And Goodput to measure the transfer of usable data.

      In this chapter it identifies the different characteristics of the UTP (Unshielded Twisted Pair) cable, STP and Coaxial cable. UTP cabling, terminated with RJ-45 connectors, is a common copper-based medium for interconnecting network devices, such as computers, with intermediate devices, such as routers and network switches. STP uses two pairs of wires that are wrapped in an overall metallic braid or foil. The coaxial cable design has been adapted for different purposes. Coax is an important type of cable that is used in wireless and cable access technologies. Coax cables are used to attach antennas to wireless devices. The coaxial cable carries radio frequency (RF) energy between the antennas and the radio equipment.