Chapter 8 - Physical Layer

What is an OSI Physical layer? It’s the layer 1 in the OSI model. Physical layer is the one who transmit binary data over media between devices. Physical layer protocols define media specification. In other words, the OSI Physical layer provides the means to transport across the media the bits that make up a Data Link layer frame. When this layer accepts the complete frame from the Data Link layer, then this layer encodes it as a series of signals that are transmitted onto the local media. And there are three basic forms of network media on which the bits are represent depend on the type of media. These are the copper cable which the signals are patterns of electrical pulses; the fiber which the signals are patterns of light; the wireless media which the signals are patterns of radio transmissions.

                There are appropriate standards in this layer that governing the hardware that relevant electrical pulses electrical and communications engineering organizations. The TCP/IP standards are set by the Internet Engineering Task Force (IETF) in RFCs. The technologies of this layer are defined by these organizations: International Organization for Standardization (ISO), Institute of Electrical and Electronics Engineers (IEEE), American National Standards Institute (ANSI), International Telecommunication Union (ITU), Electronics Industry Alliance/Telecommunications Industry Association (EIA/TIA), and the National telecommunications authorities such as the Federal Communication Commission (FCC) in the USA. These organizations defined that this layer standards are: Physical and electrical properties of the media; mechanical properties (materials, dimensions, pinouts) of the connectors; bit representation by the signals (encoding); and definition of control information signals.

                The three fundamental functions of the Physical layer are: the physical components; data encoding; and signaling.

The physical elements are the electronic hardware devices, media and connectors that transmit and carry the signals to represent the bits.

The encoding function is to convert a stream of data bits into a predefined code. The Manchester Encoding is a sample even though it represents bits as voltage transition and not efficient enough to be used at higher signaling speeds, it is the signaling method employed by 10BaseT Ethernet.

The signaling is when the physical layer generates the electrical, optical, or wireless signals that represent the "1" and "0" on the media. The signaling method can be amplitude, frequency, or phase. The NRZ signaling is a simple signaling method, its bit stream are transmitted into a series of voltage.

                In calculating the data transfer. These are the three ways: Bandwidth which is typically measured in kilobits per second (kbps) or megabits per second (Mbps); Throughput is the measure of the transfer of bits across the media over a given period of time; Goodput is the measure of usable data transferred over a given period of time, and is therefore the measure that is of most interest to network users.

The physical media or the connecting communication are the copper media, the unshielded twist pair(UTP), the coaxial cable, the other copper media, the fiber media,the wireless media and the media connectors.