OSI Physical Layer

                In the 8th chapter of the Cisco Networking Academy (CCNA) Exploration Network Fundamental Course, this explains all about the OSI physical layer which is the 1st layer of the Open System Interconnection model. It discusses all about its purpose, its job, its operation, its standards, its fundamental principles, its way of signaling and encoding of the data representing bits, and its connecting communication such as the types of physical/network media that can be used for a specific kind of connection.

The physical layer as stated is providing the means to transport access the network media the bits that make up a Data Link layer frame. This frame is a layer 2 protocol data unit which has been encoded by a data link layer protocol. As the physical layer accepts the frame, it then encodes it as a series of signals that will be transmitted to a media. As what is stated, the purpose of the Physical layer is to create the electrical, optical, or microwave signal that represents the bits in each frame.

In signaling bits for a media, it has the following characteristics to represent the signal. These are: amplitude, frequency and phase. Some of its signaling methods used to represent on the medium are the Manchester encoding, and the Non-return to zero signaling (NRZ). The data/ bit transfer can be measured in three ways: bandwidth, throughput, and the goodput.

This Layer consists of hardware in the form of media, electrical circuitry, and connectors. There are three basic forms of network/physical media on which data is represented. These are: Copper cable, Fiber, and Wireless. Copper wires are the most commonly used for data communications but in local area networks only. Some of these wires are the coaxial cable, unshielded twisted pair cable (UTP) and the shielded twisted pair cable (STP). In UTP cabling, ended with RJ-45 connectors, it has three cable types. These are: Ethernet straight-through, Ethernet crossover and the rollover. The fiber media compared to copper wire can be operated at much greater lengths because of its lower signal loss and being thin but it is more expensive and need more care in handling it.

 Five important keywords:

1.) Manchester Encoding -> bit values are represented as voltage transitions.

2.) Non-Return to Zero (NRZ) -> the bit stream is transmitted as a series of voltage values.

3.) Bit Time -> the time it takes for one bit to be transmitted from a NIC operating at some predefined    standard speed.

4.) Encoding -> method of converting a stream of data bits into a predefined code.

5.) Code Groups -> is a consecutive sequence of code bits that are interpreted and mapped as data bit patterns.