CHAPTER 8 – OSI PHYSICAL LAYER

          The discussion in this chapter (Chapter 8) is all about the OSI model’s layer 1 which is the Physical Layer and describes its purpose and functions. Before knowing deeper about the Physical Layer, let us recall that the upper layers of the OSI layers protocol prepare data from the human network for transmission from its source to its destination. The upper layers of the OSI model are implemented in software and the Physical layer is implemented in hardware. All communication from the human network becomes binary digits, which are transported individually across the Physical media.

          The Physical layer has three fundamental functions: a) physical components, includes the electronic hardware devices, media connectors that carry the signals to represent the bits; b) encoding, a method of converting data bits into a predefined code; c) signaling, the method of representing the bits.

          This chapter described also the standards of the Physical layer. These standards are defined by the organizations such as ISO, IEEE, ITU, ANSI, and etc. It also discussed about some kinds of copper media connectors such as coaxial cable, unshielded twisted-pair (UTP) cable, and RJ-45 connections, and also the safety accompanying the copper media in dealing with electrical hazards and fire hazards.

          To fully understand this chapter, here are some important points that are related to this OSI model’s Physical layer:

THE PHYSICAL LAYER
          The Physical layer consists of hardware in the form electronic circuitry, media, and connectors. And the function of the Physical layer protocol is to control how data is placed on the communication media. So, the Physical layer encodes the bits into the signals for a particular medium and it must also distinguish where one frame ends and where the next frame begins or else the devices on the media will not recognize whether it fully received the frame and not be able to reconstruct that frame. The Physical layer represents each of the bits in the frame as a signal.

THE MEDIA
          A media is a physical medium in which data is transferred. And the three basic forms of network media on which data is transferred are: copper cable, fiber, and wireless. And let us always remember that the media does not carry the frame as a single entity, in fact, the media carries signals one at a time to represent that bits that make up the frame.


THE BIT TIME
          The time that takes for a bit to be transmitted from a network adapter that is operating at some predefined standard speed.

THE SIGNALING METHODS or THE TWO WAYS TO ENCODE BITS AS VOLTAGES
          The nature of the signal representing the bits on the media depends on the signaling method that is used. The signaling method must have a standard agreement so that the receiver can detect the signals and decode them so that the communication between the media will not fail. One example of signaling is the NRZ (Non Return to Zero), in here, the bit stream is transmitted as a series of voltage values and these values includes the 0 which represents a low voltage and the 1 represents the high voltage. When there are long strings of 1s or 0s are being transmitted then the receiving nodes can not resynchronize the bit times with the transmitting nodes.
Another example of the signaling method is the Manchester Encoding. Manchester Encoding is different from NRZ. Unlike NRZ which only represents bits as voltages high or low, Manchester Encoding represents voltage transitions. In other words, 1 represents the transition bits from low voltage to high voltage, and 0 represents the transition from high voltage to a low voltage.

THE CODE GROUP
          A code group is a consecutive sequence of code bits that are interpreted and mapped as data bit patterns. It is often used as an intermediary encoding technique for higher speed LAN technologies. It is also a set of encoded symbols. Symbols on the other hand, is an encoding techniques that use bit patterns. Reducing bit level error, limiting the effective energy transmitted into the media, helping to distinguish data bits from control bits, and better media error detection are the advantages of using code groups.

THE DATA CARRYING CAPACITY
          Bandwidth is the amount of data that can be transmitted in a certain amount of time. It is usually expressed in bits per second (bps), either kilobits per second (kbps) or megabits per second (mbps) in digital bandwidth. Throughput is the measure of the transfer of bits across the media over a given period of time. Goodput is used to measure the transfer of usable data in a given period of time.


          In general, the Layer 1 of the OSI model is responsible for the physical interconnection of devices across the network. The Physical layer protocol encoded the bits and decodes it at the destination after transmission. To transmit data, bits are represented as signals using signaling methods over the different media. And there are the standards considered for the Physical layer, and these standards are defined by different organizations.