Datacom Chapter 8 ( The OSI Physical Layer )

Chapter 8

 The OSI Physical Layer

OSI was define as the international standardization program created by ISO and ITU-T to develop standards for data networking that facilitate multivendor equipment interoperability and OSI model has seven layer that provide guidelines in representation of network operation namely: Physical, Data Link, Network, Transport, Session, Presentation, and Application. In this chapter it discusses the general function of physical layer as well as the standard and protocol that manage the transmission of data across the media. It introduces the signal and the purpose of the Physical Layer which is to create the electrical, optical, or microwave signal that represents the bits in each frame. Media as we all know that it is a single cable connecting one computer to another like UTP where the signal was sent or carried. There are three basic forms of network media namely: Copper cable, Fiber and wireless. Data transfer can be measured in three ways: Bandwidth- which is the amount of data that can be transmitted in a certain amount of time, Throughput- which is the amount of digital data per tome unit that is delivered from one node to another or the actual network performance, and the last which is the Goodput- define as the number of useful bits per unit of time from a certain source address to a certain destination, excluding protocol overhead, and excluding retransmitted data packets or it is measure of the transfer of usable data after protocol overhead traffic has been removed. Physical Layer is concerned with network media and signaling figure below shows some of the characteristic of networking media.

Copper is one of the basic and common network media being used in networking. Here are some types of copper media: the Coaxial cable is a single conductor that runs through the center of the cable that is encased by, but insulated from, the other shield, the RJ-45 connections which is used widely in LANs with one type of media and in some WANs with another media type, and unshielded twisted-pair cable used in Ethernet LANs, consist of four pairs-coded wires that have been twisted together and then encased in a flexible plastic sheath. The purpose outer jacket part of the unshielded twisted pair (UTP) cable is to protects the wire copper from physical damage, while the twisted pair is to protect the signal from interference and the color-coded plastic insulation is an electrically isolates wires from each other and identifies each pair. UTP cable with RJ-45 is a common copper- based medium for interconnecting networks devices. There are three main cable types that are obtained by using specific wiring conventions namely: Ethernet Straight-through its standard arrangement is both end T568A and both end T568B and it connect to a network host to a network device such as a switch or hub, Ethernet Crossover its standard arrangement is one end T568A other end T568B and it connecting two network hosts or intermediary devices, and the last cable types is the rollover which has the standard of cisco proprietary and it connect a workstation serial port to a router console port using an adaptor. There are also two other types of copper cable the coaxial and shielded twisted pair. Fiber media is a medium that uses glass or plastic threads to transmit data that capable of transmitting electric signals into light waves. Wireless media carry electromagnetic signals at radio and microwave frequencies that represent the binary digits data communication and its technology work well in open environments and network security is a major component of wireless network administration. Here are the wireless media standards and types: WI-FI, Bluetooth, WIMAX, and the GSM.