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Ethernet

                This is the 9^th chapter of Cisco Networking Academy (CCNA) Exploration Network Fundamental Course. As stated, Ethernet is defined by Data Link layer and Physical layer protocols which are the 1^st and 2^nd layers of the OSI model.

                Robert Metcalfe and his workers designed the 1^st LAN at Xerox more than 30 years ago. The 1^st LAN was the original version of Ethernet. This Ethernet was released as an open standard. In 1985, IEEE published standards for LANs = 802. The logical topology of Ethernet is a multi-access bus. It was designed to accommodate multiple computers that were interconnected on a shared bus topology. The Ethernet usually adapted optical fiber media. It also uses UTP copper cables to interconnect network devices via intermediary devices such as hubs and switches.

                The Ethernet standard which is 802.3 operates in the lower 2 layers of the OSI model: the Data Link layer and the physical layer. The Data Link layer consists of sub layers: Media Access Control (MAC) – lower sub layer, and Logical Link Control (LLC) – upper sub layer. The MAC is a unique identifier that was created to assist in determining the source and destination address with in an Ethernet network. All devices connected to an Ethernet LAN have MAC addressed interfaces. For Ethernet, the IEEE 802.2 standard describes the LLC sub layer functions while the 802.3 standard describes the MAC sub layer and Physical layer functions. The Ethernet MAC has 2 primary responsibilities: Data encapsulation and MAC. The data encapsulation has primary functions of frame delimiting, addressing, and error detection.

             The early versions of Ethernet are: Thicknet (10BASE5) – used a thick coaxial that allowed for cabling distances of up to 500 meters before the signal required a repeater; and Thinnet (10BASE2) – used a thin coaxial cable that was smaller in diameter and more flexible than Thicknet and allowed for cabling distances of 185 meters.

             Four data rates currently defined for operation over optical fiber and twisted-pair cables: (1) 10Mbps -> for 10BASE-T Ethernet and have a bit time of 100ns; (2) 100Mbps -> for fast Ethernet and have a bit time of 10ns; (3) 1000Mbps or 1Gbps -> for gigabit Ethernet and have a bit time of 1ns; and (4) 10Gbps -> for 10 Gigabit Ethernet and have a bit time of .1ns. With 10Gbps Ethernet, flexible, efficient, reliable, relatively low cost end-to-end Ethernet networks become possible.

               The difference of using hub and switches in the network: Some related issues using hub are scalability, latency, network failure, and collisions. In 10BASE-T networks, typically the central point of the network segment was a hub. Three primary reasons for the increase of throughput using switch: dedicated bandwidth to each port, collision-free environment, and full-duplex operation. He The switches enhanced LAN performance and it can control the flow of data by isolating each port and sending a frame only to its proper destination (if the destination is known), rather than send every frame to every device. Three reasons for still using hubs: availability, economics, and requirements.

               

Important keywords:

1.)    CSMA/CD -> Carrier Sense Multiple Access w/ Collision Detection.

     -> manage the problems that result when multiple devices attempt to communicate over a           shared physical medium.

2.)    Collision -> the frames from each device impact and are damage when they meet in the physical media.

3.)    Alohanet -> a digital radio network designed to transmit information over a shared radio frequency between the Hawaiian Islands.

4.)    Half-duplex communication -> a connection of sharing the media where only one station could successfully transmit a t a time.

5.)    Full-duplex communication -> having a connection that can carry both transmitted and received signals at the same time.

6.)    Jamming signal -> used to notify the other devices of a collision, so that they will invoke a backoff algorithm.

7.)    Backoff algorithm -> causes all devices to stop transmitting for a random amount of time, which allows the collision signals to subside.

8.)  Slot time -> the time that a transmitting station waits before attempting to retransmit following a collision.

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.