CCNA INTRO Exam Certification Guide - Part 1 Networking Fundamentals - Chapter 3

C H A P T E R 3 Data Link Layer Fundamentals: Ethernet LANs As you learned in the previous chapter, OSI Layers 1 and 2 map closely to the network interface layer of TCP/IP. In this chapter, you will learn more details about the functions of each of the two lowest layers in the OSI reference model, with specific coverage of Ethernet local-area networks (LANs). The introduction to this book mentioned that the INTRO exam covers some topics lightly and covers others to great depth. As implied in the title, this chapter hits the fundamentals of Ethernet, paving the way for deeper coverage of other topics later in the book. Chapter 9, “Cisco LAN Switching Basics,” and Chapter 10, “Virtual LANs and Trunking,” delve into a much deeper examination of LAN switches and virtual LANs. Chapter 11, “LAN Cabling, Standards, and Topologies,” increases your breadth of knowledge about Ethernet, including a lot of broad details about Ethernet standards, cabling, and topologies—all of which can be on the exam. “Do I Know This Already?” Quiz The purpose of the “Do I Know This Already?” quiz is to help you decide whether you really need to read the entire chapter. If you already intend to read the entire chapter, you do not necessarily need to answer these questions now. The ten-question quiz, derived from the major sections in “Foundation Topics” portion of the chapter, helps you determine how to spend your limited study time. Table 3-1 outlines the major topics discussed in this chapter and the “Do I Know This Already?” quiz questions that correspond to those topics. Table 3-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping Foundations Topics Section OSI Perspectives on Local-Area Networks Early Ethernet Standards Ethernet Data Link Protocols Recent Ethernet Standards Questions Covered in This Section 1, 5 3, 7, 8 2, 4, 6, 9 10 44 Chapter 3: Data Link Layer Fundamentals: Ethernet LANs CAUTION The goal of self-assessment is to gauge your mastery of the topics in this chapter. If you do not know the answer to a question or are only partially sure of the answer, you should mark this question wrong for purposes of self-assessment. Giving yourself credit for an answer that you correctly guess skews your self-assessment results and might provide you with a false sense of security. 1. Which of the following best describes the main function of OSI Layer 1 protocols? a. Framing b. Delivery of bits from one device to another c. Addressing d. CSMA/CD e. Defining the size and shape of Ethernet cards 2. Which of the following are part of the functions of OSI Layer 2 protocols? a. Framing b. Delivery of bits from one device to another c. Addressing d. Error detection e. Defining the size and shape of Ethernet cards 3. Which of the following is true about Ethernet crossover cables? a. Pins 1 and 2 are reversed on the other end of the cable. b. Pins 1 and 2 connect to pins 3 and 6 on the other end of the cable. c. Pins 1 and 2 connect to pins 3 and 4 on the other end of the cable. d. The cable can be up to 1000 m to cross over between buildings. e. None of the above. 4. Which of the following are true about the format of Ethernet addresses? a. Each manufacturer puts a unique code into the first 2 bytes of the address. b. Each manufacturer puts a unique code into the first 3 bytes of the address. c. Each manufacturer puts a unique code into the first half of the address. d. The part of the address that holds this manufacturer’s code is called the MC. e. The part of the address that holds this manufacturer’s code is called the OUI. f. The part of the address that holds this manufacturer’s code has no specific name. “Do I Know This Already?” Quiz 45 5. Which of the following is true about the Ethernet FCS field? a. It is used for error recovery. b. It is 2 bytes long. c. It resides in the Ethernet trailer, not the Ethernet header. d. It is used for encryption. e. None of the above. 6. Which of the following fields can be used by Ethernet as a “type” field, to define the type of data held in the “data” portion of the Ethernet frame? a. The DIX Ethernet DSAP field b. The IEEE 802.2 Ethernet Type field c. The IEEE 802.2 Ethernet DSAP field d. The SNAP header Protocol Type field e. None of the above. 7. Which of the following are true about the CSMA/CD algorithm? a. The algorithm never allows collisions to occur. b. Collisions can happen, but the algorithm defines how the computers should notice a collision and how to recover. c. The algorithm works only with two devices on the same Ethernet. d. None of the above. 8. Which of the following would be a collision domain? a. All devices connected to an Ethernet hub b. All devices connected to an Ethernet switch c. Two PCs, with one cabled to a router Ethernet port with a crossover cable, and the other PC cabled to another router Ethernet port with a crossover cable. d. None of the above 9. Which terms describe Ethernet addresses that can be used to communicate with more than one device at a time? a. Burned-in address b. Unicast address c. Broadcast address d. Multicast address e. None of the above 46 Chapter 3: Data Link Layer Fundamentals: Ethernet LANs 10. With autonegotiation on a 10/100 card, what characteristics are negotiated if the device on the other end does not perform negotiation at all? a. 100 Mbps, half duplex b. 100 Mbps, full duplex c. 10 Mbps, half duplex d. 10 Mbps, full duplex The answers to the “Do I Know This Already?” quiz are found in Appendix A, “Answers to the ‘Do I Know This Already?’ Quizzes and Q&A Sections.” The suggested choices for your next step are as follows: 8 or less overall score—Read the entire chapter. This includes the “Foundation Topics” and “Foundation Summary” sections and the Q&A section. 9 or 10 overall score—If you want more review on these topics, skip to the “Foundation Summary” section and then go to the Q&A section. Otherwise, move to the next chapter. OSI Perspectives on Local-Area Networks 47 Foundation Topics Ethernet is the undisputed king of LAN standards today. Fifteen years ago, people wondered whether Ethernet or Token Ring would become win the battle of the LANs. Eight years ago, it looked like Ethernet would win that battle, but it might lose to an upstart called Asynchronous Transfer Mode (ATM) in the LAN. Today when you think of LANs, no one even questions what type—it’s Ethernet. Ethernet has remained a viable LAN option for many years because it has adapted to the changing needs of the marketplace while retaining some of the key features of the original protocols. From the original commercial specifications that transferred data 10 megabits per second (Mbps) to the 10 gigabits per second (Gbps) rates today, Ethernet has evolved and become the most prolific LAN protocol ever. Ethernet defines both Layer 1 and Layer 2 functions, so this chapter starts with some basic concepts in relation to OSI Layers 1 and 2. After that, the three earliest Ethernet standards are covered, focusing on the physical layer details. Next, this chapter covers data link layer functions, which are common among all the earlier Ethernet standards as well as the newer standards. Finally, two of the more recent standards, Fast Ethernet and Gigabit Ethernet, are introduced. OSI Perspectives on Local-Area Networks The OSI physical and data link layers work together to provide the function of delivery of data across a wide variety of types of physical networks. Some obvious physical details must be agreed upon before communication can happen, such as the cabling, the types of connectors used on the ends of the cables, and voltage and current levels used to encode a binary 0 or 1. The data link layer typically provides functions that are less obvious at first glance. For instance, it defines the rules (protocols) to determine when a computer is allowed to use the physical network, when the computer should not use the network, and how to recognize errors that occurred during transmission of data. Part II, “Operating Cisco Devices,” and Part III, “LAN Switching,” cover a few more details about Ethernet Layers 1 and 2. Typical LAN Features for OSI Layer 1 The OSI physical layer, or Layer 1, defines the details of how to move data from one device to another. In fact, many people think of OSI Layer 1 as “sending bits.” Higher layers encapsulate the data and decide when and what to send. But eventually, the sender of the data needs to actually transmit the bits to another device. The OSI physical layer defines the standards used to send and receive bits across a physical network. ... - tailieumienphi.vn