CCENT/CCNA ICND1 Official Exam Certification Guide - Chapter 3 potx

C H A P T E R 3 Fundamentals of LANs Physical and data link layer standards work together to allow computers to send bits to each other over a particular type of physical networking medium. The Open Systems Interconnection (OSI) physical layer (Layer 1) defines how to physically send bits over a particular physical networking medium. The data link layer (Layer 2) defines some rules about the data that is physically transmitted, including addresses that identify the sending device and the intended recipient, and rules about when a device can send (and when it should be silent), to name a few. This chapter explains some of the basics of local-area networks (LAN). The term LAN refers to a set of Layer 1 and 2 standards designed to work together for the purpose of implementing geographically small networks. This chapter introduces the concepts of LANs—in particular, Ethernet LANs. More-detailed coverage of LANs appears in Part II (Chapters 7 through 11). “Do I Know This Already?” Quiz The “Do I Know This Already?” quiz allows you to assess whether you should read the entire chapter. If you miss no more than one of these 11 self-assessment questions, you might want to move ahead to the “Exam Preparation Tasks” section. Table 3-1 lists the major headings in this chapter and the “Do I Know This Already?” quiz questions covering the material in those sections. This helps you assess your knowledge of these specific areas. The answers to the “Do I Know This Already?” quiz appear in Appendix A. Table 3-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping Foundation Topics Section Questions An Overview of Modern Ethernet LANs 1 A Brief History of Ethernet 2 Ethernet UTP Cabling 3, 4 Improving Performance by Using Switches Instead of Hubs 5–7 Ethernet Data-Link Protocols 8–11 1828xbook.fm Page 41 Thursday, July 26, 2007 3:10 PM 42 Chapter 3: Fundamentals of LANs 1. Which of the following is true about the cabling of a typical modern Ethernet LAN? a. Connect each device in series using coaxial cabling b. Connect each device in series using UTP cabling c. Connect each device to a centralized LAN hub using UTP cabling d. Connect each device to a centralized LAN switch using UTP cabling 2. Which of the following is true about the cabling of a 10BASE2 Ethernet LAN? a. Connect each device in series using coaxial cabling b. Connect each device in series using UTP cabling c. Connect each device to a centralized LAN hub using UTP cabling d. Connect each device to a centralized LAN switch using UTP cabling 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 on one end of the cable connect to pins 3 and 6 on the other end of the cable. c. Pins 1 and 2 on one end of the cable connect to pins 3 and 4 on the other end of the cable. d. The cable can be up to 1000 meters long to cross over between buildings. e. None of the other answers is correct. 4. Each answer lists two types of devices used in a 100BASE-TX network. If these devices were connected with UTP Ethernet cables, which pairs of devices would require a straight-through cable? a. PC and router b. PC and switch c. Hub and switch d. Router and hub e. Wireless access point (Ethernet port) and switch 1828xbook.fm Page 42 Thursday, July 26, 2007 3:10 PM “Do I Know This Already?” Quiz 43 5. Which of the following is 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 with only two devices on the same Ethernet. d. None of the other answers is correct. 6. Which of the following is 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 other answers is correct. 7. Which of the following describe a shortcoming of using hubs that is improved by instead using switches? a. Hubs create a single electrical bus to which all devices connect, causing the devices to share the bandwidth. b. Hubs limit the maximum cable length of individual cables (relative to switches) c. Hubs allow collisions to occur when two attached devices send data at the same time. d. Hubs restrict the number of physical ports to at most eight. 8. Which of the following 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 1828xbook.fm Page 43 Thursday, July 26, 2007 3:10 PM 44 Chapter 3: Fundamentals of LANs 9. Which of the following is one of the functions of OSI Layer 2 protocols? a. Framing b. Delivery of bits from one device to another c. Error recovery d. Defining the size and shape of Ethernet cards 10. 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 MAC. 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. 11. 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 other answers is correct. 1828xbook.fm Page 44 Thursday, July 26, 2007 3:10 PM An Overview of Modern Ethernet LANs 45 Foundation Topics A typical Enterprise network consists of several sites. The end-user devices connect to a LAN, which allows the local computers to communicate with each other. Additionally, each site has a router that connects to both the LAN and a wide-area network (WAN), with the WAN providing connectivity between the various sites. With routers and a WAN, the computers at different sites can also communicate. This chapter describes the basics of how to create LANs today, with Chapter 4, “Fundamentals of WANs,” describing the basics of creating WANs. Ethernet is the undisputed king of LAN standards today. Historically speaking, several competing LAN standards existed, including Token Ring, Fiber Distributed Data Interface (FDDI), and Asynchronous Transfer Mode (ATM). Eventually, Ethernet won out over all the competing LAN standards, so that today when you think of LANs, no one even questions what type— it’s Ethernet. An Overview of Modern Ethernet LANs The term Ethernet refers to a family of standards that together define the physical and data link layers of the world’s most popular type of LAN. The different standards vary as to the speed supported, with speeds of 10 megabits per second (Mbps), 100 Mbps, and 1000 Mbps (1 gigabit per second, or Gbps) being common today. The standards also differ as far as the types of cabling and the allowed length of the cabling. For example, the most commonly used Ethernet standards allow the use of inexpensive unshielded twisted-pair (UTP) cabling, whereas other standards call for more expensive fiber-optic cabling. Fiber-optic cabling might be worth the cost in some cases, because the cabling is more secure and allows for much longer distances between devices. To support the widely varying needs for building a LAN—needs for different speeds, different cabling types (trading off distance requirements versus cost), and other factors—many variations of Ethernet standards have been created. The Institute of Electrical and Electronics Engineers (IEEE) has defined many Ethernet standards since it took over the LAN standardization process in the early 1980s. Most of the standards define a different variation of Ethernet at the physical layer, with differences in speed and types of cabling. Additionally, for the data link layer, the IEEE separates the functions into two sublayers: ■ The 802.3 Media Access Control (MAC) sublayer ■ The 802.2 Logical Link Control (LLC) sublayer 1828xbook.fm Page 45 Thursday, July 26, 2007 3:10 PM 46 Chapter 3: Fundamentals of LANs In fact, MAC addresses get their name from the IEEE name for this lower portion of the data link layer Ethernet standards. Each new physical layer standard from the IEEE requires many differences at the physical layer. However, each of these physical layer standards uses the exact same 802.3 header, and each uses the upper LLC sublayer as well. Table 3-2 lists the most commonly used IEEE Ethernet physical layer standards. The table is convenient for study, but the terms in the table bear a little explanation. First, beware that the term Ethernet is often used to mean “all types of Ethernet,” but in some cases it is used to mean “10BASE-T Ethernet.” (Because the term Ethernet sometimes can be ambiguous, this book refers to 10-Mbps Ethernet as 10BASE-T when the specific type of Ethernet matters to the discussion.) Second, note that the alternative name for each type of Ethernet lists the speed in Mbps—namely, 10 Mbps, 100 Mbps, and 1000 Mbps. The T and TX in the alternative names refer to the fact that each of these standards defines the use of UTP cabling, with the T referring to the T in twisted pair. To build and create a modern LAN using any of the UTP-based types of Ethernet LANs listed in Table 3-2, you need the following components: ■ Computers that have an Ethernet network interface card (NIC) installed ■ Either an Ethernet hub or Ethernet switch ■ UTP cables to connect each PC to the hub or switch Figure 3-1 shows a typical LAN. The NICs cannot be seen, because they reside in the PCs. However, the lines represent the UTP cabling, and the icon in the center of the figure represents a LAN switch. Table 3-2 Today’s Most Common Types of Ethernet Common Name Speed Alternative Name Name of IEEE Standard Cable Type, Maximum Length Ethernet 10 Mbps 10BASE-T IEEE 802.3 Copper, 100 m Fast Ethernet 100 Mbps 100BASE-TX IEEE 802.3u Copper, 100 m Gigabit Ethernet 1000 Mbps 1000BASE-LX, 1000BASE-SX IEEE 802.3z Fiber, 550 m (SX) 5 km (LX) Gigabit Ethernet 1000 Mbps 1000BASE-T IEEE 802.3ab 100 m 1828xbook.fm Page 46 Thursday, July 26, 2007 3:10 PM An Overview of Modern Ethernet LANs 47 Figure 3-1 Typical Small Modern LAN Most people can build a LAN like the one shown in Figure 3-1 with practically no real knowledge of how LANs work. Most PCs contain an Ethernet NIC that was installed at the factory. Switches do not need to be configured for them to forward traffic between the computers. All you have to do is connect the switch to a power cable and plug in the UTP cables from each PC to the switch. Then the PCs should be able to send Ethernet frames to each other. You can use such a small LAN for many purposes, even without a WAN connection. Consider the following functions for which a LAN is the perfect, small-scale solution: File sharing: Each computer can be configured to share all or parts of its file system so that the other computers can read, or possibly read and write, the files on another computer. This function typically is simply part of the PC operating system. Printer sharing: Computers can share their printers as well. For example, PCs A, B, and C in Figure 3-1 could print documents on PC D’s printer. This function is also typically part of the PC’s operating system. File transfers: A computer could install a file transfer server, thereby allowing other computers to send and receive files to and from that computer. For example, PC C could install File Transfer Protocol (FTP) server software, allowing the other PCs to use FTP client software to connect to PC C and transfer files. Gaming: The PCs could install gaming software that allows multiple players to play in the same game. The gaming software would then communicate using the Ethernet. NOTE Figure 3-1 applies to all the common types of Ethernet. The same basic design and topology are used regardless of speed or cabling type. A B C D FTP Server Software Installed Here Printer Cable 1828xbook.fm Page 47 Thursday, July 26, 2007 3:10 PM 48 Chapter 3: Fundamentals of LANs The goal of the first half of this chapter is to help you understand much of the theory and practical knowledge behind simple LAN designs such as the one illustrated in Figure 3-1. To fully understand modern LANs, it is helpful to understand a bit about the history of Ethernet, which is covered in the next section. Following that, this chapter examines the physical aspects (Layer 1) of a simple Ethernet LAN, focusing on UTP cabling. Then this chapter compares the older (and slower) Ethernet hub with the newer (and faster) Ethernet switch. Finally, the LAN coverage in this chapter ends with the data-link (Layer 2) functions on Ethernet. A Brief History of Ethernet Like many early networking protocols, Ethernet began life inside a corporation that was looking to solve a specific problem. Xerox needed an effective way to allow a new invention, called the personal computer, to be connected in its offices. From that, Ethernet was born. (Go to http://inventors.about.com/library/weekly/aa111598.htm for an interesting story on the history of Ethernet.) Eventually, Xerox teamed with Intel and Digital Equipment Corp. (DEC) to further develop Ethernet, so the original Ethernet became known as DIX Ethernet, referring to DEC, Intel, and Xerox. These companies willingly transitioned the job of Ethernet standards development to the IEEE in the early 1980s. The IEEE formed two committees that worked directly on Ethernet—the IEEE 802.3 committee and the IEEE 802.2 committee. The 802.3 committee worked on physical layer standards as well as a subpart of the data link layer called Media Access Control (MAC). The IEEE assigned the other functions of the data link layer to the 802.2 committee, calling this part of the data link layer the Logical Link Control (LLC) sublayer. (The 802.2 standard applied to Ethernet as well as to other IEEE standard LANs such as Token Ring.) The Original Ethernet Standards: 10BASE2 and 10BASE5 Ethernet is best understood by first considering the two early Ethernet specifications, 10BASE5 and 10BASE2. These two Ethernet specifications defined the details of the physical and data link layers of early Ethernet networks. (10BASE2 and 10BASE5 differ in their cabling details, but for the discussion in this chapter, you can consider them as behaving identically.) With these two specifications, the network engineer installs a series of coaxial cables connecting each device on the Ethernet network. There is no hub, switch, or wiring panel. The Ethernet consists solely of the collective Ethernet NICs in the computers and the coaxial cabling. The series of cables creates an electrical circuit, called a bus, which is shared among all devices on the Ethernet. When a computer wants to send some bits to another computer on the bus, it sends an electrical signal, and the electricity propagates to all devices on the Ethernet. Figure 3-2 shows the basic logic of an old Ethernet 10BASE2 network, which uses a single electrical bus, created with coaxial cable and Ethernet cards. 1828xbook.fm Page 48 Thursday, July 26, 2007 3:10 PM A Brief History of Ethernet 49 Figure 3-2 Small Ethernet 10BASE2 Network The solid lines in the figure represent the physical network cabling. The dashed lines with arrows represent the path that Larry’s transmitted frame takes. Larry sends an electrical signal across his Ethernet NIC onto the cable, and both Bob and Archie receive the signal. The cabling creates a physical electrical bus, meaning that the transmitted signal is received by all stations on the LAN. Just like a school bus stops at every student’s house along a route, the electrical signal on a 10BASE2 or 10BASE5 network is propagated to each station on the LAN. Because the network uses a single bus, if two or more electrical signals were sent at the same time, they would overlap and collide, making both signals unintelligible. So, unsurprisingly, Ethernet also defined a specification for how to ensure that only one device sends traffic on the Ethernet at one time. Otherwise, the Ethernet would have been unusable. This algorithm, known as the carrier sense multiple access with collision detection (CSMA/CD) algorithm, defines how the bus is accessed. In human terms, CSMA/CD is similar to what happens in a meeting room with many attendees. It’s hard to understand what two people are saying at the same time, so generally, one person talks and the rest listen. Imagine that Bob and Larry both want to reply to the current speaker’s comments. As soon as the speaker takes a breath, Bob and Larry both try to speak. If Larry hears Bob’s voice before Larry makes a noise, Larry might stop and let Bob speak. Or, maybe they both start at almost the same time, so they talk over each other and no one can hear what is said. Then there’s the proverbial “Pardon me; go ahead with what you were saying,” and eventually Larry or Bob talks. Or perhaps another person jumps in and talks while Larry and Bob are both backing off. These “rules” are based on your culture; CSMA/ CD is based on Ethernet protocol specifications and achieves the same type of goal. Basically, the CSMA/CD algorithm can be summarized as follows: ■ A device that wants to send a frame waits until the LAN is silent—in other words, no frames are currently being sent—before attempting to send an electrical signal. ■ If a collision still occurs, the devices that caused the collision wait a random amount of time and then try again. Larry Archie Bob Solid Lines Represent Co-Ax Cable 10BASE2, Single Bus 1828xbook.fm Page 49 Thursday, July 26, 2007 3:10 PM 50 Chapter 3: Fundamentals of LANs In 10BASE5 and 10BASE2 Ethernet LANs, a collision occurs because the transmitted electrical signal travels along the entire length of the bus. When two stations send at the same time, their electrical signals overlap, causing a collision. So, all devices on a 10BASE5 or 10BASE2 Ethernet need to use CSMA/CD to avoid collisions and to recover when inadvertent collisions occur. Repeaters Like any type of LAN, 10BASE5 and 10BASE2 had limitations on the total length of a cable. With 10BASE5, the limit was 500 m; with 10BASE2, it was 185 m. Interestingly, the 5 and 2 in the names 10BASE5 and 10BASE2 represent the maximum cable length—with the 2 referring to 200 meters, which is pretty close to the actual maximum of 185 meters. (Both of these types of Ethernet ran at 10 Mbps.) In some cases, the maximum cable length was not enough, so a device called a repeater was developed. One of the problems that limited the length of a cable was that the signal sent by one device could attenuate too much if the cable was longer than 500 m or 185 m. Attenuation means that when electrical signals pass over a wire, the signal strength gets weaker the farther along the cable it travels. It’s the same concept behind why you can hear someone talking right next to you, but if that person speaks at the same volume and you are on the other side of a crowded room, you might not hear her because the sound waves have attenuated. Repeaters connect to multiple cable segments, receive the electrical signal on one cable, interpret the bits as 1s and 0s, and generate a brand-new, clean, strong signal out the other cable. A repeater does not simply amplify the signal, because amplifying the signal might also amplify any noise picked up along the way. You should not expect to need to implement 10BASE5 or 10BASE2 Ethernet LANs today. However, for learning purposes, keep in mind several key points from this section as you move on to concepts that relate to today’s LANs: ■ The original Ethernet LANs created an electrical bus to which all devices connected. ■ Because collisions could occur on this bus, Ethernet defined the CSMA/CD algorithm, which defined a way to both avoid collisions and take action when collisions occurred. ■ Repeaters extended the length of LANs by cleaning up the electrical signal and repeating it—a Layer 1 function—but without interpreting the meaning of the electrical signal. NOTE Because the repeater does not interpret what the bits mean, but it does examine and generate electrical signals, a repeater is considered to operate at Layer 1. 1828xbook.fm Page 50 Thursday, July 26, 2007 3:10 PM [...]... Table 3- 6 Key Topics for Chapter 3 Key Topic Element Description Page Number Table 3- 2 The four most popular types of Ethernet LANs and some details about each 46 List Summary of CSMA/CD logic 49 Figure 3- 6 EIA/TIA standard Ethernet Cabling Pinouts 55 Figure 3- 7 Straight-through cable concept 56 Figure 3- 8 Crossover cable concept 57 Table 3- 3 List of devices that transmit on wire pair 1,2 and pair 3, 6... be useful to closely examine the RJ-45 connectors and wires as you read through this section As soon as the cable has RJ-45 connectors on each end, the RJ-45 connector needs to be inserted into an RJ-45 receptacle, often called an RJ-45 port Figure 3- 4 shows photos of the cables, connectors, and ports Figure 3- 4 RJ-45 Connectors and Ports RJ-45 Connectors RJ-45 Ports NOTE The RJ-45 connector is slightly... Devices that use an opposite pair of pins to transmit need a straight-through cable Table 3- 3 lists the devices mentioned in this book and the pin pairs they use, assuming that they use 10BASE-T and 100BASE-TX Table 3- 3 10BASE-T and 100BASE-TX Pin Pairs Used Devices That Transmit on 1,2 and Receive on 3, 6 Devices That Transmit on 3, 6 and Receive on 1,2 PC NICs Hubs Routers Switches Wireless Access... switches send on the pair at pins 3, 6, and receive on the pair at pins 1,2, the cable must swap or cross the pairs Figure 3- 8 shows several conceptual views of a crossover cable Figure 3- 8 Crossover Ethernet Cable RJ-45 Pins RJ-45 Pins 1 2 3 1 2 3 6 6 3, 6 1,2 3, 6 1,2 The top part of the figure shows the pins to which each wire is connected Pin 1 on the left end connects to pin 3 on the right end, pin 2 on... used Figure 3- 7 Ethernet Straight-Through Cable Concept Larry I’ll transmit on pins 1,2 and receive on 3, 6 The pair on 1,2 on the left connects to pins 1,2 on the right! And it works! I’ll receive on 1,2 and transmit on 3, 6! PC1 Transmit Pair (1,2) Hub Receive Pair (1,2) PC1 Receive Pair (3, 6) Hub Transmit Pair (3, 6) NIC Hub Straight-Through Cable The network shown in Figure 3- 7 uses a straight-through... that remains a key design point today, is the concept of cabling each device to a centralized connection point Originally, 10BASE-T called for the use of Ethernet hubs, as shown in Figure 3- 3 Figure 3- 3 Small Ethernet 10BASE-T Network Using a Hub Larry 10BASE-T, Using Shared Hub - Acts Like Single Bus Archie Hub 1 Bob Solid Lines Represent Twisted Pair Cabling When building a LAN today, you could choose... 1828xbook.fm Page 58 Thursday, July 26, 2007 3: 10 PM 58 Chapter 3: Fundamentals of LANs For example, Figure 3- 9 shows a campus LAN in a single building In this case, several straight-through cables are used to connect PCs to switches Additionally, the cables connecting the switches—referred to as trunks—require crossover cables Figure 3- 9 Typical Uses for Straight-Through and Crossover Ethernet Cables Building... http:// www.eia.org.) Figure 3- 6 shows two pinout standards from the EIA/TIA, with the color coding and pair numbers listed Figure 3- 6 EIA/TIA Standard Ethernet Cabling Pinouts Pair 2 Pair 3 Pair 1 Pair 4 Pinouts 1 = G/W 2 = Green 3 = O/W 4 = Blue 5 = Blue/W 6 = Orange 7 = Brown/W 8 = Brown Pair 3 Pair 2 Pair 1 Pair 4 1 234 567 8 1 234 567 8 T568A T568B Pinouts 1 = O/W 2 = Orange 3 = G/W 4 = Blue 5 = Blue/W... send and receive concurrently 1828xbook.fm Page 63 Thursday, July 26, 2007 3: 10 PM Ethernet Data-Link Protocols To appreciate why collisions cannot occur, consider Figure 3- 1 2, which shows the fullduplex circuitry used with a single PC’s connection to a LAN switch Figure 3- 1 2 Full-Duplex Operation Using a Switch Receive Transmit Transmit Receive Full-Duplex NIC Switch NIC With only the switch and one... used on another card Figure 3- 1 3 shows the structure Figure 3- 1 3 Structure of Unicast Ethernet Addresses Organizationally Unique Identifier (OUI) Size, in bits Size, in hex digits Example Vendor Assigned (NIC Cards, Interfaces) 24 Bits 24 Bits 6 Hex Digits 6 Hex Digits 00 60 2F 3A 07 BC Many terms can be used to describe unicast LAN addresses Each LAN card comes with a burned-in address (BIA) that is . to the LAN) — RJ-45 Pins 6 3 2 1 6 3 2 1 1,2 1,2 3, 6 3, 6 RJ-45 Pins 1828xbook.fm Page 57 Thursday, July 26, 2007 3: 10 PM 58 Chapter 3: Fundamentals of LANs For example, Figure 3- 9 shows a campus. straight-through cable. Table 3- 3 lists the devices mentioned in this book and the pin pairs they use, assuming that they use 10BASE-T and 100BASE-TX. Table 3- 3 10BASE-T and 100BASE-TX Pin. centralized connection point. Originally, 10BASE-T called for the use of Ethernet hubs, as shown in Figure 3- 3 . Figure 3- 3 Small Ethernet 10BASE-T Network Using a Hub When building a LAN today,