Check Your Understanding 309 Token Ring A token-passing LAN developed and supported by IBM. Token Ring runs at 4 or 16 Mbps over a ring topology. trailer Controls information appended to data when encapsulating the data for network transmission. Check Your Understanding Complete all the review questions to test your understanding of the topics and con- cepts in this chapter. Answers are listed in Appendix C, “Check Your Understanding Answer Key.” 1. Which of the following is not one of the recognized IEEE sublayers? A. Media Access Control B. Data Link Control C. Logical Link Control D. None of the above 2. The recognized IEEE sublayers are concerned with what layers of the OSI reference model? A. 2 and 3 B. 1 and 2 C. 3 and 4 D. 1 and 3 3. The LLC, as a sublayer, participates in the process. A. Encryption B. Encapsulation C. Framing D. All of the above 4. The first six hexadecimal numbers in a MAC address represent an ? A. Interface serial number B. Organizationally unique identifier C. Interface unique identifier D. None of the above 1102.book Page 309 Tuesday, May 20, 2003 2:53 PM 310 Chapter 5: Ethernet Fundamentals 5. MAC addresses are bits in length. A. 12 B. 24 C. 48 D. 64 6. What is the name of the access method used in Ethernet that explains how Ethernet works? A. TCP/IP B. CSMA/CD C. CMDA/CS D. CSMA/CA 7. Where does the MAC address reside? A. Transceiver B. Computer BIOS C. NIC D. CMOS 8. Which of the following statements best describes communication between two devices on a LAN? A. The source device encapsulates data in a frame with the MAC address of the destination device and then transmits it. Everyone on the LAN sees it, but the devices with nonmatching addresses otherwise ignore the frame. B. The source encapsulates the data and places a destination MAC address in the frame. It puts the frame on the LAN, where only the device with the matching address can check the address field. C. The destination device encapsulates data in a frame with the MAC address of the source device and puts it on the LAN. The device with the matching address removes the frame. D. Each device on the LAN receives the frame and passes it up to the computer, where software decides whether to keep or to discard the frame. 1102.book Page 310 Tuesday, May 20, 2003 2:53 PM Check Your Understanding 311 9. Which functions are associated with framing? A. Identifies which computers are communicating with one another B. Signals when communication between individual computers begins and when it ends C. Flags corrupted frames D. All of the above 10. How does a computer on a LAN detect an error in a frame? A. It sends a copy of the frame back to the sender for verification. B. It checks the destination address to verify that the frame really was intended for it. C. It compares an FCS in the frame to one that the computer calculates from the contents of the frame. D. It calculates a checksum from the data in the frame and then sends it back to the source for verification. 11. Media Access Control refers to what? A. The state in which a NIC has captured the networking medium and is ready to transmit. B. Rules that govern media capture and release. C. Protocols that determine which computer on a shared-medium environment is allowed to transmit the data. D. A formal byte sequence has been transmitted. 12. Which best describes a CSMA/CD network? A. One node’s transmission traverses the entire network and is received and examined by every node. B. Signals are sent directly to the destination if the source knows both the MAC and IP addresses. C. One node’s transmission goes to the nearest router, which sends it directly to the destination. D. Signals always are sent in broadcast mode. 1102.book Page 311 Tuesday, May 20, 2003 2:53 PM 312 Chapter 5: Ethernet Fundamentals 13. In an Ethernet or IEEE 802.3 LAN, when do collisions occur? A. When one node places a packet on a network without informing the other nodes B. When two stations listen for a traffic, hear none, and transmit simultaneously C. When two network nodes send packets to a node that no longer is broadcasting D. When jitter is detected and traffic is disrupted during normal transmission 14. Which is an important Layer 2 data link layer function? A. Logical link control B. Addressing C. Media access control D. All of the above 15. Which of the following is an Ethernet frame error type? A. Local collision B. Remote collision C. Late collision D. All of the above 16. Which protocol is a nondeterministic protocol? A. Token Ring B. CSMA/CD C. IPX D. RIP 17. Which is true of a deterministic MAC protocol? A. It defines collisions and specifies what to do about them. B. It allows the hub to determine the number of users active at any one time. C. It allows hosts to “take turns” sending data. D. It allows the use of a “talking stick” by network administers to control the media access of any users considered “troublemakers.” 1102.book Page 312 Tuesday, May 20, 2003 2:53 PM Check Your Understanding 313 18. The network area within which data packets originated and collide is called a ? A. Collision domain B. Network domain C. Broadcast domain D. Network segment 19. Which best describes broadcasting? A. Sending a single frame to many stations at the same time B. Sending a single frame to all routers to simultaneously update their routing tables C. Sending a single frame to all routers at the same time D. Sending a single frame to all hubs and bridges at the same time 20. Using repeaters the collision domain. A. Reduces B. Has no effect on C. Extends D. None of the above 21. The process of using the complex networking devices, such as bridges, switches, and routers, to break up the collision domains is known as A. Sectioning B. Segmentation C. Collision domain reduction D. None of the above 1102.book Page 313 Tuesday, May 20, 2003 2:53 PM Objectives Upon completion of this chapter, you will be able to ■ Understand the evolution of Ethernet technology ■ Understand the MAC method, frame format, and transmission process of Ethernet technologies ■ Understand the specific media and encoding used in each Ethernet technology ■ Understand the pinouts and wiring typical of each Ethernet technology ■ Understand the basic architectural considerations of each Ethernet technology ■ Define microsegmentation ■ Describe Layer 2 bridging ■ Describe how a LAN switch operates ■ Describe full-duplex transmission ■ Identify the common switching methods: cut-through switching, store-and- forward switching, and fragment-free switching ■ Describe the functions and features of the Spanning Tree Protocol (STP) ■ Describe how STP works ■ Describe the different STP port states 1102.book Page 314 Tuesday, May 20, 2003 2:53 PM Chapter 6 Ethernet Technologies and Ethernet Switching Ethernet, along with its associated IEEE 802.3 protocols, is one of the world’s most important networking standards. Because of the great success of the original Ethernet and the soundness of its design, it has evolved over time. This evolution was in response to the developing needs of modern LANs. Ethernet most likely will continue to evolve in response to future demands for network capability. The previous chapter introduced both the history of Ethernet and the standards associated with Ethernet. You also learned that the term Ethernet refers a family of the Ethernet technologies. This chapter discusses the Ethernet technologies in more detail. In addition, this chapter introduces Layer 2 bridging and switching techniques. Switching and bridging are techniques that decrease congestion in LANs by reducing traffic and increasing bandwidth. Finally, this chapter introduces the Spanning Tree Protocol (STP), tells how STP works, and covers the STP switch port states. Please be sure to look at this chapter’s associated e-Lab Activities, Videos, and PhotoZooms that you will find on the CD-ROM accompanying this book. These CD elements are designed to supplement the material and reinforce the concepts introduced in this chapter. 10-Mbps and 100-Mbps Ethernet This section introduces the specifics of the most important varieties of Ethernet. The goal is not to memorize all the facts about each type of Ethernet, but rather to develop a sense of what is common to all forms of Ethernet and what are the specific strengths and weak- nesses of the commercially important forms of Ethernet. 1102.book Page 315 Tuesday, May 20, 2003 2:53 PM 316 Chapter 6: Ethernet Technologies and Ethernet Switching Ethernet’s popularity began with the thick coaxial 10BASE5. However, Thicknet cable had undesirable installation properties. 10BASE2 used Thinnet—thinner coaxial cable that was easier to install and terminate; the distance decreased from 500 to 185 meters (m). This trend toward easy installation and lower cost took a great step forward with the introduction of UTP-based 10BASE-T. However, the length that an unrepeated signal could travel decreased to 100m, necessitating the introduction of repeaters and then multiport repeaters (hubs). The repeater concept allowed 10BASE-T networks of up to 500m. As workgroups grew in size and applications increased in complexity, the shared bandwidth of the hub became a limiting factor. The introduction of Ethernet 10BASE-T switches addressed both the length and bandwidth limitations—station-to- switch links were now point-to-point. The power, versatility, and cost-effectiveness of 10BASE-T coincided with an explosion in the number of LAN users, the number of Internet users (which also increased LAN traffic), and the complexity of applications. Demand for higher bandwidth grew, and Fast Ethernet was introduced. The copper cable version of Fast Ethernet that became commercially successful was 100BASE-TX, and many clever features were developed for interoperability with 10BASE-T systems (the emergence of 10/100 interfaces, for example). To compete with the backbone/LAN technology of FDDI, fiber-based 100BASE-FX was introduced. Throughout all of these Ethernet technologies, the MAC addressing concept, the frame format, and the CSMA/CD MAC method were maintained. 10-Mbps Versions of Ethernet Figure 6-1 shows a subset of physical layer implementations that you can deploy to support Ethernet. The 10BASE5, 10BASE2, and 10BASE-T implementations of Ether- net are considered legacy implementations and are referred to as such in the sections that follow. Figure 6-1 Types of Ethernet Logical Link Control Sublayer Physical Signaling Sublayer Physical Medium 10BASES5 (500 m) 50 Ohm Coax N-Style 1000BASE-LX (550-5000 m) MM Fiber Sc 10BASE2 (185 m) 50 Ohm Coax BNC 10BASE-T (100 m) 100 Ohm UTP RJ45 10BASE-TX (100 m) 100 Ohm UTP RJ45 100BASE-FX (228_412 m) MM Fiber SC 1000BASE-T (100 m) 100 Ohm UTP RJ45 1000BASE-SX (220-550 m) MM Fiber SC 10BASE-(Various) MM or Sm Fiber SC 802.3 Media Access Control 1102.book Page 316 Tuesday, May 20, 2003 2:53 PM 10-Mbps and 100-Mbps Ethernet 317 Four things are common among legacy Ethernet: ■ Timing parameters ■ Frame format ■ Transmission process ■ A basic design rule After you learn about what these three historically important versions have in common, you will examine each in more detail. 10BASE5, 10BASE2, and 10BASE-T all share the same timing parameters, as shown in Table 6-1. Note that 1 bit-time at 10 Mbps = 100 nanosecond = 0.1 µsecond = 1 ten-millionth of a second. The frame format is common to 10BASE5, 10BASE2, and 10BASE-T. Figure 6-2 shows an Ethernet frame as observed at the MAC sublayer. Figure 6-2 Ethernet Frame Table 6-1 Parameters for 10-Mbps Ethernet Operation Parameter Value Bit-time 100 nsec Slot time 512 bit-times Interframe spacing 96 bits* *The value listed is the official interframe spacing. Collision attempt limit 16 Collision backoff limit 10 Collision jam size 32 bits Maximum untagged frame size 1518 octets Minimum frame size 512 bits (64 octets) 1102.book Page 317 Tuesday, May 20, 2003 2:53 PM 318 Chapter 6: Ethernet Technologies and Ethernet Switching The Legacy Ethernet transmission process is identical until the lower part of the OSI physical layer. As the frame passes from the MAC sublayer to the physical layer, further processes occur before the bits are placed on the medium from the physical layer. One process that is particularly important at this level is the signal quality error (SQE) signal. This signal is typical of what you will see in many networking technologies. At the phys- ical layer, the network is “alive” with communications other than the user data to ensure a properly functioning network. SQE always is used in half duplex; it is not required but is permitted in full-duplex operation. SQE is active in the following instances: ■ Within 4 to 8 microseconds following a normal transmission, to indicate that the outbound frame successfully was transmitted. ■ Whenever there is a collision on the medium. ■ Whenever there is an improper signal on the medium. Improper signals might include detected jabber, or the reflections that result from a cable fault, such as a short. (There are separate conditions depending on which medium is attached.) ■ Whenever a transmission has been interrupted as jabber—that is, it has transmitted longer than allowed. All 10-Mbps forms of Ethernet take octets received from the MAC sublayer and per- form a process called line encoding. Line encoding describes how the bits actually are signaled on the wire. The simplest encodings (such as nonreturn to zero, or NRZ, in which a 1 bit is 5 volts [V] and the 0 bits are 0V) typically have undesirable timing and electrical characteristics. Therefore, line codes have been engineered to have desirable transmission properties and tailored to each medium. The form of line encoding used in 10-Mbps systems is called Manchester encoding. Figure 6-3 shows a Manchester encoding example. The y-axis is voltage; the x-axis is time. Figure 6-3 Manchester Encoding Examples N O TE In NRZ encoding, sig- nals are maintained at constant voltage levels, with no signal transitions (no return to a 0V level) during a bit interval. 1102.book Page 318 Tuesday, May 20, 2003 2:53 PM . (22 8_ 4 12 m) MM Fiber SC 10 00BASE-T (10 0 m) 10 0 Ohm UTP RJ45 10 00BASE-SX (22 0-550 m) MM Fiber SC 10 BASE-(Various) MM or Sm Fiber SC 8 02. 3 Media Access Control 11 02. book Page 316 Tuesday, May 20 , 20 03. Sublayer Physical Signaling Sublayer Physical Medium 10 BASES5 (500 m) 50 Ohm Coax N-Style 10 00BASE-LX (550-5000 m) MM Fiber Sc 10 BASE2 (18 5 m) 50 Ohm Coax BNC 10 BASE-T (10 0 m) 10 0 Ohm UTP RJ45 10 BASE-TX (10 0 m) 10 0 Ohm UTP RJ45 10 0BASE-FX (22 8_ 4 12 . bits Maximum untagged frame size 15 18 octets Minimum frame size 5 12 bits (64 octets) 11 02. book Page 317 Tuesday, May 20 , 20 03 2: 53 PM 318 Chapter 6: Ethernet Technologies and Ethernet Switching The