Check Your Understanding 249 11. Which of the following media is used to interconnect the ISDN BRI port to the service-provider device? A. CAT 5 UTP straight-through B. CAT 5 UTP crossover C. Coaxial D. Fiber optic 12. What type of connector is used for DSL connection? A. RJ-45 B. RJ-11 C. F D. DB-9 13. What type of connector is used to connect a router and a cable system? A. RJ-45 B. RJ-11 C. F D. AUI 14. What type of cable is used to connect a terminal and a console port? A. Straight-through B. Rollover C. Crossover D. Coaxial chpt_04.fm Page 249 Tuesday, May 27, 2003 9:01 AM Objectives Upon completion of this chapter, you will be able to ■ Describe briefly the history of Ethernet ■ Identify the IEEE standards ■ Understand MAC addressing ■ Identify the common fields of the data link layer frames ■ Describe Media Access Control (MAC) ■ Describe CSMA/CD ■ Understand the operation of Ethernet ■ Identify different types of collisions ■ Explain collisions, collision domains, and broadcast domains ■ Identify the Layer 1, 2, and 3 devices used to create collision domains and broadcast domains ■ Discuss data flow and the problem with broadcasts ■ Understand collisions and collision domains ■ Understand broadcasts and broadcast domains ■ Understand segmentation of a network and the devices used to create the segments 1102.book Page 250 Tuesday, May 20, 2003 2:53 PM Chapter 5 Ethernet Fundamentals Ethernet, in its various forms, is the most widely used local-area network (LAN) tech- nology. Ethernet was designed to fill the middle ground between long-distance, low-speed networks and specialized, computer-room networks carrying data at high speeds for very limited distance. Ethernet is well suited to applications in which a local communication medium must carry sporadic, occasionally heavy traffic at high-pack data rates. It was designed to enable sharing resources on a local workgroup level. Design goals include simplicity, low cost, compatibility, fairness, low delay, and high speed. In this chapter, you learn about the history of Ethernet and IEEE Ethernet standards. This chapter discusses the operation of Ethernet, Ethernet framing, and error handling, as well as the different types of the collisions on Ethernet networks. In addition, this chapter introduces collision domains and broadcast domains. Finally, this chapter describes seg- mentation and the devices used to create network segments. Please be sure to look at this chapter’s associated e-Lab Activities, Videos, and Photo- Zooms 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. History and Evolution of Ethernet LANs are high-speed, low-error data networks that cover a relatively small geographic area (up to a few thousand meters). LANs connect workstations, peripherals, terminals, and other devices in a single building or other geographically limited area. 1102.book Page 251 Tuesday, May 20, 2003 2:53 PM 252 Chapter 5: Ethernet Fundamentals Ethernet is the dominant LAN technology in the world. Most of the traffic on the Inter- net originates and ends with an Ethernet connection. From its beginning in the 1970s, Ethernet has evolved to meet the increasing demand for high-speed LANs. When a new medium, fiber optics, was produced, Ethernet adapted to take advantage of fiber’s great bandwidth and low error rate. Now the same basic protocol that transported data at 3 megabits per second (Mbps) in 1973 is carrying data at 10 gigabits per second (Gbps). Ethernet’s success is a result of its simplicity and ease of maintenance, its capability to incorporate new technologies, its reliability, and its low cost of installation and upgrade. With the introduction of Gigabit Ethernet, what started as a LAN technology has now had its reach extended to distances that make Ethernet a metropolitan-area and even a wide-area networking standard. This section provides an overview of the Ethernet, including the history of Ethernet, Ethernet naming convention, and Ethernet frame formats. Introduction to Ethernet The original idea for Ethernet grew out of the problem of allowing two or more users to use the same medium without each user’s signals interfering with each other. This problem of multiple user access to a shared medium was studied in the early 1970s at the University of Hawaii. A system called Alohanet was developed to allow various stations on the Hawaiian Islands to each have structured access to the shared radio fre- quency band in the atmosphere. The original technology that today’s Ethernet is based on was wireless. This work later formed the basis for the famous Ethernet MAC method known as carrier sense multiple access collision detect (CSMA/CD). CSMA/CD is dis- cussed in more detail later in this chapter. The original version of Ethernet was the world’s first LAN. It was designed more than 30 years ago by Robert Metcalfe and his coworkers at Xerox. The first Ethernet standard was published by a consortium of Digital Equipment Company, Intel, and Xerox (DIX) in 1980. Metcalfe wanted Ethernet to be a shared standard from which everyone could benefit. Therefore, DIX made the new standard an open standard, meaning that it was available to any company. This was not often done in the computer industry. The first products developed using the Ethernet standard were sold during the early 1980s. Ether- net products transmitted at 10 Mbps over thick (about the diameter of your smallest finger) coaxial cable up to a distance of 2 km. Ethernet was an instant success. The Institute of Electrical and Electronic Engineers (IEEE) is a professional organiza- tion that defines network standards. In 1985, the IEEE standards committee for local and metropolitan networks published its standards for LANs. The IEEE LAN standards are the predominant and best-known LAN standards in the world today. These stan- dards start with the number 802. The standard that was based on Ethernet is standard 1102.book Page 252 Tuesday, May 20, 2003 2:53 PM History and Evolution of Ethernet 253 802.3. The IEEE wanted to make sure that its standards were compatible with and fit into the ISO’s OSI reference model. The IEEE divides the OSI data link layer into two separate sublayers: Media Access Control (MAC) and Logical Link Control (LLC). As a result, some small modifications to the original Ethernet standard were made in the 802.3 standard. Some differences exist between the DIX Ethernet and the 802.3 specifications. However, the differences between the two standards are so minor that any Ethernet network interface card (NIC) can transmit and receive Ethernet and 802.3 packets and frames. Essentially, Ethernet and IEEE 802.3 are the same standards. Just remember that 802.3 is now the official IEEE Ethernet standard. During the mid-1980s, Ethernet’s 10-Mbps bandwidth was more than enough for the PCs of that era. By the early 1990s, PCs had become much faster and people were beginning to complain about the bottleneck caused by the small bandwidth of Ether- net LANs. In 1995, the IEEE announced a standard for a 100-Mbps Ethernet. This was followed by standards for Gigabit (1 billion bits per second) Ethernet in 1998 and 1999. IEEE approved the standards for 10-Gb Ethernet in June 2002. These more modern standards are still Ethernet (802.3). All the new Ethernet standards are essentially compatible with the original Ethernet standard. An Ethernet packet (a frame) could leave an older 10-Mbps NIC in a PC, eventually be placed by a router onto a 10-Gbps Ethernet fiber link, and then end up at a 100-Mbps Ethernet card. As long as the packet stayed on Ethernet networks, it would not be changed. This illustrates one of the main reasons for Ethernet’s great success—it is very scalable. That means that the bandwidth of the network could be increased again and again without changing the underlying Ethernet technology. The original Ethernet (IEEE 802.3) has been supplemented a number of times to incor- porate new transmission media and to enable higher transmission rates. However, it is important to understand that the essential qualities of the original Ethernet have been retained. The Ethernet 802.3 standards all belong to the same family. Differences exist between the standards, but their similarities are greater than their differences. What has been retained from the original in each new standard means that the 802.3 family of protocols are all compatible. IEEE Ethernet Naming Rules The term Ethernet refers to a family of networking technologies that include original Ethernet, Fast Ethernet, Gigabit Ethernet (or Gig-E), and 10-Gb Ethernet (or 10-G). These various types of Ethernet are discussed in detail in Chapter 6, “Ethernet Tech- nologies and Ethernet Switching.” 1102.book Page 253 Tuesday, May 20, 2003 2:53 PM 254 Chapter 5: Ethernet Fundamentals Ethernet interfaces can range from US$10 to $100,000. Ethernet speeds can be 10, 100, 1000, or 10,000 Mbps. This section explores more details of original Ethernet (10BASE-T), Fast Ethernet (100BASE-TX and 100BASE-FX), Gigabit Ethernet (1000BASE-T and 1000BASE-X), and 10-Gb Ethernet. Two features of Ethernet remain consistent across all forms of Ethernet: the basic frame format and the IEEE sublayers of OSI Layer 2. When Ethernet needs to be expanded to add a new medium or capability, the IEEE issues a new supplement to the 802.3 standard. The new supplements are given a one- or two-letter designation—for example, 802.3u is Fast Ethernet. An abbreviated descrip- tion (called an identifier) also is assigned to the supplement. The following are examples of some of the supplements: ■ 10BASE2 (IEEE 802.3a) ■ 10BASE5 (IEEE 802.3) ■ 100BASE-T (IEEE 802.3i) ■ 1000BASE-TX (IEEE 802.3X) As you can see, the abbreviated description consists of these parts: ■ A number indicating the number of megabits per second transmitted ■ The word BASE, indicating that baseband signaling is used ■ Numbers (the 2 and 5) that refer to the coaxial cable segment length (the 185m length has been rounded up to 2, for 200) ■ One or more letters of the alphabet indicating the type of medium used (F = fiber optical cable, T = copper unshielded twisted pair) Baseband signaling is the simplest method of signaling. In baseband signaling, the whole bandwidth of the transmission medium is used for the signal. The data signal (a voltage on UTP or a flash of light on fiber) is transmitted directly over the transmission medium. No other special signal (known as a carrier signal) is required. Ethernet uses baseband signaling. A second method of signaling, broadband signaling, is not used in Ethernet. In broad- band signaling, the data signal is never placed directly on the transmission medium. Instead, an analog signal called the carrier signal is modulated by the data signal. Then this modulated carrier signal is transmitted. Radio broadcasts and cable TV use broad- band signaling. 1102.book Page 254 Tuesday, May 20, 2003 2:53 PM History and Evolution of Ethernet 255 Like the International Organization for Standardization (ISO), the IEEE is a standards- making organization. The manufacturers of networking equipment are not required to fully comply with all the specifications of any standard. The goals of IEEE are as follows: ■ To supply the engineering information necessary to build devices that comply with an Ethernet standard ■ To not stifle innovation by manufacturers If you are setting up or maintaining a small LAN, you can buy all your equipment from one reputable manufacturer. Then you can be assured of the compatibility of all the devices. However, if you are responsible for a large network made up of many smaller Ethernet LANs with equipment from a mix of vendors, your situation is very different. The capability to use equipment from a variety of vendors to interoperate reliably is a very important issue. It is unfortunate that no industry or governmental agency exists to test and certify that a device fully meets an IEEE standard. However, that is a very good reason for you to educate yourself about the standards and the industry. The fact is, you have only your own knowledge and that of your coworkers on which to base your design and purchasing decisions. IEEE 802.3/Ethernet and the OSI Model LAN standards define the physical media and the connectors used to connect devices to media at the physical layer of the OSI reference model. LAN standards also define the way devices communicate at the data link layer. In addition, LAN standards define how to encapsulate protocol-specific traffic in such as way that traffic going to differ- ent upper-layer protocols can use the same channel that passes though that layers of the OSI model. To provide these functions, the IEEE Ethernet data link layer has two sublayers: ■ Media Access Control (MAC) (802.3)—As the name implies, the MAC sublayer defines how to transmit frames on the physical wire. It handles physical address- ing associated with each device, network topology definition, and line discipline. ■ Logical Link Control (LLC) (802.2)—As the name implies, the LLC sublayer is responsible for logically identifying different protocol types and then encapsulat- ing them. A type code or a service access point (SAP) identifier performs the logical identification. The type of LLC frame used by an end station depends on what identifier the upper-layer protocol (such as IP) expects. Although IEEE 802.2 represents one standard type of frame encapsulation, there are others, such as Ethernet II (used primarily with TCP/IP–based Ethernet LANS. These are dis- cussed later in the chapter. 1102.book Page 255 Tuesday, May 20, 2003 2:53 PM 256 Chapter 5: Ethernet Fundamentals As shown in Figure 5-1, the IEEE 802.3 standard defines the physical layer (Layer 1) and the MAC portion of the data link layer (Layer 2). Figure 5-1 802.3 Ethernet and the OSI Model Figure 5-2 maps a variety of technologies to OSI Layer 1 and the lower half of Layer 2. In this book, we focus primarily on Ethernet LAN technology. Layer 1, the physical layer, involves interfacing with media, signals, bit streams that travel on media, com- ponents that put signals on media, and various topologies. The physical layer performs a key role in the communication that takes place between computers, but its efforts alone are not enough. Each of its functions has its limitations. Layer 2 addresses these limitations. Figure 5-2 LAN Specifications and OSI Model 1102.book Page 256 Tuesday, May 20, 2003 2:53 PM History and Evolution of Ethernet 257 For each limitation in Layer 1, Layer 2 has a solution, as documented in Table 5-1. The Layer 2 sublayers, LLC and MAC, are active, vital agreements that make technol- ogy compatible and computer communication possible. The MAC sublayer is concerned with the physical components that will be used to communicate the information. Like the other layers, the LLC remains relatively independent of the physical equipment that will be used for the communicative process. The LLC allows multiple Layer 3 protocols, such as IP and IPX, to be simultaneously supported along with multiple frame types. Figure 5-3 maps a variety of Ethernet technologies to the lower half of OSI Layer 2, and all of Layer 1. Although there are other varieties of Ethernet, the ones depicted are the most widely used and are the focus of this course. Figure 5-3 Ethernet Technologies and OSI Model Table 5-1 Layer 1 Limitations Versus Layer 2 Solutions Layer 1 Limitation Layer 2 Solution Layer 1 cannot communicate with the upper-level layers. Layer 2 communicates with upper- level layers via the LLC sublayer. Layer 1 cannot identify computers. Layer 2 identifies computers using the MAC addressing scheme. Layer 1 can only describe streams of bits. Layer 2 uses framing to organize or group the bits. (This process ulti- mately provides a way for the bits to convey meaning.) Layer 1 cannot decide which computer will transmit binary data from a group that is all trying to transmit at the same time. Layer 2 uses the MAC sublayer to accomplish this. Logical Link Control Sublayer 802.3 Media Access Control Physical Signaling Layer Physical Medium 10BASE5 (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 1102.book Page 257 Tuesday, May 20, 2003 2:53 PM 258 Chapter 5: Ethernet Fundamentals MAC Addressing To allow for local delivery of frames on the Ethernet, there must be an addressing sys- tem, a way of naming the computers and interfaces. Every computer has a unique way of identifying itself. Each computer on a network has a physical address. No two physical addresses on a network should ever be alike. Referred to as the Media Access Control (MAC) address, the physical address is located on the NIC. Other terms for the MAC address include the hardware address, the NIC address, the Layer 2 address, and the Ethernet address. Ethernet uses MAC addresses to uniquely identify individual devices. Every device (PC, router, switch) with an Ethernet interface to the LAN must have a MAC address; otherwise, other devices cannot communicate with it. A MAC address is 48 bits in length and is expressed as 12 hexadecimal digits. The first six hexadecimal digits, which are administered by the IEEE, identify the manufacturer or vendor and thus comprise the organizationally unique identifier (OUI). The remaining six hexadecimal digits comprise the interface serial number, or another value administered by the specific vendor. MAC addresses sometimes are referred to as burned-in addresses (BIAs) because they are burned into read-only memory (ROM) and are copied into random-access memory (RAM) when the NIC initializes. Figure 5-4 illustrates the MAC address format. Figure 5-4 MAC Address Format Without MAC addresses, the LAN would be a group of computers without identifiers, and it would be impossible to deliver an Ethernet frame. Therefore, at the data link layer, a header and a trailer are added to upper-layer data. The header and trailer con- tain control information intended for the data link layer entity in the destination sys- tem. Data from upper-layer entities is encapsulated in the data link layer header and trailer. Ethernet and 802.3 LANs are broadcast networks. All stations see all frames. Each station must examine every frame to determine whether that station is the desired destination. On an Ethernet network, when one device wants to send data to another device, it can open a communication pathway to the other device by using its MAC address. When a Organizational Unique Identifier (OUI) Vendor Assigned (NIC Cards, Interfaces) 24 Bits 6 Hex Digits 00 60 2F Cisco 24 Bits 6 Hex Digits 3A 07 BC Particular Device 1102.book Page 258 Tuesday, May 20, 2003 2:53 PM . 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 m) MM Fiber SC 10 00BASE-T (10 0 m) 10 0 Ohm UTP RJ45 10 00BASE-SX (22 0-550. 10 0, 10 00, or 10 ,000 Mbps. This section explores more details of original Ethernet (10 BASE-T), Fast Ethernet (10 0BASE-TX and 10 0BASE-FX), Gigabit Ethernet (10 00BASE-T and 10 00BASE-X), and 10 -Gb. broadcasts and cable TV use broad- band signaling. 11 02. book Page 25 4 Tuesday, May 20 , 20 03 2: 53 PM History and Evolution of Ethernet 25 5 Like the International Organization for Standardization