Switched, Fast, and Gigabit Ethernet - CH 3 - Ethernet, Fast Ethernet, and Gigabit Ethern.. Page 1 of 36 [Figures are not included in this sample chapter] Switched, Fast, and Gigabit Ethernet - 3 - Ethernet, Fast Ethernet, and Gigabit Ethernet Standards This chapter discusses the theory and standards of the three versions of Ethernet around today: regular 10Mbps Ethernet, 100Mbps Fast Ethernet, and 1000Mbps Gigabit Ethernet. The goal of this chapter is to educate you as a LAN manager or IT professional about essential differences between shared 10Mbps Ethernet and these newer technologies. This chapter focuses on aspects of Fast Ethernet and Gigabit Ethernet that are relevant to you and doesn’t get into too much technical detail. Read this chapter and the following two (Chapter 4, "Layer 2 Ethernet Switching," and Chapter 5, "VLANs and Layer 3 Switching") together. This chapter focuses on the different Ethernet MAC and PHY standards, as well as repeaters, also known as hubs. Chapter 4 examines Ethernet bridging, also known as Layer 2 switching. Chapter 5 discusses VLANs, some basics of routing, and Layer 3 switching. These three chapters serve as a precursor to the second half of this book, namely the hands-on implementation in Chapters 8 through 12. After you understand the key differences between yesterday’s shared Ethernet and today’s Switched, Fast, and Gigabit Ethernet, evaluating products and building a network with these products should be relatively straightforward. The chapter is split into seven sections: l "Ethernet and the OSI Reference Model" discusses the OSI Reference Model and how Ethernet relates to the physical (PHY) and Media Access Control (MAC) layers of the OSI model. l The second section, "10Mbps Ethernet MAC and PHY Standards," delves into the 10Mbps Ethernet MAC in more detail and discusses the various Ethernet physical layer implementations available today: Thick, Thin, Twisted-Pair, and Fiber Ethernet. You can skip this section if you are very familiar with Ethernet. We included it as a refresher to set the stage for our discussion of the new Fast Ethernet standard and switching technology. l Ethernet was designed to be a shared-media, half-duplex technology. The third section of this chapter, "Half- and Full-Duplex Ethernet," discusses the transition from half-duplex to full- duplex transmission. l The fourth section of this chapter, "100BASE-T/Fast Ethernet," intro-duces the IEEE 802.3u Fast Ethernet standard and discusses how it differs from the regular 10Mbps Ethernet standard. This section explains the Fast Ethernet MAC and the four different PHY standards in more detail. l In the section "Gigabit Ethernet," you learn about the new Gigabit Ethernet MAC and PHY standard, as well as the new 1000BASE-T PHY for Category 5 or better UTP cable. file://I:\chapters\z\zd594.html 3/21/01 Switched, Fast, and Gigabit Ethernet - CH 3 - Ethernet, Fast Ethernet, and Gigabit Ethern.. Page 2 of 36 l Next, we discuss the auto-negotiation standard. Auto-negotiation was first introduced as part of the Fast Ethernet standard. It is an automatic speed-selection method, connecting different devices together. l In the final section, "Ethernet Repeaters," we start explaining deployment by means of repeaters. In particular, this section discusses Ethernet and Fast Ethernet repeater standards, as well as the Gigabit Ethernet buffered distributor/repeater design specification. We also cover repeater design rules. Chapter 6, "Cabling and More on Physical Layers," covers the Fast and Gigabit Ethernet physical layers and the corresponding encoding and cabling specifications in much more detail. Ethernet and the OSI Reference Model Most data communications protocols in use today are defined in terms of a layered model called the Open Systems Interconnection (OSI) Reference Model. Table 3.1 describes the layers of this model. TABLE 3.1 NOVELL'S NETWARE OPERATING SYSTEM ADHERES TO THE ISO/OSI REFERENCE MODEL MORE CLOSELY THAN MOST OTHER OPERATING SYSTEMS Layer Layer Examples Comment Name Number Application 7 NetWare NLMs Provides access to the network. Presentation 6 Formats data for workstation-based interpretation/display. Session 5 NetWare Core Protocol (NCP) Layers 5-7 are often not clearly defined and vary by operating system. Some applications span all three layers. Transport 4 SPX (Sequenced Packet Provides end-to-end management, including Exchange) acknowledgment, error correction. Network 3 IPX (Internetwork Packet Routes packets. Dissimilar LANs Exchange), IPX RIP (IPX communicate through Layer 3 routing. Routing Information Protocol) Data link 2 LLC: IEEE 802.2 LLC Bridges are Layer 2 devices. Generates standard source and destination addresses. AUI interfaces between Layers 1 and 2. MAC: IEEE 802.3 Ethernet, Fast Ethernet, Gigabit Ethernet Physical 1 UTP, coax, fiber Specifies the electrical and signal/coding characteristics on the cable. Repeaters operate at this level. Both ANSI and the IEEE have used this seven-layer model in the past for good reason. Breaking down a technology into different layers allows a given layer to be changed without impacting the file://I:\chapters\z\zd594.html 3/21/01 Switched, Fast, and Gigabit Ethernet - CH 3 - Ethernet, Fast Ethernet, and Gigabit Ethern.. Page 3 of 36 remainder of the model. The IEEE was able to add unshielded twisted-pair support to Ethernet while still keeping Ethernet’s core intact--just as different software protocols, such as IPX, TCP/IP, and NetBEUI, can be used with the same hardware because each component forms an independent layer. In this way, interoperability between network applications is greatly improved. Next, we discuss Layers 1 through 3 in more detail. (Layers 4 through 7 are not relevant to this book, so we do not cover them here.) Layer 1: The Physical Layer The physical layer, or PHY, protocol defines the electrical signaling, symbols, line states, clocking requirements, encoding of data, and connectors for data transmission. An example of a PHY layer is 10BASE-T, which uses Manchester encoding to transmit data. Repeaters are Layer 1 devices in that they only retransmit signals without decoding them. That means a repeater has no MAC, and the data is only handled by a PHY on the receiving and transmitting ports. All higher layers talk to the physical layer through a predefined interface. For 10Mbps Ethernet, this is the attachment unit interface (AUI). You can use a DB-15 connector to connect Layer 1 to Layer 2. 100Mbps Ethernet calls this interface the media-independent interface (MII), whereas Gigabit Ethernet calls it the Gigabit media-independent interface (GMII). Layer 1 interfaces to the actual cable by means of the media-dependent interface (MDI). The MDI for 10BASE-T is the RJ-45 connector, for example. Layer 2: The Data Link Layer The data link layer actually consists of two separate pieces: MAC and the Logical Link Control (LLC). The LLC is a standardized interface between a hardware-specific MAC and Layer 3. The IEEE has published a rather complex LLC specification, called 802.2. All IEEE-approved MACs, such as Ethernet and Token Ring, use this LLC interface to talk to Layer 3. Even some non- IEEE defined MACs, such as FDDI, chose to use this LLC as a standardized, independent interface. The LLC function happens primarily in software. The MAC layer is of particular interest in this book. The MAC describes how a station schedules, transmits, and receives data in a shared-media environment. All Layer 2 MACs send and receive frames. The MAC generates the physical source and destination addresses for a particular frame, ensures reliable transfer of information across the link, synchronizes data transmission, recognizes errors, and controls the flow of data. The IEEE has defined a number of MAC/Layer 2 specifications over the years, with Ethernet being the most common one. Other IEEE-defined MACs are Token Ring/802.5, Token-Bus/802.4, and 100VG-AnyLAN/802.11. In general, MACs are very important in shared-media environments where multiple nodes can connect to the same transmission medium. The MAC sends and receives data via Layer 1, the media or physical layer. You can link two different LAN segments via a bridge. You can connect a Thinnet segment (10BASE2) to a 10BASE-T segment by means of a bridge, for example. Bridging is often called a Layer 2 function these days. A bridge contains a MAC and PHY for every port. We discuss fast multiport bridges, better known as Layer 2 switches, in more detail in Chapter 4. Layer 3: The Network Layer file://I:\chapters\z\zd594.html 3/21/01 Switched, Fast, and Gigabit Ethernet - CH 3 - Ethernet, Fast Ethernet, and Gigabit Ethern.. Page 4 of 36 Ethernet, the focus of this book, operates at Layer 2. With Layer 3 switches, however, you can now buy a Layer 2 and Layer 3 device in one box, so we have added a chapter to cover this topic. Layer 2 uses frames, and Layer 3 uses packets. A packet comprises one or more frames: It might take more than one frame to transfer a complete packet. The network layer is responsible for setting up the actual connection between source and destination. Often, different paths exist between two endpoints, and Layer 3 needs to determine the best routes for getting the packet to the final destination. Therefore, Layer 3 is synonymous with routing. Sometimes, dissimilar networks consisting of different types of MAC standards need to be connected together. A company may have an FDDI backbone, for example, but workgroups are connected using Ethernet. In this case, the network layer is required to convert between the dissimilar frames present in the Ethernet and FDDI networks. You can do this by taking the individual frames, reassembling the original packet, and then sending out frames in the new framing format again.