Ethernet Basics Chapter 4

Objectives Define and describe Ethernet Explain early Ethernet implementations Describe ways to extend and enhance Ethernet networks

Introduction Networks did not exist when computers were first created Sneakernet was the method of moving files A more efficient method of sharing data was developed

Historical/Conceptual Ethernet

The First Ethernet Implementation Developed by Xerox in 1973 Based on bus topology Transferred data at 3Mbps max Remained in-house technology until 1979 Evolved into today’s Ethernet standards

The Next Iteration of the Ethernet DIX (Digital-Intel-Xerox) standard Transferred data at 10Mbps max DEC, Intel, and Xerox transferred control of the Ethernet standard to IEEE 802.3 (Ethernet) committee Tech Tip: IEEE (p. 63) The source for all things Ethernet is but a short click away on the Internet. For starters, check out www.ieee802.org

Issues Faced by Ethernet’s Designers How to send data across the wire How to identify the sending and receiving computers How to determine which computer should use shared cable at what time Tech Tip: Defining Ethernet (p. 63) Providing a clear and concise definition of Ethernet has long been one of the major challenges in teaching networking. This difficulty stems from the fact that Ethernet has changed over the years to incorporate new and improved technology. Most folks won’t even try to define Ethernet, but here’s my best attempt at a current definition. Ethernet is a standard for a family of network technologies that share the same basic bus topology, frame type, and network access method. Because the technologies share these essential components, you can communicate between them just fine. The implementation of the network might be different, but the frames remain the same. This is true for Ethernet running on a physical bus topology—the ancient 10Base5 and 10Base2—and a logical bus topology—10BaseT and later.

Topology Hybrid star-bus Hub at the center Electronic repeater Repeats the same signal out to the other connected ports Does not send signal back down the originating port Repeaters are not amplifiers

Figure 4.1 Ethernet hub

Organizing the Data: Ethernet Frames Test Specific Organizing the Data: Ethernet Frames

Ethernet Frames Smaller pieces of data transmitted between computers Using frames addresses two networking issues Prevents any single machine from monopolizing the shared bus cable Makes retransmitting lost data more efficient Used by all networking technologies (including Ethernet) Exam Tip (p. 64): The terms frame and packet are often used interchangeably, especially on exams! This book uses the terms more strictly. You’ll recall from Chapter 2 that frames are based on MAC addresses; packets are generally associated with data assembled by the IP protocol at Layer 3 of the OSI seven-layer model.

Figure 4.2 Ethernet frame

Preamble and MAC addresses Beginning of each frame Seven bytes of alternating ones and zeros Start frame Follows the preamble One byte MAC address Unique identifying address for each node Exam Tip (p. 65): The CompTIA Network+ exam might describe MAC addresses as 48-bit binary addresses or 6-byte binary addresses. Cross Check: NICs and OSI (p. 65) You learned about NICs and MAC addresses in Chapter 2, “so check your memory with these questions. Where does the NIC get its MAC address? How does the MAC address manifest on the card? At what layer or layers of the OSI seven-layer model does the NIC operate?

Figure 4.3 Frames propagating on a network Note: (p. 65): There are many situations in which one computer might have two or more NICs, so one physical system might represent more than one node. Figure 4.3 Frames propagating on a network

NICs Ethernet security vulnerability Sniffers can order a NIC to run in promiscuous mode NIC processes all frames, not only those intended for its MAC address Sniffers have legitimate uses, but may also be used unscrupulously Note (p. 65): There are many situations in which one computer might have two or more NICs, so one physical system might represent more than one node.

Type and Data Type Helps receiving computer interpret the frame contents at a basic level ►IPv4 or IPv6 data Data Part of the frame that contains the payload If an IP packet, packet contains extra information such as the IP addresses of both systems

Pad and the Frame Check Sequence Minimum Ethernet frame size is 64 bytes Extra data added if frame has fewer than 64 bytes Frame check sequence Aids in determining if the data has been damaged in transit Calculation used at the beginning and at the end of transmission must give same result

Network Access Carrier sense Multiple access Each node checks to see whether cable is in use Sends the frame when cable is free Multiple access All machines have equal access to the wire Collision occurs if two machines send frame simultaneously

CSMA/CD Carrier Sense Each NIC on the network examines the wire before sending a frame. If the node detects traffic, it will pause a random amount of time and try again. Multiple Access All machines have equal access to the wire. Access to the wire is on a first-come, first-served basis Collision Detection If two NICs transmit at the same time, a collision results. NICs may listen to detect a collision. Exam Tip (p. 66): CSMA/CD is a network access method that maps to the IEEE 802.3 standard for Ethernet networks.

Figure 4.4 No one else is talking—send the frame!

Figure 4.5 Collision!

Collisions When collision occurs Both machines generate a random number to determine delay time before resending packet Properly running Ethernet network has a maximum collision rate of 10 percent Collision domain A group of nodes that could send frames at the same time ►could potentially cause a collision

Figure 4.6 Rolling for timing

Early Ethernet Networks

Bus Ethernet Original Ethernet networks used a true bus topology Thicknet (10Base5) Thinnet (10Base2) The T connector enabled the bus to carry a single electrical signal that connected every device on the network The ends of the bus have to be terminated

Figure 4.7 Thicknet vampire tap

Figure 4.8 10Base2 T connector in action

Figure 4.9 Terminating resistor

10BaseT: Physical vs. Logical Over 99 percent of all networks use 10BaseT or its newer versions Consists of two or more computers connected to a central hub NICs connect with wires per 802.3 standards Hubs for 10BaseT Vary in size, shape, and number of ports All need electrical power Cross Check: Physical vs. Logical (p. 69) You might be tempted at this moment to define 10BaseT in terms of physical topology versus logical topology—after all, 10BaseT uses a physical star, but a logical bus. Refer to Chapter 3, however, and cross-check your memory. What’s a physical topology? And a logical topology? What would you say if you walked into an office building that implemented a 10BaseT network? Yes, if you actually walked into it, you’d probably say “Ouch!” But beyond that, think about how you would describe the wires and connectors you would see in terms of physical or logical topology. Exam Tip (p. 69): If you ever run into a situation on a 10BaseT or later network in which none of the computers can get on the network, always check the hub first!

10BaseT Signal Type Type of cable Baseband Twisted Pair Speed A single signal on the cable Type of cable Twisted Pair Speed 10 Mbps Exam Tip (p. 70): The names of two earlier physical bus versions of Ethernet, 10Base5 and 10Base2, gave the maximum length of the bus. 10Base5 networks could be up to 500 meters long, for example, whereas 10Base2 topped out at 185 meters. (What, you were expecting 200 meters?).

Figure 4.10 Two 10BaseT hubs

10BaseT: UTP Uses CAT 3 or higher Two pairs of wires required (four-pair cable commonly used) One pair of wires sends data to the hub The other pair receives data from the hub Cross Check: Check Your CATs! (p. 70) You’ve already seen CAT levels in Chapter 3, so check your memory and review the different speeds of the various CAT levels. Could 10BaseT use CAT 2? Could it use CAT 6? What types of devices can use CAT 1?

Figure 4.11 A typical four-pair CAT 5e unshielded twisted-pair cable

Check Your CATS! RJ-45 Connector Used in 10BaseT Each pin connects to a single wire inside the cable Pins are numbered from one to eight Note (p. 71): The real name for RJ-45 is “8 Position 8 Contact (8P8C) modular plug.” The term RJ-45 is so dominant, however, that nobody but the nerdiest of nerds calls it by its real name. Stick to RJ-45.

Figure 4.12 Two views of an RJ-45 connector

Figure 4.13 The pins on an RJ-45 connector are numbered 1 through 8

Check Your CATS! (cont’d.) RJ-45 pin assignments 1 and 2 send data 3 and 6 receive data Duplex versus half-duplex mode RJ-45 connector is called a crimp Crimping is the act of installing an RJ-45 connector A crimper is the tool used Wires are color-coded Teaching Tip: Although it is stated in the text, be sure to emphasize that in spite of having separate wires for sending and receiving, 10BaseT does not allow for simultaneous send-receive. Rules of CSMA/CD apply. Later versions of Ethernet changed this rule.

Figure 4.14 Color-coded pairs

TIA/EIA 568A and 568B TIA/EIA defines industry standards for correct crimping Advantage of following an established color code scheme Ensures wires match up correctly at each end of the cable Network technicians can make their own Ethernet cables

Figure 4.15 The TIA/EIA 568A and 568B standards Note (p. 71): TIA/EIA 568C, the current standard, includes the same wiring standards as TIA/EIA 568A and TIA/EIA 568B. It’s all just wrapped up in a new name: ANSI/TIA-568-C. When the EIA left the planet in 2011 the names of the standards changed. CompTIA continues to use the older name on exams. Tech Tip: 568A and 568B (p. 71) An easy trick to remembering the difference between 568A and 568B is the word “GO.” The green and orange pairs are swapped between 568A and 568B, whereas the blue and brown pairs stay in the same place! Exam Tip (p. 71): For the CompTIA Network+ exam, you will be tested on the TIA/EIA 568A or 568B color codes. Memorize them. You’ll see the standards listed as EIA/TIA 568A, TIA/EIA568A, T568A, or just 568A. Know the A and B and you’ll be fine. Figure 4.15 The TIA/EIA 568A and 568B standards

10BaseT Limits and Specifications Maximum distance between hub and computer: 100 meters No more than 1024 computers connected to one hub Such a high number is too expensive and not practical Excessive collisions can easily bog down Ethernet performance

10BaseT Summary Speed: 10 Mbps Signal type: Baseband Distance: 100 meters between hub/node Node limit: 1024 nodes per hub Topology: Star-bus topology: physical star, logical bus Cable type: CAT3 or better UTP cabling with RJ-45 connectors Note: There are many situations where one computer might have two or more NICs, so one system might have more than one node!

10BaseFL Fiber-optic version Increased maximum distance Two kilometers between the hub and the node Immune to electrical interference More secure because difficult to tap into Multimode fiber-optic cables with ST or SC connectors Note (p. 72): 10BaseFL is often simply called “10BaseF.”

Figure 4.16 Typical 10BaseFL card

10BaseFL Summary Speed: 10Mbps Signal type: Baseband Distance: 2000 meters between hub/node Node limit: 1024 nodes per hub Topology: Star-bus topology: physical star, logical bus Cable type: Multimode fiber-optic cabling with ST or SC connectors

Media Converters 10BaseT and 10BaseFL have different cabling and hubs but same Ethernet packets A media converter connects different Ethernet types

Figure 4.17 Typical copper-to-fiber Ethernet media converter (photo courtesy of TRENDnet)

Extending and Enhancing Ethernet Networks

General Tips Install additional hubs to connect multiple LANs Use a network bridge to connect two Ethernet networks Replace hubs with better devices to reduce collisions

Coupler Device with female connectors on both ends Used to connect a machine in a location not planned for in original network Examples of coupler types BNC couplers UTP couplers

Connecting Ethernet Segments When all ports on an existing hub have been used, add another hub or a bridge Hubs can be connected using an uplink port or a crossover cable Uplink ports Connect two hubs using a straight-through cable

Figure 4.19 Typical uplink port

Connecting Hubs When connecting hubs: You can only daisy-chain hubs Take time to figure out the uplink ports If you plug hubs in incorrectly, no damage will occur (they just won’t work) Exam Tip (p. 74): Two terms you might see on hubs and switches and, consequently, on the exams: MDI and MDIX. A media dependent interface (MDI) is a regular port on a hub or switch. A media dependent interface crossover (MDIX) is an uplink port.

Figure 4.20 Daisy-chained hubs

Figure 4.21 A hierarchical hub configuration will not work!

Figure 4.22 Press-button port

Crossover Cables Another way to connect two hubs Connect via two normal ports using one crossover cable Reverse sending and receiving pairs on one end One end crimped per TIA/EIA 568A Second end crimped per TIA/EIA 568B Exam Tip (p. 75): The CompTIA Network+ objectives compare three types of copper cables: straight-through vs. crossover vs. rollover. The first two you should have now, right? Straight-through uses the same standard for the RJ-45 on both ends; crossover uses 568A on one end and 568B on the other. A rollover cable has an RJ-45 on one end and a class RS-232 serial port on the other. They’re used to connect a laptop or other computer directly to a Cisco switch or router. A rollover cable, therefore, is completely different from the other two. You’ll see rollover cables in Chapter 8, “Routing,” and the discussion of managed devices. Don’t get fooled on the exam if rollover is added as possible answer for connecting computers. Tech Tip: Crossing Crossovers (p. 75) If you mess up your crossover connections, you won’t cause any damage, but the connection will not work. Think about it. If you take a straight-through cable (that is, not a crossover cable) and try to connect two PCs directly, it won’t work. Both PCs will try to use the same send and receive wires. When you plug the two PCs into a hub, the hub electronically crosses the data wires, so one NIC sends and the other can receive. If you plug a second hub to the first hub using regular ports, you essentially cross the cross and create a straight connection again between the two PCs! That won’t work. Luckily, nothing gets hurt—except your reputation if one of your colleagues notes your mistake.

Figure 4.23 A crossover cable reverses the sending and receiving pairs Try This: Examine Your Uplink Ports (p. 75) Although most hubs come with uplink ports, they all seem to have different ways to use them. Some hubs have dedicated uplink ports, and some have uplink ports that convert to regular ports at the press of a button. Take a look at some hubs and try to figure out how you would use an uplink port to connect it to another hub. Figure 4.23 A crossover cable reverses the sending and receiving pairs

Bridge Acts like a repeater or hub to connect two Ethernet segments Goes one step beyond a repeater or hub Filters and forwards traffic At first, acts like a repeater or hub Monitors and records network traffic Then begins to filter and forward Exam Tip (p. 76): Because bridges work with MAC addresses, they operate at Layer 2, the Data Link layer, of the OSI networking model. They function in the Link/Network Interface layer of the TCP/IP model.

Switched Ethernet

The Trouble with Hubs Classic 10BaseT network can only have one message on the wire at a time Collisions slow the effective transmission speed for the whole network Ethernet switch Creates point-to-point connections between two conversing computers Note (p. 76): SAP originally stood for Systems Applications and Products when the company formed in the early 1970s. Like IBM, SAP is now just referred to by the letters.

Figure 4.24 Hub (top) and switch (bottom) comparison Exam Tip (p. 77): One classic difference between a hub and a switch is in the repeating of packets during normal use. Although it’s true that switches initially forward all frames, they filter by MAC address in regular use. Hubs never learn and always forward all frames Figure 4.24 Hub (top) and switch (bottom) comparison

Switches to the Rescue Ethernet switches give every conversation the full bandwidth of the network Source Address Table (SAT) A switch copies the source MAC addresses and builds a table of MAC addresses of each connected computer Note (p. 78): Because a switch filters traffic on MAC addresses (and MAC addresses run at Layer 2 of the OSI seven-layer model), they are sometimes called Layer 2 switches. Exam Tip (p. 78): When presented with a question about broadcast domains vs. collision domains, here’s what you need to know. In early pre-switched Ethernet networks, there was no difference between the two. All broadcast traffic went to all nodes; all nodes connected to the same bus and thus collisions occurred. Switches essentially eliminated collisions among the nodes attached to the switches within a broadcast domain.

Figure 4.25 A switch tracking MAC addresses

Figure 4.26 A switch making two separate connections

Figure 4.27 Switches are very commonly connected in a tree organization

Spanning Tree Protocol (STP) Eliminates the problem of accidental bridge loops (i.e., redundant connections in a network) With STP enabled: Loops are detected Looped port’s state is set to blocking Note (p. 79): Administrators can manually change STP settings for specific ports by using the management interface for the switch. On a switch port directly connected to a port on a busy server, for example, could be set as portfast—meaning it always forwards traffic. Likewise, and administrator could apply BPDU filtering to the port so it doesn’t send or receive BPDU traffic.

Exam Tip (p. 78): The CompTIA Network+ exam refers to bridging loops as switching loops. The terms mean the same thing, but bridging loop is more common. Be prepared for either term on the exam. Figure 4.28 A bridging loop

Spanning Tree Protocol (cont’d.) STP-enabled switches use a Bridge Protocol Data Unit (BPDU) frame Determines distance between them Helps keep track of changes on the network Rapid Spanning Tree Protocol (RSTP), 802.1w replaced the original STP in 2001 Note: Switches today all have STP enabled and network designers create bridging loops in their networks to provide fault tolerance. Ports set as blocking still listen to the traffic on the network. If a link fails, the blocking port can become a forwarding port, thus enabling traffic to flow properly.

Troubleshooting Hubs and Switches Problem categories Physical damage, dead ports, or general flakiness Hub or switch might have problems if device can’t connect to the network Check for link lights Check cables Replace hub or switch with a known-good device Note (p. 79): When we get to modern higher-end switches in Chapter 12, you’ll need to follow other procedures to do proper diagnostic work. We’ll get there soon enough!