5 The First Ethernet Implementation Developed by Xerox in 1973Based on bus topologyTransferred data at 3Mbps maxRemained in-house technology until 1979Evolved into today’s Ethernet standards
6 The Next Iteration of the Ethernet DIX (Digital-Intel-Xerox) standardTransferred data at 10Mbps maxDEC, Intel, and Xerox transferred control of the Ethernet standard to IEEE802.3 (Ethernet) committeeTech Tip: IEEE (p. 53)The source for all things Ethernet is but a short click away on the Internet. For starters, check out
7 802.3 Standards Several variant standards Similar issues 802.3i, 802.3ab, 802.3by, etc.Similar issuesHow to send data across the wireHow to identify the sending and receiving computersHow to determine which computer should use shared cable at what timeNote (p.53): There have been four different Ethernet frame types defined over the years, but only one, the Ethernet II frame type, is used today. Every version of Ethernet uses the same Ethernet II frame.
8 Organizing the Data: Ethernet Frames Test SpecificOrganizing the Data: Ethernet Frames
9 Ethernet Frames (1 of 2)Frames are smaller pieces of data transmitted between computersUsing frames addresses two networking issuesPrevents any single machine from monopolizing the shared bus cableMakes retransmitting lost data more efficientSmall frames means computers can share the cable easily—each listens on the segment.Exam Tip (p. 54): The terms frame and packet are often used interchangeably, especially on exams! This book uses the terms more strictly. You’ll recall from Chapter 1 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.
11 Preamble and MAC addresses Beginning of each frameSeven bytes of alternating ones and zerosStart frame delimiterFollows the preambleOne byteMAC addressUnique identifying address for each nodeExam Tip (p. 54): The CompTIA Network+ exam might describe MAC addresses as 48-bit binary addresses or 6-byte binary addresses.Cross Check: NICs and OSI (p. 55)You learned about NICs and MAC addresses in Chapter 1, “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?
12 Type and DataTypeHelps receiving computer interpret the frame contents at a basic level, e.g., IPv4 or IPv6 dataDataPart of the frame that contains the payloadIf an IP packet, packet contains extra information such as the IP addresses of both systems
13 Pad and the Frame Check Sequence Minimum Ethernet frame size is 64 bytesExtra data added if frame has fewer than 64 bytesFrame check sequenceHelps determine if the data was damaged in transitCalculation used at the beginning and at the end of transmission must give same resultCyclic redundancy check (CRC)
15 Bus Ethernet (1 of 2) Hybrid star-bus Hub at the center Electronic repeaterRepeats the same signal out to the other connected portsDoes not send signal back down the originating portRepeaters are not amplifiersExam Tip (p.56): The typical scenario involving installing early networks put the placement of the hub near the center of the network.
17 10BaseT: Physical vs. Logical Over 99 percent of all networks use 10BaseT or its newer versionsConsists of two or more computers connected to a central hubNICs connect with wires per standardsHubs for 10BaseTVary in size, shape, and number of portsAll need electrical powerCross Check: Physical vs. Logical (p. 56)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.
18 10BaseT: UTP (1 of 7) Uses CAT 3 or higher Two pairs of wires required (four-pair cable commonly used)One pair of wires sends data to the hubThe other pair receives data from the hub
19 Figure 3.3 A typical four-pair CAT 5e unshielded twisted-pair cable 10BaseT: UTP (2 of 7)Figure A typical four-pair CAT 5e unshielded twisted-pair cable
20 10BaseT: UTP (3 of 7) RJ-45 Connector Used in 10BaseT Each pin connects to a single wire inside the cablePins are numbered from one to eightNote (p. 57): As noted in Chapter 2, the real name for RJ-45 is 8 position 8 contact (8P8C) modular plug. The term RJ-45 is so prevalent, however, that nobody but the nerdiest of nerds calls it by its real name. Stick to RJ-45.
21 Figure 3.4 Two views of an RJ-45 connector 10BaseT: UTP (4 of 7)Figure Two views of an RJ-45 connector
22 Figure 3.5 The pins on an RJ-45 connector 10BaseT: UTP (5 of 7)Figure The pins on an RJ-45 connectorare numbered 1 through 8
23 10BaseT: UTP (6 of 7) RJ-45 pin assignments 1 and 2 send data3 and 6 receive dataFull duplex versus half-duplex modeRJ-45 connector is called a crimpCrimping is the act of installing an RJ-45 connectorA crimper is the tool usedWires are color-codedTeaching 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.
25 TIA/EIA 568A and 568B (1 of 2)TIA/EIA defines industry standards for correct crimpingAdvantage of following an established color code schemeEnsures wires match up correctly at each end of the cableNetwork technicians can make their own Ethernet cables
26 Figure 3.7 The TIA/EIA 568A and 568B standards TIA/EIA 568A and 568B (2 of 2)Exam Tip (p. 58): 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. 58)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. 58):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 The TIA/EIA 568A and 568B standards
27 10BaseT Summary Speed: 10 Mbps Signal type: Baseband Distance: 100 meters between hub/nodeNode limit: 1024 nodes per hubTopology: Star-bus topology: physical star, logical busCable type: CAT3 or better UTP cabling with RJ-45 connectorsNote: There are many situations where one computer might have two or more NICs, so one system might have more than one node!Check out the Chapter 3 Challenge! sim “T-568B” at It’s a great tool for getting the colors set in your head.
28 10BaseFL (1 of 2) Fiber-optic version Increased maximum distance Two kilometers between the hub and the nodeImmune to electrical interferenceMore secure because difficult to tap intoMultimode fiber-optic cables with ST or SC connectorsNote (p. 59): 10BaseFL is often simply called “10BaseF.”
30 10BaseFL Summary Speed: 10Mbps Signal type: Baseband Distance: meters between hub/nodeNode limit: 1024 nodes per hubTopology: Star-bus topology: physical star, logical busCable type: Multimode fiber-optic cabling with ST or SC connectors
31 Media Converters (1 of 2)10BaseT and 10BaseFL have different cabling and hubs but same Ethernet packetsA media converter connects different Ethernet types
32 Media Converters (2 of 2)Figure Typical copper-to-fiber Ethernet media converter(photo courtesy of TRENDnet)
33 CSMA/CD (1 of 3)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 (first-come, first-served)Collision Detection: if two NICs transmit at the same time, a collision results.NICs may listen to detect a collision.Exam Tip (p. 60): CSMA/CD is a network access method that maps to the IEEE standard for Ethernet networks.
34 Figure 3.10 No one else is talking—send the frame! CSMA/CD (2 of 3)Figure No one else is talking—send the frame!
37 The Trouble with HubsClassic 10BaseT network can only have one message on the wire at a timeCollisions slow the effective transmission speed for the whole networkEthernet switchCreates point-to-point connections between two conversing computers
38 Switches to the Rescue (1 of 4) Ethernet switches give every conversation the full bandwidth of the networkSource Address Table (SAT)A switch copies the source MAC addresses and builds a table of MAC addresses of each connected computerExam Tip (p. 62):Adding another hub or two to an early Ethernet network enabled you to add more devices, but also compounded the problem with collisions. One option was to connect networks using a bridge. A bridge acted like a repeater to connect two networks, but then went a step further—filtering and forwarding traffic between those segments based on the MAC addresses of the computers on those segments. This preserved precious bandwidth, making larger Ethernet networks possible. You’ll see the term bridge applied to modern devices, primarily in wireless networking. The interconnectedness of network segments is similar, but the devices are fundamentally different. See Chapter 14, “Wireless Networking,” for the scoop on wireless.Exam Tip (p. 62): 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.
39 Switches to the Rescue (2 of 4) Exam Tip (p. 62): 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 Hub (top) and switch (bottom) comparison
40 Switches to the Rescue (3 of 4) Figure A switch tracking MAC addresses
41 Switches to the Rescue (4 of 4) Figure A switch making four separate connections
42 Connecting Ethernet Segments (1 of 2) When all ports on an existing switch have been used, add another switchSwitches can be connected using an uplink port or a crossover cableUplink portsConnect two switches using a straight-through cable
43 Connecting Ethernet Segments (2 of 2) Figure Typical uplink port
44 Crossover Cables (1 of 2) Connect switches without uplink port Connect via two normal ports using one crossover cableReverse sending and receiving pairs on one endOne end crimped per TIA/EIA 568ASecond end crimped per TIA/EIA 568B
45 Crossover Cables (2 of 2) Figure 3.17 A crossover cable reverses the sending and receiving pairs
46 Spanning Tree Protocol (STP) (1 of 3) Eliminates the problem of accidental bridge loops (i.e., redundant connections in a network)With STP enabled:Loops are detectedLooped port’s state is set to blocking
47 Spanning Tree Protocol (STP) (2 of 3) Exam Tip (p. 64): 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 A bridging loop
48 Spanning Tree Protocol (STP) (3 of 3) STP-enabled switches use a Bridge Protocol Data Unit (BPDU) frameDetermines distance between themHelps keep track of changes on the networkRapid Spanning Tree Protocol (RSTP), 802.1w replaced the original STP in 2001Exam Tip (p.65): The CompTIA Network+ exam objectives refer to STP, BPDU guard, and root guard as mitigation techniques. That’s a fancy way of saying that the technologies make negative issues less destructive. They help preserve the network.
49 Troubleshooting Switches Problem categoriesPhysical damage, dead portsSwitch might have problems if device can’t connect to the networkCheck for link lightsCheck cablesReplace switch with a known-good deviceNote (p. 65): When we get to modern higher-end switches in Chapter 11, “Advanced Networking Devices,” you’ll need to follow other procedures to do proper diagnostic work. We’ll get there soon enough!