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Ethernet Basics Chapter 3.

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1 Ethernet Basics Chapter 3

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

3 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

4 Historical/Conceptual

5 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

6 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. 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 issues 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 Note (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 Specific Organizing the Data: Ethernet Frames

9 Ethernet Frames (1 of 2) Frames are 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 Small 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.

10 Ethernet Frames (2 of 2) Figure Ethernet frame

11 Preamble and MAC addresses
Beginning of each frame Seven bytes of alternating ones and zeros Start frame delimiter Follows the preamble One byte MAC address Unique identifying address for each node Exam 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 Data Type Helps receiving computer interpret the frame contents at a basic level, e.g., 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

13 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 Helps determine if the data was damaged in transit Calculation used at the beginning and at the end of transmission must give same result Cyclic redundancy check (CRC)

14 Early Ethernet Standards

15 Bus Ethernet (1 of 2) 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 Exam Tip (p.56): The typical scenario involving installing early networks put the placement of the hub near the center of the network.

16 Bus Ethernet (2 of 2) Figure Ethernet hub

17 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 standards Hubs for 10BaseT Vary in size, shape, and number of ports All need electrical power Cross 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 hub The 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 cable Pins are numbered from one to eight Note (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 connector are numbered 1 through 8

23 10BaseT: UTP (6 of 7) RJ-45 pin assignments
1 and 2 send data 3 and 6 receive data Full 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.

24 Figure 3.6 Color-coded pairs
10BaseT: UTP (7 of 7) Figure 3.6 Color-coded pairs

25 TIA/EIA 568A and 568B (1 of 2) 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

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/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! 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 node Immune to electrical interference More secure because difficult to tap into Multimode fiber-optic cables with ST or SC connectors Note (p. 59): 10BaseFL is often simply called “10BaseF.”

29 Figure 3.8 Typical 10BaseFL card
10BaseFL (2 of 2) Figure Typical 10BaseFL card

30 10BaseFL Summary Speed: 10Mbps Signal type: Baseband
Distance: 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

31 Media Converters (1 of 2) 10BaseT and 10BaseFL have different cabling and hubs but same Ethernet packets A 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!

35 CSMA/CD (3 of 3) Figure Collision!

36 Extending and Enhancing Ethernet Networks

37 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

38 Switches to the Rescue (1 of 4)
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 Exam 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 switch Switches can be connected using an uplink port or a crossover cable Uplink ports Connect 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 cable Reverse sending and receiving pairs on one end One end crimped per TIA/EIA 568A Second 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 detected Looped 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) 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 Exam 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 categories Physical damage, dead ports Switch might have problems if device can’t connect to the network Check for link lights Check cables Replace switch with a known-good device Note (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!

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