1. Ethernet Basics
Chapter 4

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
Ethernet

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. 63)
The source for all things Ethernet is but a short click away on the Internet. For starters, check out

7. 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.

8. 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

9. Figure Ethernet hub

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

11. 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.

12. Figure Ethernet frame

13. 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?

14. 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 Frames propagating on a network

15. 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.

16. 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

17. 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

18. 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

19. 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 standard for Ethernet networks.

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

21. Figure Collision!

22. 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

23. Figure 4.6 Rolling for timing

24. Early Ethernet Networks

25. 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

26. Figure 4.7 Thicknet vampire tap

27. Figure 4.8 10Base2 T connector in action

28. Figure 4.9 Terminating resistor

29. 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. 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!

30. 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?).

31. Figure Two 10BaseT hubs

32. 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?

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

34. 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.

35. Figure 4.12 Two views of an RJ-45 connector

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

37. 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.

38. Figure 4.14 Color-coded pairs

39. 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

40. 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 The TIA/EIA 568A and 568B standards

41. 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

42. 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!

43. 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.”

44. Figure 4.16 Typical 10BaseFL card

45. 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

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

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

48. Extending and Enhancing Ethernet Networks

49. 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

50. 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

51. 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

52. Figure 4.19 Typical uplink port

53. 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.

54. Figure 4.20 Daisy-chained hubs

55. Figure 4.21 A hierarchical hub configuration will not work!

56. Figure 4.22 Press-button port

57. 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.

58. 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 A crossover cable reverses the sending and receiving pairs

59. 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.

60. Switched Ethernet

61. 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.

62. 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 Hub (top) and switch (bottom) comparison

63. 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.

64. Figure 4.25 A switch tracking MAC addresses

65. Figure 4.26 A switch making two separate connections

66. Figure 4.27 Switches are very commonly connected in a tree organization

67. 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.

68. 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 A bridging loop

69. 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.

70. 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!