Check Your Understanding 309
Token Ring A token-passing LAN developed and supported by IBM. Token Ring
runs at 4 or 16 Mbps over a ring topology.
trailer Controls information appended to data when encapsulating the data for
network transmission.
Check Your Understanding
Complete all the review questions to test your understanding of the topics and con-
cepts in this chapter. Answers are listed in Appendix C, “Check Your Understanding
Answer Key.”
1. Which of the following is not one of the recognized IEEE sublayers?
A. Media Access Control
B. Data Link Control
C. Logical Link Control
D. None of the above
2. The recognized IEEE sublayers are concerned with what layers of the OSI
reference model?
A. 2 and 3
B. 1 and 2
C. 3 and 4
D. 1 and 3
3. The LLC, as a sublayer, participates in the process.
A. Encryption
B. Encapsulation
C. Framing
D. All of the above
4. The first six hexadecimal numbers in a MAC address represent an
?
A. Interface serial number
B. Organizationally unique identifier
C. Interface unique identifier
D. None of the above

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310 Chapter 5: Ethernet Fundamentals
5. MAC addresses are bits in length.
A. 12
B. 24
C. 48
D. 64
6. What is the name of the access method used in Ethernet that explains how
Ethernet works?
A. TCP/IP
B. CSMA/CD
C. CMDA/CS
D. CSMA/CA
7. Where does the MAC address reside?
A. Transceiver
B. Computer BIOS
C. NIC
D. CMOS
8. Which of the following statements best describes communication between two
devices on a LAN?
A. The source device encapsulates data in a frame with the MAC address of
the destination device and then transmits it. Everyone on the LAN sees it,
but the devices with nonmatching addresses otherwise ignore the frame.
B. The source encapsulates the data and places a destination MAC address in
the frame. It puts the frame on the LAN, where only the device with the
matching address can check the address field.
C. The destination device encapsulates data in a frame with the MAC address
of the source device and puts it on the LAN. The device with the matching
address removes the frame.
D. Each device on the LAN receives the frame and passes it up to the computer,

where software decides whether to keep or to discard the frame.
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Check Your Understanding 311
9. Which functions are associated with framing?
A. Identifies which computers are communicating with one another
B. Signals when communication between individual computers begins and
when it ends
C. Flags corrupted frames
D. All of the above
10. How does a computer on a LAN detect an error in a frame?
A. It sends a copy of the frame back to the sender for verification.
B. It checks the destination address to verify that the frame really was intended
for it.
C. It compares an FCS in the frame to one that the computer calculates from the
contents of the frame.
D. It calculates a checksum from the data in the frame and then sends it back to
the source for verification.
11. Media Access Control refers to what?
A. The state in which a NIC has captured the networking medium and is ready
to transmit.
B. Rules that govern media capture and release.
C. Protocols that determine which computer on a shared-medium environment
is allowed to transmit the data.
D. A formal byte sequence has been transmitted.
12. Which best describes a CSMA/CD network?
A. One node’s transmission traverses the entire network and is received and
examined by every node.
B. Signals are sent directly to the destination if the source knows both the MAC
and IP addresses.
C. One node’s transmission goes to the nearest router, which sends it directly to

the destination.
D. Signals always are sent in broadcast mode.
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312 Chapter 5: Ethernet Fundamentals
13. In an Ethernet or IEEE 802.3 LAN, when do collisions occur?
A. When one node places a packet on a network without informing the other
nodes
B. When two stations listen for a traffic, hear none, and transmit simultaneously
C. When two network nodes send packets to a node that no longer is broadcasting
D. When jitter is detected and traffic is disrupted during normal transmission
14. Which is an important Layer 2 data link layer function?
A. Logical link control
B. Addressing
C. Media access control
D. All of the above
15. Which of the following is an Ethernet frame error type?
A. Local collision
B. Remote collision
C. Late collision
D. All of the above
16. Which protocol is a nondeterministic protocol?
A. Token Ring
B. CSMA/CD
C. IPX
D. RIP
17. Which is true of a deterministic MAC protocol?
A. It defines collisions and specifies what to do about them.
B. It allows the hub to determine the number of users active at any one time.
C. It allows hosts to “take turns” sending data.
D. It allows the use of a “talking stick” by network administers to control the

media access of any users considered “troublemakers.”
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Check Your Understanding 313
18. The network area within which data packets originated and collide is called a
?
A. Collision domain
B. Network domain
C. Broadcast domain
D. Network segment
19. Which best describes broadcasting?
A. Sending a single frame to many stations at the same time
B. Sending a single frame to all routers to simultaneously update their
routing tables
C. Sending a single frame to all routers at the same time
D. Sending a single frame to all hubs and bridges at the same time
20. Using repeaters the collision domain.
A. Reduces
B. Has no effect on
C. Extends
D. None of the above
21. The process of using the complex networking devices, such as bridges, switches,
and routers, to break up the collision domains is known as
A. Sectioning
B. Segmentation
C. Collision domain reduction
D. None of the above
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Objectives
Upon completion of this chapter, you will be able to
■ Understand the evolution of Ethernet technology

■ Understand the MAC method, frame format, and transmission process of
Ethernet technologies
■ Understand the specific media and encoding used in each Ethernet technology
■ Understand the pinouts and wiring typical of each Ethernet technology
■ Understand the basic architectural considerations of each Ethernet technology
■ Define microsegmentation
■ Describe Layer 2 bridging
■ Describe how a LAN switch operates
■ Describe full-duplex transmission
■ Identify the common switching methods: cut-through switching, store-and-
forward switching, and fragment-free switching
■ Describe the functions and features of the Spanning Tree Protocol (STP)
■ Describe how STP works
■ Describe the different STP port states
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Chapter 6
Ethernet Technologies and
Ethernet Switching
Ethernet, along with its associated IEEE 802.3 protocols, is one of the world’s most
important networking standards. Because of the great success of the original Ethernet
and the soundness of its design, it has evolved over time. This evolution was in response
to the developing needs of modern LANs. Ethernet most likely will continue to evolve in
response to future demands for network capability.
The previous chapter introduced both the history of Ethernet and the standards associated
with Ethernet. You also learned that the term Ethernet refers a family of the Ethernet
technologies. This chapter discusses the Ethernet technologies in more detail.
In addition, this chapter introduces Layer 2 bridging and switching techniques. Switching
and bridging are techniques that decrease congestion in LANs by reducing traffic and
increasing bandwidth.
Finally, this chapter introduces the Spanning Tree Protocol (STP), tells how STP works,

and covers the STP switch port states.
Please be sure to look at this chapter’s associated e-Lab Activities, Videos, and PhotoZooms
that you will find on the CD-ROM accompanying this book. These CD elements are
designed to supplement the material and reinforce the concepts introduced in this chapter.
10-Mbps and 100-Mbps Ethernet
This section introduces the specifics of the most important varieties of Ethernet. The goal
is not to memorize all the facts about each type of Ethernet, but rather to develop a sense
of what is common to all forms of Ethernet and what are the specific strengths and weak-
nesses of the commercially important forms of Ethernet.
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316 Chapter 6: Ethernet Technologies and Ethernet Switching
Ethernet’s popularity began with the thick coaxial 10BASE5. However, Thicknet cable
had undesirable installation properties. 10BASE2 used Thinnet—thinner coaxial cable that
was easier to install and terminate; the distance decreased from 500 to 185 meters (m).
This trend toward easy installation and lower cost took a great step forward with the
introduction of UTP-based 10BASE-T. However, the length that an unrepeated signal
could travel decreased to 100m, necessitating the introduction of repeaters and then
multiport repeaters (hubs). The repeater concept allowed 10BASE-T networks of up
to 500m. As workgroups grew in size and applications increased in complexity, the
shared bandwidth of the hub became a limiting factor. The introduction of Ethernet
10BASE-T switches addressed both the length and bandwidth limitations—station-to-
switch links were now point-to-point.
The power, versatility, and cost-effectiveness of 10BASE-T coincided with an explosion
in the number of LAN users, the number of Internet users (which also increased LAN
traffic), and the complexity of applications. Demand for higher bandwidth grew, and
Fast Ethernet was introduced. The copper cable version of Fast Ethernet that became
commercially successful was 100BASE-TX, and many clever features were developed for
interoperability with 10BASE-T systems (the emergence of 10/100 interfaces, for example).
To compete with the backbone/LAN technology of FDDI, fiber-based 100BASE-FX was
introduced. Throughout all of these Ethernet technologies, the MAC addressing concept,

the frame format, and the CSMA/CD MAC method were maintained.
10-Mbps Versions of Ethernet
Figure 6-1 shows a subset of physical layer implementations that you can deploy to
support Ethernet. The 10BASE5, 10BASE2, and 10BASE-T implementations of Ether-
net are considered legacy implementations and are referred to as such in the sections
that follow.
Figure 6-1 Types of Ethernet
Logical Link Control Sublayer
Physical
Signaling
Sublayer
Physical
Medium
10BASES5 (500 m)
50 Ohm Coax N-Style
1000BASE-LX (550-5000 m)
MM Fiber Sc
10BASE2 (185 m)
50 Ohm Coax BNC
10BASE-T (100 m)
100 Ohm UTP RJ45
10BASE-TX (100 m)
100 Ohm UTP RJ45
100BASE-FX (228_412 m)
MM Fiber SC
1000BASE-T (100 m)
100 Ohm UTP RJ45
1000BASE-SX (220-550 m)
MM Fiber SC
10BASE-(Various)

MM or Sm Fiber SC
802.3 Media Access Control
1102.book Page 316 Tuesday, May 20, 2003 2:53 PM
10-Mbps and 100-Mbps Ethernet 317
Four things are common among legacy Ethernet:
■ Timing parameters
■ Frame format
■ Transmission process
■ A basic design rule
After you learn about what these three historically important versions have in common,
you will examine each in more detail.
10BASE5, 10BASE2, and 10BASE-T all share the same timing parameters, as shown
in Table 6-1. Note that 1 bit-time at 10 Mbps = 100 nanosecond = 0.1 µsecond = 1
ten-millionth of a second.
The frame format is common to 10BASE5, 10BASE2, and 10BASE-T. Figure 6-2
shows an Ethernet frame as observed at the MAC sublayer.
Figure 6-2 Ethernet Frame
Table 6-1 Parameters for 10-Mbps Ethernet Operation
Parameter Value
Bit-time 100 nsec
Slot time 512 bit-times
Interframe spacing 96 bits*
*The value listed is the official interframe spacing.
Collision attempt limit 16
Collision backoff limit 10
Collision jam size 32 bits
Maximum untagged frame size 1518 octets
Minimum frame size 512 bits (64 octets)
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318 Chapter 6: Ethernet Technologies and Ethernet Switching

The Legacy Ethernet transmission process is identical until the lower part of the OSI
physical layer. As the frame passes from the MAC sublayer to the physical layer, further
processes occur before the bits are placed on the medium from the physical layer. One
process that is particularly important at this level is the signal quality error (SQE) signal.
This signal is typical of what you will see in many networking technologies. At the phys-
ical layer, the network is “alive” with communications other than the user data to ensure
a properly functioning network. SQE always is used in half duplex; it is not required but
is permitted in full-duplex operation. SQE is active in the following instances:
■ Within 4 to 8 microseconds following a normal transmission, to indicate that
the outbound frame successfully was transmitted.
■ Whenever there is a collision on the medium.
■ Whenever there is an improper signal on the medium. Improper signals might
include detected jabber, or the reflections that result from a cable fault, such as a
short. (There are separate conditions depending on which medium is attached.)
■ Whenever a transmission has been interrupted as jabber—that is, it has transmitted
longer than allowed.
All 10-Mbps forms of Ethernet take octets received from the MAC sublayer and per-
form a process called line encoding. Line encoding describes how the bits actually are
signaled on the wire. The simplest encodings (such as nonreturn to zero, or NRZ, in
which a 1 bit is 5 volts [V] and the 0 bits are 0V) typically have undesirable timing and
electrical characteristics. Therefore, line codes have been engineered to have desirable
transmission properties and tailored to each medium. The form of line encoding used
in 10-Mbps systems is called Manchester encoding. Figure 6-3 shows a Manchester
encoding example. The y-axis is voltage; the x-axis is time.
Figure 6-3 Manchester Encoding Examples
N
O
TE
In NRZ encoding, sig-
nals are maintained

at constant voltage
levels, with no signal
transitions (no return
to a 0V level) during a
bit interval.
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