Network+ Guide to Networks, Fourth Edition
Chapter 6
Topologies and Access Methods
At a Glance
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Objectives
Teaching Tips
Quick Quizzes
Class Discussion Topics
Additional Projects
Additional Resources
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Lecture Notes
Chapter Objectives
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Describe the basic and hybrid LAN physical topologies, and their uses, advantages and disadvantages
Describe the backbone structure that form the foundation for most LANs
Compare the different types of switching used in data transmission
Understand the transmission methods underlying Ethernet, Token Ring, FDDI, and ATM networks
Describe the characteristics of different wireless network technologies, including Bluetooth and the
three IEEE 802.11 standards
Simple Physical Topologies
Define physical topology. Stress that physical topology does not specify device types, connectivity
methods, or addressing schemes for the network.
Present a brief overview of the basic physical topologies. Mention that hybrid topologies may be created.
Teaching
Tip
Make sure that students understand that they must thoroughly understand physical topologies for
the Network+ exam.
Teaching
Tip
Physical topologies and logical topologies are two different networking concepts. Students
should be aware that, when used alone, the word topology often refers to a network’s physical
topology.
Bus
Present an overview of the bus topology, using Figure 6-1 to illustrate.
Describe the characteristics of the bus topology. Stress that devices share the responsibility for getting data
from one point to another.
Discuss the concept of terminators and signal bounce. Illustrate with Figure 6-1.
Discuss the advantages and disadvantages of using a bus topology. Stress that a defect in the bus affects the
entire network.
Ring
Describe the ring topology, using Figure 6-2 to illustrate. Explain the manner in which packets are
transmitted in this topology. Define active topology.
Discuss the advantages and disadvantages of the ring topology. Stress that this topology is not very flexible
or scalable.
Network+ Guide to Networks, Fourth Edition
Star
Describe the basic structure of a star network, using Figure 6-3 to illustrate.
Discuss the advantages and disadvantages of using the star topology. Mention that more cabling is required,
but that a cabling defect will affect two nodes at most. Stress that this type of network is scalable.
Hybrid Physical Topologies
Describe the reasons for creating hybrid networks. Explain that simple topologies are generally too
restrictive, but can be used in more complex configurations.
Star-Wired Ring
Describe the Star-Wired Ring topology, using Figure 6-4 to illustrate. Mention the advantages of using this
type of topology, including fault tolerance and reliability.
Mention that Token Ring networks use this topology.
Star-Wired Bus
Describe the Star-Wired Bus topology, using Figure 6-5 to illustrate. Discuss the advantages and
disadvantages of this topology.
Mention that Ethernet and Fast Ethernet networks use this topology.
Backbone Networks
Discuss the concept of a backbone network. Mention that they are typically capable of higher throughputs,
and that they generally carry a great deal of network traffic.
Define enterprise. Stress that the backbone is the most significant building block of an enterprise-wide
network.
Serial Backbone
Describe the serial backbone configuration, using Figure 6-6 to illustrate. Define daisy chain and discuss
how daisy chaining is used in a serial backbone.
Discuss the advantages and disadvantages of the serial backbone configuration. Mention the limitations in
the extent that hubs can be connected on a serial backbone.
Distributed Backbone
Describe the distributed backbone configuration, using Figures 6-7 and 6-8 to illustrate. Explain the
advantages of using this type of backbone configuration.
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Discuss how this type of backbone can be used to connect multiple LAN segments. Use Figure 6-8 to
illustrate.
Mention that a distributed backbone configuration allows workgroups to be segregated on a network.
Mention that the distribute backbone configuration may contain daisy-chained hubs and, in such cases, this
configuration will experience some of the same limitations as serial backbones.
Collapsed Backbone
Present an overview of the collapsed backbone configuration. Use Figure 6-9 to illustrate.
Contrast the collapsed backbone in Figure 6-9 to the distributed backbone in Figure 6-8.
Discuss the advantages and disadvantages of using a collapsed backbone. Stress that this arrangement
allows you to interconnect different types of subnetworks.
Parallel Backbone
Discuss the parallel backbone configuration, using Figure 6-10 to illustrate. Mention that this is the most
robust backbone configuration.
Explain the situations in which a network administrator might choose to use a parallel backbone, and
discuss the advantages of selectively implementing the parallel structure.
Teaching
Tip
LANs contain four distinct functional areas: station connectivity, server connectivity, WAN
connectivity, and backbone. Each may be best served by a different basic or complex topology.
Logical Topologies
Define logical topology. Mention that a network’s logical and physical topologies will not necessarily
match.
Provide an overview of the bus logical topology. Explain the physical topologies that it is usually
associated with.
Provide an overview of the ring logical topology. Explain the physical topologies that it is usually
associated with.
Mention that Ethernet networks use the bus logical topology, while Token Ring networks use the ring
logical topology.
Discuss how understanding logical topologies can be useful when designing or troubleshooting a network.
Switching
Define switching. Explain that it is a component of a network’s logical topology.
Network+ Guide to Networks, Fourth Edition
Circuit Switching
Present an overview of circuit switching. Stress that this type of switching monopolizes a piece of
bandwidth.
Discuss the advantages and disadvantages of circuit switching. Mention the applications and technologies
that use circuit switching.
Message Switching
Present a brief overview of message switching. Explain the store and forward routine used to transfer data.
Mention the similarities to and differences from circuit switching.
Packet Switching
Provide an overview of packet switching. Stress that this is the most commonly used method for connecting
nodes on a network.
Explain the advantages of using packet switching. Explain why packets can travel any path on a network
and stress the benefits of this ability.
Briefly explain how packets are reassembled when they reach their destination. Point out situations where
packet switching is not optimal. Stress that packet switching efficiently uses bandwidth.
Ethernet
Review the history and typical uses of Ethernet. Explain that it is the most popular network technology
used on modern LANs.
Explain that all Ethernet networks share the same access method, Carrier Sense Multiple Access with
Collision Detection (CSMA/CD).
CSMA/CD (Carrier Sense Multiple Access with Collision Detection)
Define access method. Explain that the access method used in Ethernet is called CSMA/CD.
Discuss the characteristics of the CSMA/CD access method.
Define collision and jamming. Explain the CSMA/CD process, using Figure 6-11 to illustrate.
Explain the concept of a collision domain. Mention that repeaters propagate collisions, and discuss the
consequences of connecting multiple parts of a network with repeaters.
Discuss the relationship between collision domains and cabling distance limitations. Define data
propagation delay.
Explain that NICs cannot keep up with collision detection on high-speed networks. Discuss the limitations
that this fact imposes on high speed networks.
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Switched Ethernet
Discuss the concept of shared Ethernet, stressing its limitations. Explain that stations cannot send and
receive data simultaneously, nor can they transmit a signal when another station on the same segment is
sending or receiving data.
Provide an overview of switched Ethernet. Stress that it enables multiple nodes to simultaneously transmit
and receive data over different logical network segments. Illustrate with Figure 6-12.
Describe the advantages of using switched Ethernet. Explain that it effectively increases bandwidth.
Ethernet Frames
Provide an overview of Ethernet frames. List the four types of data frames that are available for use on
Ethernet.
Explain that Ethernet frame types have no relation to the topology or cabling characteristics of the network
and that framing also takes place independently of the higher-level layers.
Using and Configuring Frames
Explain that interoperability among the frame types should not be expected.
Discuss the importance of LAN administrators ensuring that all devices on a LAN use the same, correct
frame type. Explain that node’s Data Link layer services must be properly configured to expect the types of
frames it might receive. Mention that Ethernet_II is the most commonly used frame type.
Explain how frame types are specified using NIC configuration software. Mention that there is often an
auto-detect feature that allows the NIC to detect the types of frames used on the network and to adjust its
configuration accordingly.
Frame Fields
Discuss the frame fields that all frame types have in common, including the preamble, start-of-frame
delimiter (SFD), header, and FCS field. Describe the purpose of each field. Explain that the FCS and the
header make up the 18-byte frame for the data.
Describe the characteristics of the data portion of an Ethernet frame. Mention the number of bytes that the
data portion many contain. Explain that the minimum total size of an Ethernet frame is 64 bytes and how
padding is used to enforce this minimum. Also mention that the maximum size of an Ethernet frame is
1518 bytes.
Explain that larger frame sizes generally improve network efficiency. Mention why network administrators
should try to minimize the number of broadcast frames on their networks.
Explain that it is most efficient to support only one frame type on a network.
Ethernet_II (DIX)
Discuss the history and characteristics of Ethernet_II. Explain the differences and advantages from older
Ethernet frame types. Stress the importance and use of the type field.
Briefly describe the Ethernet_SNAP frame type and discuss its differences from the Ethernet_II frame type.
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PoE (Power over Ethernet)
Provide an overview of the PoE standard. Mention its similarity to standard telephone lines.
Discuss the role that the power sourcing equipment (PSE) and powered devices (PDs) play in PoE. Mention
that PoE requires CAT 5 or better copper cabling.
Quick Quiz 1
1.
A(n) _____ topology consists of a single cable connecting all nodes on a network without
intervening connectivity devices.
Answer: bus
2.
In a(n) _____ topology, every node on the network is connected through a central device, such as
a hub or a switch.
Answer: star
3.
Which is the most robust type of network backbone?
a. Serial
b. Distributed
c. Collapsed
d. Parallel
Answer: d
4.
In _____ switching, a connection is established between two network nodes before they begin
transmitting data.
Answer: circuit
5.
What is the minimum Ethernet frame size?
a. 32 bytes
b. 46 bytes
c. 64 bytes
d. 128 bytes
Answer: c
Token Ring
Provide a brief overview of the history of the Token Ring network access method.
Explain the advantages and disadvantages of Token Ring networks versus Ethernet networks. Mention that
Token Ring networks are more expensive, but more reliable than Ethernet networks.
Explain that Token Ring networks use the token-passing routine and a star-ring hybrid physical topology.
Discuss the token-passing mechanism in detail. Stress that this mechanism avoids the possibility of
collisions and that it does not impose distance limitations on the length of LAN segments.
Discuss the role of the active monitor on a Token Ring network.
Describe the specification for a Token Ring network, as specified by the IEEE 802.5 standard. These
specifications are described on page 306 of the text. Mention the role that Multistation Access Units
(MAUs) play on a Token Ring network, and illustrate with Figure 6-14.
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Discuss the self-circuiting feature that MAU ports provide and the benefits associated with this feature.
Illustrate the connector types used on Token Ring networks with Figure 6-15.
Teaching
Tip
Make sure that students understand that Token Ring’s logical topology is a ring, its physical
topology is a star-ring hybrid in which data circulate in a ring fashion, and the layout of the
network is a star.
FDDI (Fiber Distributed Data Interface)
Provide an overview of FDDI, including its history. Explain the basic layout of a FDDI network, using
Figure 6-16 to illustrate.
Explain the advantages and disadvantages of using FDDI networks. Stress the reliability and security
implicit to this type of network.
Explain the similarities between Token Ring networks and FDDI networks. Discuss the benefits provided
by the secondary ring in a FDDI network.
ATM (Asynchronous Transfer Mode)
Provide a brief overview of ATM, including its history. Mention types of cables that ATM may run over.
Explain the reasons why ATM uses a fixed packet size and discuss the advantages that this provides.
Discuss the concept of virtual circuits and explain the advantages that they provide.
Explain why ATM is considered to be a connection-oriented packet switching technology. Discuss the
concept of quality of service (QoS) and explain its relationship to ATM.
Explain how ATM is compatible with other leading network technologies. Discuss the purpose of LAN
emulation (LANE).
Mention that ATM is expensive and may be pushed out of use by Gigabit Ethernet.
Wireless Networks
Present a brief overview of wireless access methods. Explain that each wireless technology is defined by a
standard that describes unique functions at both the Physical and the Data Link layers of the OSI Model.
802.11
Provide an overview of the 802.11 standards, specifically mentioning 802.11a, 802.11b, and 802.11g.
Explain that these standards share many characteristics.
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Access Method
Explain that the 802.11 MAC services append 48-bit physical addresses to a frame to identify its source
and destination. Mention that, by using the same physical addressing scheme as Ethernet, 802.11 networks
can be easily combined with Ethernet networks.
Provide an overview of the CSMA/CA access method. Discuss how transmission is achieved over a
wireless network, as described on page 311 of the text. Mention that CSMA/CA reduces but does not
eliminate the possibility of collision.
Discuss the overhead required on a wireless network. Explain that the theoretical maximum throughput on
a wireless network is rarely achieved in practice.
Discuss the Request to Send/Clear to Send (RTS/CTS) protocol.
Association
Describe the process of association in wireless networks. Explain that association involves a number of
packet exchanges between a station and an access point (AP).
Give an overview of active scanning. Explain the concept of a probe frame and describe the information
contained in a reply to a probe frame.
Give an overview of passive scanning. Explain the purpose of and the information contained in a beacon
frame. Mention the purpose of the Service Set Identifier (SSID).
Explain that wireless networks can have multiple APs. Describe how a station picks an AP on such a
network. Also discuss the concept of reassociation.
Teaching
Tip
Make sure the students understand that the 802.11 standard does not specify how two APs should
communicate. Explain that, when designing an 802.11 network, it is best to use APs
manufactured by the same company, to ensure full compatibility.
Frames
Explain that, for each function such as ACKs, probes, and beacons, the 802.11 standard specifies a frame
type at the MAC sublayer. Explain that each of these frame types is subdivided into three groups and
present a short discussion of each group.
Illustrate an 802.11 data frame using Figure 6-18. Discuss each of the fields in this frame.
Emphasize the fields that are unique to the 802.11 data frame, such as the Sequence Control field.
802.11b
Present an overview of the 802.11b standard. Mention the characteristics of this standard, such as
frequency range and maximum throughput. Also mention that this is the least expensive of all the 802.11
technologies.
Network+ Guide to Networks, Fourth Edition
Teaching
Tip
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Table 6-1 at the end of this chapter summarizes the characteristics of a number of wireless
standards, including 802.11a, 802.11b, 802.11g, Bluetooth, and IrDA. Rather than discussing the
characteristics of all these standards individually, it is probably easier to show Table 6-1 to the
students and describe them simultaneously. This will make it easier to compare and contrast the
standards.
802.11a
Present an overview of the 802.11a standard. Mention the characteristics of this standard, such as frequency
range and maximum throughput.
802.11g
Present an overview of the 802.11g standard. Mention the characteristics of this standard, such as
frequency range and maximum throughput.
Bluetooth
Provide an overview of Bluetooth, including it history. Explain that it is a mobile wireless networking
standard that uses FHSS RF signaling in the 2.4-GHz band.
Discuss the characteristics of Bluetooth versions 1.1, 1.2, and 2.0. Explain the advantages of Bluetooth 2.0.
Explain the concept of piconets and scatternets. Use Figures 6-20 and 6-21 to illustrate.
Infrared (IR)
Provide an overview of IR technology. Describe the characteristics of IR, including frequency ranges and
throughput. Stress that IR is only viable over short distances and cannot circumnavigate obstacles.
Explain the ways that IR technology is used.
Mention that the specifications for using IR signaling between devices on a network have been established
by the IrDA (Infrared Data Association), and that IrDA refers to the most popular IR networking
specifications.
Teaching
Tip
Make sure that the students understand that the actual geographic range of any wireless
technology depends on several factors, including the power of the antenna, physical barriers or
obstacles between sending and receiving nodes, and interference in the environment. Therefore,
although a technology is rated for a certain average geographic range, it may actually transmit
signals in a shorter or longer range.
Network+ Guide to Networks, Fourth Edition
Quick Quiz 2
1.
In Token Ring networks, the _____ maintains the timing for ring passing, monitors token and
frame transmission, detects lost tokens, and corrects errors when a timing error or other disruption
occurs.
Answer: active monitor
2.
True or False: ATM employs a fixed packet size.
Answer: True
3.
802.11 standards specify the use of _____ to access a shared medium.
Answer: Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)
4.
In _____ scanning, a wireless station listens on all channels within its frequency range for a
special signal, known as a beacon frame, issued from an AP.
Answer: passive
Class Discussion Topics
1.
One network topology that was not discussed in this chapter is the mesh topology. In this
topology, there is a point-to-point connection between every device on the network. Have the
students discuss the advantages and disadvantages of this topology and compare it to the simple
topologies described in this chapter.
2.
Wireless technology is constantly improving in the areas of range, throughput, bandwidth, and
accessibility. For instance, it has been claimed that the development of the WiMAX standard
(IEEE 802.16) could result in almost blanket high-speed wireless coverage of the United States
within the next
10 to 15 years. Ask students if they think that wireless networking will completely replace wirebound networking technologies in the future.
Additional Projects
1.
A useful and important tool for a network administrator is a network diagramming software
package. There are many of these packages and tools available. Have the students research online
to find one or two such tools. Have them submit descriptions of the tools that they found, prices
for the tools, and reviews of the tools (this may or may not be possible, depending on which
software tool the student identifies). Compile the students’ results into a list that can be
redistributed to the class.
2.
Identify a reasonably sized network, such as a LAN in the building where the class is taught, that
the students would be able to physically examine. Have the students determine, via physical
examination or by interviewing the network administrator, the topologies employed in this
network. Have them diagram the network, identifying all connectivity hardware and all nodes on
the network. When they are done, check to make sure that the students identified the correct
topologies used on this network. If a suitable network cannot be found, draw a diagram of a
moderately sized network and have the students perform this exercise based on the diagram.
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Additional Resources
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LAN Topologies: http://docs.rinet.ru/NeHi/ch05/ch05.htm
Packet Switching Demo (Flash animation):
http://www.pbs.org/opb/nerds2.0.1/geek_glossary/packet_switching_flash.html
The Evolution of Packet Switching (historically interesting paper from 1973 by Dr. Lawrence G.
Roberts): http://www.packet.cc/files/ev-packet-sw.html
CSMA/CD Transmission Simulation Applet:
http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/csmacd/csmacd.html
Ethernet Frame Types: http://telecom.tbi.net/frmlan.html
Token Ring: http://www.datacottage.com/nch/troperation.htm
FDDI: http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/fddi.htm
The ATM Forum: http://www.atmforum.com/
IEEE Wireless Standards Zone: http://standards.ieee.org/wireless/
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