Network+ Guide to Networks
Third Edition
Chapter 6:
Topologies and Access Methods
Objectives
Describe the basic and hybrid LAN physical
topologies, and their uses, advantages, and
disadvantages
Describe the backbone structures that form
the foundation for most LANs
Compare the different types of switching
used in data transmission
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Objectives (continued)
Understand the transmission methods
underlying Ethernet, LocalTalk, Token Ring,
FDDI, and ATM networks
Describe the characteristics of different
wireless network technologies, including the
three IEEE 802.11 standards
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Simple Physical Topologies
• Physical topology is the physical layout,
or pattern, of the nodes on a network
• Physical topologies are divided into three
fundamental geometric shapes: bus, ring,
and star
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Simple Physical Topologies
(continued)
• Bus
• A bus topology consists of a single cable
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connecting all nodes on a network without
intervening connectivity devices
The single cable is called the bus and can support
only one channel for communication
Most bus networks use coaxial cable as their
physical medium
At the ends of each bus network are 50-ohm
resistors known as terminators
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Simple Physical Topologies
(continued)
• Ring
• In a ring topology, each node is connected to the
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two nearest nodes so that the entire network forms
a circle
Data is transmitted clockwise, in one direction
(unidirectional), around the ring
The fact that all workstations participate in delivery
makes the ring topology an active topology
A ring topology also differs in that it has no “ends”
and data stops at its destination and, twisted-pair
or fiber-optic cabling is used as the physical
medium22
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Simple Physical Topologies
(continued)
• In a star topology, every node on the
network is connected through a central
device, such as a hub or switch
• Star topologies are usually built with twisted-pair
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or fiber-optic cabling
Star topologies require more cabling than ring or
bus networks
Each node is separately connected to a central
connectivity device, they are more fault-tolerant
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Hybrid Physical Topologies
• Star-Wired Ring
• The star-wired ring topology uses the physical
layout of a star in conjunction with the ring
topology’s data transmission method
• Data is sent around the star in a circular pattern
• This hybrid topology benefits from the fault
tolerance of the star topology
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Hybrid Physical Topologies
(continued)
• In a star-wired bus topology, groups of
workstations are star-connected to hubs
and then networked via a single bus
• With this design, you can cover longer distances
and easily interconnect or isolate different network
segments
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Hybrid Physical Topologies
(continued)
• More expensive than using either the star or,
especially, the bus topology alone because it
requires more cabling and potentially more
connectivity devices
• The star-wired bus topology forms the basis for
modern Ethernet and Fast Ethernet networks
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Backbone Networks
• A network backbone is the cabling that connects
the hubs, switches, and routers on a network
• Backbones usually are capable of more
throughput than the cabling that connects
workstations to hubs
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Backbone Networks (continued)
• In networking, the term enterprise refers to an
entire organization, including its local and remote
offices, a mixture of computer systems, and a
number of departments
• The backbone is the most significant building
block of enterprise-wide networks
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Backbone Networks (continued)
• Serial Backbone
• The simplest kind of backbone
• It consists of two or more internetworking devices
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connected to each other by a single cable in a
daisy-chain fashion
In networking, a daisy chain is simply a linked
series of devices
Hubs and switches are often connected in a daisy
chain to extend a network
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Backbone Networks (continued)
• Distributed Backbone
• Consists of a number of connectivity devices connected
to a series of central connectivity devices such as hubs,
switches, or routers, in a hierarchy
• This kind of topology allows for simple expansion and
limited capital outlay for growth, because more layers of
devices can be added to existing layers
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Backbone Networks (continued)
• A more complicated distributed backbone connects
multiple LANs or LAN segments using routers
• Provides network administrators with the ability to
segregate workgroups and therefore manage them
more easily
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Backbone Networks (continued)
• Collapsed Backbone
• Uses a router or switch as the single central
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connection point for multiple subnetworks
A single router or switch is the highest layer of the
backbone6
The router or switch that makes up the collapsed
backbone must contain multiprocessors to handle
the heavy traffic going through it
This arrangement allows you to interconnect
different types of subnetworks
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Backbone Networks (continued)
• Parallel Backbone
• The most robust type of network backbone
• The most significant advantage of using a parallel
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backbone is that its redundant (duplicate) links
ensure network connectivity to any area of the
enterprise
Parallel backbones are more expensive than other
enterprise-wide topologies
They make up for the additional cost by offering
increased performance and better fault tolerance
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Logical Topologies
• Logical topology refers to the way in which data is
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transmitted between nodes
The most common logical topologies are bus and
ring
In a bus logical topology, signals travel from one
network device to all other devices on the network
In a ring logical topology signals follow a circular
path between sender and receiver
Logical topologies is useful when troubleshooting
and designing networks
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Switching
• A component of a network’s logical topology that
determines how connections are created between
nodes
• There are three methods for switching: circuit
switching, message switching, and packet
switching
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Switching (continued)
• Circuit Switching
• A connection is established between two network
nodes before they begin transmitting data
• Bandwidth is dedicated to this connection and
remains available until the users terminate
communication between the two nodes
• While the nodes remain connected, all data
follows the same path initially selected by the
switch
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Switching (continued)
• Message Switching
• Establishes a connection between two devices,
transfers the information to the second device,
and then breaks the connection
• The information is stored and forwarded from the
second device once a connection between that
device and a third device on the path is
established
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Switching (continued)
• This “store and forward” routine continues until the
message reaches its destination
• Message switching requires that each device in
the data’s path have sufficient memory and
processing power to accept and store the
information before passing it to the next node
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Switching (continued)
• Packet Switching is the most popular
method for connecting nodes on a
network
• Breaks data into packets before they are
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transported
Packets can travel any path on the network to their
destination
When packets reach their destination node, the
node reassembles them based on their control
information
Does not waste bandwidth by holding a
connection open until a message reaches its
destination
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Ethernet
• Carrier Sense Multiple Access with
Collision Detection (CSMA/CD)
• The access method used in Ethernet
• The term “Carrier Sense” refers to the fact that
Ethernet NICs listen on the network and wait until
they detect (or sense) that no other nodes are
transmitting data over the signal (or carrier) on the
communications channel before they begin to
transmit
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Ethernet (continued)
• The term “Multiple Access” refers to the fact that several
Ethernet nodes can be connected to a network and can
monitor traffic, or access the media, simultaneously
• The last part of the term CSMA/CD, “collision detection,”
refers to the way nodes respond to a collision
• When two transmissions interfere with each other; this is
known as a collision
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Ethernet (continued)
• The NIC will issue a special 32-bit sequence that
indicates to the rest of the network nodes that the
its previous transmission was faulty and that those
data frames are invalid which is called jamming
• A collision domain is the portion of a network in
which collisions occur if two nodes transmit data at
the same time
• A data propagation delay is the length of time data
takes to travel from one point on the segment to
another point
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Ethernet (continued)
• Switched Ethernet
• Traditional Ethernet LANs, called shared Ethernet,
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supply a fixed amount of bandwidth that must be
shared by all devices on a segment, and all nodes
on that segment belong to the same collision
domain
Switched Ethernet enables multiple nodes to
simultaneously transmit and receive data over
different logical network segments
Using switched Ethernet increases the effective
bandwidth of a network segment because fewer
workstations must vie for the same time on the
wire
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Ethernet (continued)
• Ethernet Frames
• Ethernet networks may use one (or a combination)
of four kinds of data frames: Ethernet_802.2
(“Raw”), Ethernet_802.3 (“Novell proprietary”),
Ethernet_II (“DIX”), and Ethernet_SNAP
• Each frame type differs slightly in the way it codes
and decodes packets of data traveling from one
device to another
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Ethernet (continued)
• Using and Configuring Frames
• You can use multiple frame types on a network, but
you cannot expect interoperability between the
frame types
• Frame types are typically specified through a
device’s NIC configuration software
• Most NICs can automatically sense what types of
frames are running on a network and adjust
themselves to that specification which is a feature is
called autodetect, or autosense
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Ethernet (continued)
• The preamble signals to the receiving node that
data is incoming and indicates when the data flow
is about to begin
• The start-of-frame delimiter (SFD) identifies where
the data field begins
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Ethernet (continued)
• Each Ethernet frame also contains a 14-byte header,
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which includes a destination address, a source
address, and an additional field that varies in function
and size, depending on the frame type
The extra bytes are known as padding and have no
significance other than to fill out the frame
Ethernet_II (“DIX”) and Ethernet_SNAP
• An Ethernet frame type developed by DEC, Intel, and
Xerox (abbreviated as DIX) before the IEEE began to
standardize Ethernet
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Ethernet (continued)
• Ethernet_II frame type contains a 2-byte type field.
This type field identifies the Network layer protocol
(such as IP,ARP, RARP, or IPX) contained in the frame
• The Ethernet_SNAP standard calls for additional
control fields
• Ethernet_SNAP frames allow less room for data
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Ethernet (continued)
• Power over Ethernet
• Recently, IEEE has finalized a new standard,
•
802.3af, that specifies a method for supplying
electrical power over Ethernet connections, also
known as Power over Ethernet (PoE)
The PoE standard specifies two types of devices:
power sourcing equipment (PSE) and powered
devices (PDs)
• Power sourcing equipment (PSE)
• Powered devices (PDs)
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LocalTalk
• LocalTalk is a network access method designed by
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Apple Computer, Inc. specifically for networking
Macintosh computers
It provided a simple, cost-effective way of
interconnecting Macintosh devices
LocalTalk uses a transmission method called Carrier
Sense Multiple Access with Collision Avoidance
(CSMA/CA)
LocalTalk relies on the AppleTalk protocol, but it may
also support the Macintosh version of TCP/IP called
MacTCP
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Token Ring
• A network technology first developed by IBM in the
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1980s
Token Ring networks have traditionally been more
expensive to implement than Ethernet networks
The 100-Mbps Token Ring standard, finalized in
1999, is known as High-Speed Token Ring (HSTR)
In token passing, a 3-byte packet, called a token, is
transmitted from one node to another in a circular
fashion around the ring
The active monitor maintains the timing for ring
passing
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Token Ring (continued)
• Token Ring Switching
• Token Ring networks can take advantage of
switching to better utilize limited bandwidth
• A Token Ring switch can subdivide a large network
ring into several smaller network rings
• Token Ring technology does not allow collisions
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Token Ring (continued)
• Token Ring Frames
• Token Ring networks may use one of two
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types of frames: the IEEE 802.5 or the IBM
Token Ring frame
Every Token Ring frame includes Start Delimiter
(SD), Access Control (AC), and End Delimiter
(ED) fields
Token Ring frames use a Frame Status (FS) to
provide low-level acknowledgment that the frame
was received whole
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Fiber Distributed Data Interface
(FDDI)
• A network technology whose standard was originally
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specified by ANSI in the mid-1980s and later refined
by ISO
FDDI (pronounced “fiddy”) uses a double ring of
multimode or single mode fiber to transmit data at
speeds of 100 Mbps
FDDI is more reliable and more secure than
transmission methods that depend on copper wiring
FDDI works well with Ethernet 100BaseTX
technology
FDDI technology has a high cost relative to Fast
Ethernet
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Asynchronous Transfer Mode (ATM)
• An ITU networking standard describing Data Link layer
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protocols for both network access and signal multiplexing
ATM may run over fiber-optic or CAT 5 or higher UTP or
STP cable
In ATM, a packet is called a cell and always consists of 48
bytes of data plus a 5-byte header
ATM technology is that it relies on virtual circuits
ATM a connection-oriented technology using virtual circuits
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ATM (continued)
• Establishing a reliable connection allows ATM to
guarantee a specific Quality of Service (QoS) for
certain transmissions
• QoS is a standard that specifies that data will be
delivered within a certain period of time after it is
sent
• ATM networks can be integrated with Ethernet or
Token Ring networks through the use of LAN
Emulation (LANE)
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Wireless Networks
• 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
• These standards differ in their specified signaling
methods, geographic ranges, and frequency
usages, among other things.
• The most popular wireless standards used on
contemporary LANs are those developed by
IEEE’s 802.11 committee
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Wireless Networks (continued)
• 802.11 Another name for Wireless Local
Area Networks (WLAN) standards
committee
• Access Method
• 802.11 standards specify the use of Carrier Sense
Multiple Access with Collision Avoidance
(CSMA/CA) to access a shared medium
• Use of ACK packets to verify every transmission
• RTS/CTS enables a source node to issue an RTS
signal to an access point requesting the exclusive
opportunity to transmit
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Wireless Networks (continued)
• Association
• In the context of wireless networking,
communication that occurs between a station and
an access point to enable the station to connect to
the network via that access point
• As long as a station is on and has its wireless
protocols running, it periodically surveys its
surroundings for evidence of an access point, a
task known as scanning
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Wireless Networks (continued)
• There are two types of scanning: active and passive
• In active scanning, the station transmits a special
frame, known as a probe, on all available
channels within its frequency range
• In passive scanning, a wireless station listens on
all channels within its frequency range for a
special signal, known as a beacon frame, issued
from an access point
• Service Set Identifier (SSID), a unique character
string used to identify an access point
• A station might choose a different access point
through a process called re-association
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Wireless Networks (continued)
• Frames
• For each function, the 802.11 standard specifies a
frame type at the MAC sublayer
• These multiple frame types are divided into three
groups: management, control and data
• Management frames are those involved in association
and re-association, such as the probe and beacon
frames
• Control frames are those related to medium access
and data delivery, such as the ACK and RTS/CTS
frames
• Data frames are those that carry the data sent
between stations
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Wireless Networks (continued)
• 802.11b
• Also known as “Wi-Fi,” for Wireless Fidelity
• Uses direct sequence spread spectrum (DSSS)
signaling
• 802.11b was the first to take hold and remains the
most popular
• It is also the least expensive of all the 802.11 WLAN
technologies
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Wireless Networks (continued)
• 802.11a
• 802.11a’s high throughput is attributable to its use
of higher frequencies, its unique method of
encoding data, and more available bandwidth
• Higher frequency signals require more power to
transmit and travel shorter distances than lower
frequency signals
• The additional access points, as well as the nature
of 802.11a equipment, make this standard more
expensive than either 802.11b or 802.11g
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Wireless Networks (continued)
• 802.11g
• 802.11g benefits from being compatible with 802.11b
networks
• 802.11g has high throughput
• 802.11g’s compatibility with the more established 802.11b
has caused many network managers to choose it over
802.11a, despite 802.11a’s comparative advantages
• Laptops could roam between the ranges of the 802.11b and
802.11g access points without an interruption in service
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Wireless Networks (continued)
• Bluetooth
• Bluetooth is a mobile wireless networking
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standard that uses DSSS signaling in the 2.4-GHz
band to achieve a maximum theoretical
throughput of 1 Mbps
Bluetooth was designed to be used on small
networks composed of personal communications
devices, also known as personal area networks
(PANs)
Bluetooth’s low throughput and short range makes
it impractical for business LANs.
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Wireless Networks (continued)
• HomeRF
• HomeRF is a wireless networking specification
developed by the HomeRF Working Group
• The most unique aspect of the HomeRF standard
is that it was designed to allow both traditional
telephone signals and data signals to be
exchanged over the same wireless network
• Its working group was disbanded in January 2003
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Summary
• Basic and hybrid LAN physical topologies,
and their uses, advantages, and
disadvantages
• Describe the backbone structures
• Compared the different types of switching
used in data transmission
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Summary (continued)
• Transmission methods underlying
Ethernet, LocalTalk, Token Ring, FDDI, and
ATM networks
• Characteristics of different wireless
network technologies, including the three
IEEE 802.11 standards
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