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Guide to Networking Essentials
Fifth Edition
Chapter 7
Network Architectures
Objectives
• Compare and contrast media access methods used
in network architectures
• Describe the operation of Ethernet
• Differentiate between Ethernet standards and
speeds
• Explain the four Ethernet frame types and how they
are used
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Objectives (continued)
• Describe the token ring architecture and its
components
• Describe the AppleTalk network architecture
• Explain the function of Fiber Distributed Data
Interface
• Describe other LAN and WAN architectures and their
role in today’s networks
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Putting Data on the Cable: Access
Methods
• Given that network architectures communicate in a
number of different ways, some factors in network
communications must be considered
– How computers put data on the cable
– How they ensure that the data reaches its destination
undamaged
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Function of Access Methods
• The way in which computers attached to a network
share the cable must be defined
• A collision results from two or more devices sending
a signal along the same channel at the same time
– Splitting data in small chunks helps prevent collisions
• Channel access methods specify when computers
can access the cable or data channel
– Ensure that data reaches destination by preventing
computers from sending messages that might collide
– Every computer on a network must use the same
access method
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Major Access Methods
• Channel access is handled at the MAC sublayer of
the Data Link layer in the OSI model
• Five major types of channel access
–
–
–
–
–
Contention
Switching
Token passing
Demand priority
Polling
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Contention
• In early networks based on contention, computers
sent data whenever they had data to send
• As networks grow, outgoing messages collide more
frequently, must be sent again, and then collide
again
• To organize contention-based networks, two carrier
access methods were created
– CSMA/CD
– CSMA/CA
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Carrier Sense Multiple Access with
Collision Detection (CSMA/CD)
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Carrier Sense Multiple Access with
Collision Avoidance (CSMA/CA)
• When the computer senses that no other computer
is using the network, it signals its intent to transmit
– Other computers with data to send must wait when
they receive the “intent-to-transmit” signal and send
their “intent-to-transmit” only when channel is free
• The overhead created by intent-to-transmit packets
reduces network speed significantly
• Used in wireless LANs with an access point
– Wireless NIC tells access point its intents to transmit
– Access point hears transmissions from all devices,
so it can determine whether it’s okay to transmit
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Switching
• Switching: nodes are interconnected through a a
switch, which controls access to the media
– Contention occurs only when multiple senders ask to
reach the same receiver simultaneously or when the
simultaneous transmission requests exceed the
switch’s capability to handle multiple connections
• Advantages: fairer, centralized management
(enables QoS), switch can have connection ports
that operate at different speeds
• Disadvantage: higher cost
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Token Passing
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Demand Priority
• Demand priority: channel access method used
solely by the 100VG-AnyLAN 100 Mbps Ethernet
standard (IEEE 802.12)
– 100VG-AnyLAN runs on a star bus topology
– Intelligent hubs control access to the network
• Hub searches all connections in a round-robin fashion
• When an end node has data to send, it transmits a
demand signal to the hub
• The hub then sends an acknowledgement that the
computer can start transmitting its data
– The major disadvantage of demand priority is price
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Polling
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Choosing an Access Method
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Choosing an Access Method
(continued)
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Choosing an Access Method
(continued)
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The Ethernet Architecture
• 1960s and 1970s: many organizations worked on
methods to connect computers and share data
– E.g., the ALOHA network at the University of Hawaii
– 1972: Robert Metcalf and David Boggs, from Xerox’s
PARC, developed an early version of Ethernet
• 1975: PARC released first commercial version (3
Mbps, up to 100 computers, max. 1 km of total cable)
• DIX developed standard based on Xerox’s Ethernet
(10 Mbps)
• 1990: IEEE defined the 802.3 specification
– Defines how Ethernet networks operate at layers 1-2
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Overview of Ethernet
• Ethernet is the most popular network architecture
– Advantages: easy to install, scalable, broad media
support, and low cost
– Supported transmission speeds: 10 Mbps to 10 Gbps
– Uses the NIC’s MAC address to address frames
– Ethernet variations are compatible with one another
• Basic operation and frame formatting is the same
• Cabling, speed of transmission, and method by which
bits are encoded on the medium differ
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Ethernet Operation
• Ethernet is a best-effort delivery system
– It works at the Data Link layer of the OSI model
• Relies on the upper-layer protocols to ensure reliable
delivery of data
• Understanding the following concepts is important:
–
–
–
–
How Ethernet accesses network media
Collisions and collision domains
How Ethernet handles errors
Half-duplex and full-duplex communications
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Accessing Network Media
• Ethernet uses CSMA/CD in a shared-media
environment (a logical bus)
– Ethernet device listens for a signal or carrier (carrier
sense) on the medium first
– If no signal is present, no other device is using the
medium, so a frame can be sent
– Ethernet devices have circuitry that detects collisions
and automatically resends the frame that was
involved in the collision
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Collisions and Collision Domains
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Ethernet Error Handling
• Collisions are the only type of error for which
Ethernet automatically attempts to resend the data
• Errors can occur when data is altered in medium
– Usually caused by noise or faulty media connections
– When the destination computer receives a frame,
the CRC is recalculated and compared against the
CRC value in the FCS
– If values match, the data is assumed to be okay
– If values don’t match, the data was corrupted
• Destination computer discards the frame
• No notice is given to the sender
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Half-Duplex Versus Full-Duplex
Communications
• When half-duplex communication is used with
Ethernet, CSMA/CD must also be used
• When using a switched topology, a computer can
send and receive data simultaneously (full-duplex
communication)
– The collision detection circuitry is turned off because
collisions aren’t possible
– Results in a considerable performance advantage
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Ethernet Standards
• Each Ethernet variation is associated with an IEEE
standard
• The following sections discuss many of the
standards, some of which are obsolete or had
limited use
• Keep in mind that Ethernet over UTP cabling has
been the dominant technology since the early
1990s, and will likely to continue to be for the
foreseeable future
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100 Mbps IEEE Standards
• The most widely accepted Ethernet standard today
is 100BaseT, which is also called fast Ethernet
– The current IEEE standard for 100BaseT is 802.3u
• Subcategories:
– 100BaseTX: Two-pair Category 5 or higher UTP
– 100BaseT4: Four-pair Category 3 or higher UTP
– 100BaseFX: Two-strand fiber-optic cable
– Because of its widespread use, the cable and
equipment in fast Ethernet are inexpensive
– Architecture of choice for all but heavily used servers
and multimedia applications
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100BaseTX
• 100BaseTX is the standard that’s usually in mind
when discussing 100 Mbps Ethernet
• Requires two of the four pairs bundled in a
Category 5 twisted-pair cable
• Although three cable types are available for
100BaseT, 100BaseTX is the most widely accepted
– Generally called fast Ethernet
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100BaseT4
• 100BaseT4 Ethernet uses all four pairs of wires
bundled in a UTP cable
• Advantage: capability to run over Category 3 cable
– One of the biggest expenses of building a network is
cable installation, so many organizations with
Category 3 cabling chose to get the higher speed
with the existing cable plant by using 100BaseT4
instead of 100BaseTX
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100BaseFX
• 100BaseFX uses two strands of fiber-optic cable
– Advantages:
• Impervious to electrical noise and electronic
eavesdropping
• Can span much greater distances between devices
– Disadvantage: far more expensive than twisted-pair
– Rarely used as a complete 100BaseTX replacement
• Used as backbone cabling between hubs or switches
and to connect wiring closets between floors or
buildings
• Connect client or server computers to the network
when immunity to noise and eavesdropping is
required
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100BaseT Design Considerations
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100BaseT Design Considerations
(continued)
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10 Mbps IEEE Standards
• Four major implementations of 10 Mbps Ethernet
–
–
–
–
10Base5: Ethernet using thicknet coaxial cable
10Base2: Ethernet using thinnet coaxial cable
10BaseT: Ethernet over UTP cable
10BaseF: Ethernet over fiber-optic cable
• Of these 10 Mbps standards, only 10BaseT and
10BaseF are seen today
• 10Base2 and 10Base5 are essentially obsolete
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10BaseT
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10BaseF
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Gigabit Ethernet: IEEE 802.3ab and
802.3z Standards
• Gigabit Ethernet implementations
– 802.3z-1998 covers 1000BaseX specifications,
including the L (long wavelength laser/fiber-optic), S
(short wavelength laser/fiber-optic), and C (copper
jumper cables)
– 802.3ab-1999 covers 1000BaseT specifications,
which require four pairs of 100 ohm Category 5 or
higher cable
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1000BaseT
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1000BaseLX
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1000BaseSX
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1000BaseCX
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10 Gigabit Ethernet: 10 Gbps IEEE
802.3ae Standard
• Defined to run only on fiber-optic cabling, both SMF
and MMF, on a maximum distance of 40 km
– Provides bandwidth that can transform how WAN
speeds are thought of
• Runs in full-duplex mode only
– CSMA/CD is not necessary
• Primary use: as network backbone
– It also has its place in storage area networks (SANs)
– Will be the interface for enterprise-level servers
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10 Gigabit Ethernet: 10 Gbps IEEE
802.3ae Standard (continued)
• Standards
– 10GBASE-SR: Runs over short lengths (between 26
and 82 meters) over MMF
• For high-speed servers, SANs, etc.
– 10GBASE-LR: Runs up to 10 km on SMF
• For campus backbones and MANs
– 10GBASE-ER: Runs up to 40 km over SMF
• Primary applications are for MANs
– 10GBASE-SW: Uses MMF for distances up to 300 m
– 10GBASE-LW: Uses SMF for distances up to 10 km
– 10GBASE-EW: Uses SMF for distances up to 40 km
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What’s Next for Ethernet?
• Implementations of 40 Gbps Ethernet are underway
• Ethernet could increase tenfold every 4-6 years
– 100 Gbps Ethernet available by 2006 to 2008, terabit
Ethernet by 2011, and 10 terabit Ethernet by 2015
• In October 2005, Lucent Technologies
demonstrated for the first time the transmission of
Ethernet over fiber-optic cable at 100 Gbps
– It will be able to transfer data across the city faster
than today’s CPUs can transfer data to memory
– This level of speed has major implications for the
entertainment industry and many other areas
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Ethernet Frame Types
• Ethernet supports four non-compatible frame types
– Ethernet 802.3: used by IPX/SPX on Novell NetWare
2.x and 3.x networks
– Ethernet 802.2: used by IPX/SPX on Novell NetWare
3.12 and 4.x networks
• Supported by default in Microsoft NWLink
– Ethernet SNAP: used in EtherTalk and mainframes
– Ethernet II is used by TCP/IP
• All Ethernet frame types support a packet size
between 64 and 1518 bytes, and can be used by all
network architectures mentioned previously
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Ethernet 802.3
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Ethernet 802.2
• Ethernet 802.2 frames comply completely with the
Ethernet 802.3 standard
• The IEEE 802.2 group didn’t address Ethernet,
only the LLC sublayer of the OSI model’s layer 2
– Since Novell had already decided to use the term
Ethernet 802.3 to describe Ethernet raw, it’s
generally accepted that Ethernet 802.2 means a fully
802.3- and 802.2-compliant Ethernet frame
• Ethernet 802.2 frames contain similar fields to
802.3, with three additional LLC fields
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Ethernet SNAP
• Ethernet SubNetwork Address Protocol (SNAP)
is generally used on the AppleTalk Phase 2
• It contains enhancements to the 802.2 frame,
including a protocol type field, which indicates the
network protocol used in the frame’s data section
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Ethernet II
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Wireless Ethernet: IEEE 802.11b, a,
and g
• AP serves as the center of a star topology network
• Stations can’t send and receive at the same time
– CSMA/CA is used instead of CSMA/CD
• 802.11b/a/g use handshaking before transmission
– Station sends AP an RTS and it responds with CTS
• Standards define a maximum transmission rate, but
speeds might be dropped to increase reliability
• No fixed segment length
– Maximum of 300 feet without obstructions
• Can be extended with large, high-quality antennas
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The Token Ring Architecture
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Token Ring Function
• A token passes around the ring
– If an “in use” token is received from NAUN, and the
computer has data to send, it attaches its data to the
token and sends it to its NADN
– If received token is in use, NIC verifies if it is the
destination station
• If not, the computer re-creates the token and the data
exactly and sends them to its NADN
• If it is, data is sent to the upper-layer protocols
– Two bits in data packet are toggled and token is
sent to NADN; when original sender receives it, it
frees the token and then passes it along
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Beaconing
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Hardware Components
• A hub can be a multistation access unit (MSAU)
or smart multistation access unit (SMAU)
• IBM’s token ring implementation is the most
popular adaptation of the IEEE 802.5 standard
– Minor variations but very similar to IEEE specs
• IBM equipment is most often used
– 8228 MSAU has 10 connection ports, eight of which
can be used for connecting computers
– The RO port on one hub connects to RI port on the
next hub, and so on, to form a ring among the hubs
• IBM allows connecting 33 hubs
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Cabling in a Token Ring Environment
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The Token Ring Architecture
(summary)
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The AppleTalk Environment
• Designed for use in Macintosh networks (1983)
• Can be run over several physical architectures;
commonly run over Ethernet (EtherTalk)
• Easy to implement
• Dynamic scheme used to determine device’s
address
• AppleTalk Phase 1 supported only 32 computers
per network, and only with LocalTalk cabling
– With hubs/repeaters, increased the number to 254
• AppleTalk Phase 2, EtherTalk, and TokenTalk
(1989) allow more than 16 million computers
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LocalTalk
• LocalTalk uses STP in a bus topology to allow
users to share peripherals and data in a small
home or office environment
– CSMA/CA channel access method
• Avoids more collisions, but cumbersome
– Maximum transmission speed of 230.4 Kbps
• Thus, this architecture was used primarily in small,
Macintosh-only environments
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EtherTalk and TokenTalk
• EtherTalk is the AppleTalk protocol running over a
10 Mbps IEEE 802.3 Ethernet network
• TokenTalk is the AppleTalk protocol running over a
4 or 16 Mbps IEEE 802.5 token ring network
• Both implementations require using a different NIC
– Since 1996, Apple Computer has offered systems
with built-in Ethernet NICs or with options to add
Ethernet or token ring to its systems at a low cost
– Mac OS X with an Ethernet interface can freely
participate in a Windows-based network
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The Fiber Distributed Data Interface
(FDDI) Architecture
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The Fiber Distributed Data Interface
(FDDI) Architecture (continued)
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Networking Alternatives
• Many other network architectures are available
• Some are good for specialized applications, and
others are emerging as new standards
• Topics
–
–
–
–
–
Broadband technologies (cable modem and DSL)
Broadcast technologies
ATM
ATM and SONET Signaling Rates
High Performance Parallel Interface (HIPPI)
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Broadband Technologies
• Baseband systems use a digital encoding scheme
at a single fixed frequency
• Broadband systems use analog techniques to
encode information across a continuous range of
values
– Signals move across the medium in the form of
continuous electromagnetic or optical waves
– Data flows one way only, so two channels are
necessary for computers to send and receive data
– E.g., cable TV
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Cable Modem Technology
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Digital Subscriber Line (DSL)
• Competes with cable modem for Internet access
– Broadband technology that uses existing phone lines
to carry voice and data simultaneously
– Most prominent variation for home Internet access is
Asymmetric DSL (ADSL)
• Splits phone line in two ranges: Frequencies below 4
KHz are used for voice transmission, and frequencies
above 4 KHz are used to transmit data
• Typical connection speeds for downloading data range
from 256 Kbps to 8 Mbps; upload speeds are in the
range of 16 Kbps to 640 Kbps
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Broadcast Technologies
• By definition: one-way transmissions
– This changed in Internet access by satellite
television systems
• Work on the principle that most traffic a user
generates is to receive files, text, and graphics
– The average user’s computer sends very little traffic
– User connects to service provider through a modem
– Service provider sends data by satellite to the user’s
home at speeds up to 400 Kbps
– E.g., service offered by DirectTV, through its
DirectPC add-on products
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Asynchronous Transfer Mode (ATM)
• High-speed network technology for LANs and WANs
– Connection-oriented switches
• Dedicated circuits are set up before communicating
– Data travels in fixed-size 53-byte cells (5 byte-header)
• Enables ATM to work at extremely high speeds
– Quick switching
– Predictable traffic flow
• Enables ATM to guarantee QoS
– Used quite heavily for the backbone and
infrastructure in large communications companies
– LAN emulation (LANE) required for LAN applications
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ATM and SONET Signaling Rates
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High Performance Parallel Interface
(HIPPI)
• HIPPI (late 1980s): high-speed interface developed
for supercomputers and high-end workstations
– Serial HIPPI is a fiber-optic version that uses pointto-point optical links for bandwidth up to 800 Mbps
• In early 1990s, it was used as a network backbone
and for interconnecting supercomputers
– With the advent of Gigabit Ethernet, interest in HIPPI as
a LAN backbone decreased
– HIPPI-6400 (1998): up to 6.4 Gbps transfer rates
• Known as Gigabyte System Network (GSN)
– HIPPI and GSN are now exotic networking products
and aren’t often found in typical corporate networks
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Summary
• Cable access methods determine how a network
architecture gains access to the network medium
• A network architecture defines how data is placed,
transmitted, and at what speed, and how problems in
the network are handled
• DIX introduced Ethernet, which later became the
IEEE 802.3 standard, transmitting data at 10 Mbps
– Standards for 10Mbps, 100Mbps, 1000Mbps (Gigabit),
and 10G indicate the supported network mediums
• 10 Gigabit Ethernet runs only over fiber-optic cable and
only in full-duplex mode
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Summary (continued)
• Token ring networks are reliable, fast, and efficient
– Capable of transmitting at 4 Mbps or 16 Mbps
• Macintosh computers use AppleTalk to communicate
• FDDI is an extremely reliable, fast network
architecture that uses dual counter-rotating rings
• Cable modem technology delivers high-speed
Internet access to homes and businesses
• ATM, a high-speed network technology designed
both for LANs and WANs, uses connection-oriented
switches
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