Transcript Chapter 5

CEG 2400 FALL 2012
Chapter 5
Topologies and Ethernet Standards
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Physical Topologies
• Physical topology
– Describes the physical network nodes layout
– Does not specify:
• Device types
• Connectivity methods
• Addressing schemes
• Fundamental shapes
– Bus, ring, star
– Hybrid (combination of fundamental shapes)
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Bus
• Bus topology
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Single cable
Connects all network nodes
No intervening connectivity devices
They share the communication channel
• Physical medium
– Usually coaxial cable
• Passive topology
– Node listens for, accepts data
– Uses broadcast to send
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Bus
• Terminators
– 50-ohm resistors used to stop signal at end of wire
• Signal bounce
– Signal travels endlessly between two network ends
– Happens if no terminator
• One end grounded to removes static electricity
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A terminated bus topology network
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Bus
• Advantage
– Relatively inexpensive
• Disadvantages
– Does not scale well (adding more clients)
– Difficult to troubleshoot (hard to tell where problem is)
– Not very fault tolerant (one client can bring it down)
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Ring
• Ring topology
– Node connects to nearest two nodes
– Clockwise data transmission (circular network)
• One direction (unidirectional) around ring
– Active topology
• Each workstation participates in data delivery
– Physical medium
• Twisted pair or fiber-optic cabling
• Drawbacks
– Malfunctioning workstation can disable network
– Not very flexible or scalable
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Ring
A ring topology network
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Star
• Star topology
– Node connects through central device
• Usually a router or switch
• Physical medium
– Twisted pair or fiber-optic cabling
• Single cable connecting two devices
• Advantage
– Fault tolerant
– Flexible
• Most popular fundamental layout
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Star
• A star topology network
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Hybrid Topologies
• Pure bus, ring, star topologies rarely exist
• Hybrid topology
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More likely
Complex combination of pure topologies
Several options
Star-wired ring
Star-wired bus (most common)
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Star-Wired Ring
• Star-wired ring topology
– Star physical topology
– Ring logical topology
• Benefit
– Star’s fault tolerance
• Network use
– Token Ring networks (not common anymore)
• IEEE 802.5
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Star-Wired Ring
A star-wired ring topology network
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Star-Wired Bus
• Star-wired bus topology
– Star-connected devices
– Central device networked via single bus
• Advantage
– Cover longer distances
– Easily interconnect, isolate different segments
• Drawback
– More cabling, connectivity device expense
• Basis for modern Ethernet networks
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Star-Wired Bus
A star-wired bus topology network
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Logical Topologies
• Refers to way data transmitted between nodes
rather than physical layout
• Does not necessarily match physical topology
• Most common: bus and ring
• Bus – signal travels from one device to all other
devices
– Broadcast domain
• All nodes connected to single repeating device or
switch
• Ring – signal follows a circular path between sender
and receiver
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Network Backbone
• Cabling that connects hubs, switches, routers
• Has more throughput
• Large organizations
– Fiber-optic backbone
– Cat 5 or better for hubs, switches
• In an Enterprise
– Significant building block: backbone
• Enterprise-wide network backbones are
– Complex, difficult to plan
• Several different types – Serial, Distributed,
Collapsed, and Parallel
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Serial Backbone
A serial backbone
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Serial Backbone
• Simplest backbone
– Two or more devices connected using single medium
in daisy-chain fashion
• Benefit
– Logical growth solution
• Modular additions
– Low-cost LAN infrastructure expansion
• Easily attach switches
• Backbone components
– Gateways, routers, switches
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Serial Backbone
• Standards
– Limited number of repeating devices allowed
– Limited distance spanned between each
– Exceed standards
• Intermittent, unpredictable data transmission errors
• Not used in modern networks
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Distributed Backbone
A distributed backbone connecting multiple LANs
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Distributed Backbone
• Intermediate connectivity devices connected to
hierarchy of central connectivity devices
• Benefit
– Simple expansion, limited capital outlay
• More complicated distributed backbone connects
multiple LANs, LAN segments using routers
• Additional benefits
– Workgroup segregation (troubleshooting)
– May include daisy-chain linked repeating devices
• Drawback
– Potential for single failure points
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Collapsed Backbone
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Collapsed Backbone
• Uses router or switch
– Single central connection point for multiple
subnetworks
– Single router or switch with multiprocessors to handle
traffic
• Disadvantage
– Central router failure risk
– Routers may slow data transmission
• Advantages
– Interconnect different subnetwork types
– Central management
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Parallel Backbone
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Parallel Backbone
• Most robust network backbone
• A variation of collapsed backbone
• Requires duplicate connections between
connectivity devices
• Advantage
– Redundant links
– Increased performance
– Better fault tolerance
• Disadvantage
– Minor cost, more cabling (but usually worth it)
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Switching
• Logical network topology component
• Three methods
1. Circuit switching
2. Packet switching
3. Multiprotocol label switching
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Circuit Switching
• Connection established between two network nodes
– Before transmitting data
• Dedicated bandwidth – nodes stay connected
• Data follows same initial path selected by switch
• Monopolizes bandwidth while connected even if not
sending data
– Resource wasted
• Uses
– Live audio, videoconferencing
– Traditional telephone calls
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Packet Switching
• Most popular
• Breaks data into packets before transporting
• Packets
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Travel any network path to destination
Find fastest circuit available at any instant
Need not follow each other
Need not arrive in sequence
Reassembled at destination
• Ethernet networks and the internet are the most
common to use this type
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Multiprotocol Label Switching(MPLS)
• Based on short path labels rather than long network
addresses thus avoiding complex lookups in a
routing table
• The labels identify virtual links (paths) between
distant nodes rather than endpoints
• Packet-forwarding decisions are made solely on the
contents of this label, without the need to examine
the packet itself.
• Routers interpret label to predefined paths
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MPLS
• MPLS can encapsulate packets of various network
protocols
• Supports IP
• MPLS operates at a layer that is generally
considered to lie between traditional definitions of
layer 2 (data link layer) and layer 3 (network layer),
and is often referred to as a "layer 2.5" protocol.
MPLS shim within a frame
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Ethernet
• Most popular networking technology used on
modern LANs
• Benefits
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Flexible
Can run on various network media
Excellent throughput
Reasonable cost
• All variations
– Share common access method called CSMA/CD
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CSMA/CD (Carrier Sense Multiple Access
with Collision Detection)
• Network access method
– Controls how nodes access communications channel
• Carrier sense (CS)
– Ethernet NICs listen, wait until free channel detected
• Multiple access (MA)
– Ethernet nodes simultaneously monitor traffic or can
access the media
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CSMA/CD
• Collision
– Two nodes simultaneously:
• Check channel, determine it is free, begin transmission
• Collision detection (CD)
– Way nodes respond to collision
– Collision detection routine
• Enacted if node detects collision
– Jamming – What happens if collision
• NIC issues 32-bit sequence
• Indicates previous message faulty
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CSMA/CD
• Heavily trafficked network segments
– Collisions are common
• Collisions corrupt data, truncate data frames
– Network must detect and compensate
• Segment growth – too many devices
– Performance suffers
– “Critical mass”
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CSMA/CD process
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CSMA/CD
• Collision domain
– Portion of network where collisions occur
• Ethernet network design
– Repeaters repeat collisions
• Result in larger collision domain
– Switches and routers
• Separate collision domains
• Collision domains differ from broadcast domains
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Broadcast domains and collision domains
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Ethernet Standards for Copper Cable
• IEEE Physical layer standards
– Specify how signals transmit to media
• How to specify
– Number transmission type cable
• Ex. 10 base T
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Ethernet Standards for Copper Cable
• 10Base-T
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10 represents maximum throughput: 10 Mbps
Base indicates baseband transmission
T stands for twisted pair
Two pairs of wires: transmit and receive
• Full-duplex transmission
• Two wires for transmit
• Two wires for receive
– Baseband transmission, star topology, RJ-45
connectors
– Not common anymore
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5,4,3 rule – max 5 segments, 4 repeating devices, 3 segments populated,
500 meters max between nodes
A 10 Base-T network
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Ethernet Standards for Copper Cable
• 100Base-T (Fast Ethernet)
– IEEE 802.3u standard
– Similarities with 10Base-T
• Baseband transmission, star topology, RJ-45
connectors
– 100Base-TX (most common)
• 100-Mbps throughput over twisted pair
• Full-duplex transmission: doubles effective bandwidth
• where X is a placeholder for the FX and TX variants
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max 3 segments, 2 repeating devices, 300 meters max between nodes
A 100 Base-T network
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Ethernet Standards for Copper Cable
• 1000Base-T (Gigabit Ethernet)
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IEEE 802.3ab standard
1000 represents 1000 Mbps
Base indicates baseband transmission
T indicates twisted pair wiring
Four pairs of wires in Cat 5 or higher cable
• Transmit and receive signals
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Ethernet Standards for Copper Cable
• 10GBase-T
– IEEE 802.3an
– Pushing limits of twisted pair
• Requires Cat 6, 6a, or 7 cabling
– Benefits
• Very fast data transmission
• Cheaper than fiber-optic
– Uses
• Connect network devices
• Connect servers, workstations to LAN
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Ethernet Standards for Fiber-Optic
Cable
• 100Base-FX (Fast Ethernet)
– IEEE 802.3u standard
– 100-Mbps throughput, baseband, fiber-optic cabling
• Multimode fiber containing at least two strands
– Half-duplex mode
• One strand receives; one strand transmits
– Full duplex-mode
• Both strands send and receive
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Ethernet Standards for Fiber-Optic
Cable
• 1000Base-LX (1-Gigabit Ethernet)
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IEEE 802.3z standard
1000-Mbps throughput
Base: baseband transmission
LX: Long wavelengths
Single-mode fiber: 5000 meters maximum segment
(3.1 miles)
– Multimode fiber: 550 meters maximum segment (0.34
miles)
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Ethernet Standards for Fiber-Optic
Cable
• 1000Base-SX (1-Gigabit Ethernet)
– Differences from 1000Base-LX
• Multimode fiber-optic cable
• Uses short wavelengths
– Maximum segment length dependencies
• Fiber diameter
– 50 micron fibers: 550 meter maximum length (0.34 miles)
– 62.5 micron fibers: 275 meter maximum length (0.17
miles)
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10-Gigabit Fiber-Optic Standards
• 802.3ae standard
– Fiber-optic Ethernet networks transmitting data at 10
Gbps
– Several variations (will discuss next)
– Common characteristics
• Star topology, allow one repeater, full-duplex mode
– Differences
• Signal’s light wavelength; maximum allowable segment
length
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10-Gigabit Fiber-Optic Standards
• 10GBase-SR and 10GBase-SW
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10 Gbps
Base: baseband transmission
S: short reach
Physical layer encoding
• R works with LAN fiber connections
• W works with SONET fiber connections
– Multimode fiber
– Shortest segment length of 10G Fiber (300 meters)
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10-Gigabit Fiber-Optic Standards
• 10GBase-LR and 10GBase-LW
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10G: 10 Gbps
Base: baseband transmission
L: long reach
Single-mode fiber
Medium segment length of 10G Fiber (10,000 meters,
6.2 miles)
– 10GBase-LR: WAN or MAN
– 10GBase-LW: SONET WAN links
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10-Gigabit Fiber-Optic Standards
• 10GBase-ER and 10GBase-EW
– E: extended reach
– Single-mode fiber
– Longest fiber-optic segment reach
• 40,000 meters (25 miles)
– 10GBase-EW
• Encoding for SONET transmission format
– Best suited for WAN use
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Summary of Common Ethernet Standards
Common Ethernet standards
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Ethernet Frames
• Four Ethernet frame types
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2.
3.
4.
Ethernet_802.2 (Raw)
Ethernet_802.3 (Novell proprietary)
Ethernet_II (DIX)
Ethernet_SNAP
• Frame types differ slightly in format
– Coding and decoding packets
• Framing
– Independent of higher-level layers
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Using and Configuring Frames
• Ensure all devices use same, correct frame type
– Node communication
• Ethernet_II used today
• Frame type configuration
– Specified using NIC configuration software
– NIC autodetect
• Importance
– Know frame type for troubleshooting
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Frame Fields
• Common fields
– 7-byte preamble, 1-byte start-of-frame delimiter
– SFD (start-of-frame delimiter) identifies where data
field begins
– 14-byte header
– 4-byte FCS (frame check sequence)
– Frame size range: 64 to 1518 total bytes
• Larger frame sizes result in faster throughput
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Ethernet_II (DIX)
• Developed by DEC, Intel, Xerox (abbreviated DIX)
• Contains 2-byte type field
– Identifies the Network layer protocol (ex. IP)
• Most commonly used on contemporary Ethernet
networks
Ethernet II (DIX) frame
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PoE (Power over Ethernet)
• IEEE 802.3af standard
– Supplying electrical power over Ethernet connections
• Two device types
– PSE (power sourcing equipment)
– PDs (powered devices)
• Ex. Wireless access point, outdoor camera
• Requires Cat 5 or better copper cable
• Connectivity devices must support PoE
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PoE
PoE capable switch
PoE adapters
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Summary
• Physical topology describes basic network physical
layout
– Examples: bus, ring, star, hybrid
• Logical topology describes signal transmission
• Network backbones
– Serial, distributed, collapsed, parallel
• Switching
– Circuit switching, Packet switching, MPLS
• Ethernet
– Cabling specifications, data frames, PoE
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End of Chapter 5
Questions
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