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TCOM 5272
Telecomm Lab
Dr. Mostafa Dahshan
OU-Tulsa 4W 2nd floor
660-3713
mdahshan@ou.edu
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Acknowledgements
 Some notes and figures in this
presentation are imported from
 Notes by Dr. Anindya Das
 Textbook supplemental material
 CCNA Intro Exam Certification Guide
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Ethernet/802.3
 Most common LAN architecture
 Used to transport data between
devices connected to the same
delivery medium
 Uses a data frame broadcast method
 Frame is sent to the entire bus, intended
destination processes the frame, while all
other devices discard it
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Ethernet 802.3 (2)
 Negative effects of a shared LAN
 broadcast delivery of all frames
 CSMA/CD: collisions are inherent
 distance limitation requires using
repeaters to extend
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Repeaters
 Connect two or more cable segments
 Retransmit incoming signal to all
other segments
 Cable segment is run within IEEE
specifications
 Ethernet segment in star-bus network
 Repeater hub is a multiport repeater
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Repeaters (2)
 Filter out signal disturbance caused
by EMI and RFI
 Amplify and reshape incoming signal
 Retime the signal (in Ethernet
applications)
 Reproduce the signal on all cable runs
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Repeaters (3)
 Benefits of Using a Repeater
 a layer 1 device that cleans up and
boosts the signal
 extends the coverage area of a LAN
segment
 Negative Effects of Using a Repeater
 increases the collision domain size
 increases the broadcast domain size
 can’t filter traffic based on Layer 2 or 3
addressing
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Half-Duplex Ethernet
 Only one host can transmit at a time because the
NIC needs to listen for collisions
 The NIC provides several circuits.
Most important are:
 receive (RX), transmit (TX), and collision detection
 bandwidth usage = 50% to 60%
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Collision Domains
 Group of Ethernet devices connected
by repeaters (or repeater hubs)
 Only one device can transmit at a
time
 Simultaneous transmissions result in
a collision
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Full-Duplex Ethernet
 Transmission and the reception at
the same time
 Requires using two pairs of wires in
the cable and a switched connection
between each node
 Connection is considered point-topoint and is collision free
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Full-Duplex Ethernet (2)
 Because both nodes can transmit and
receive at the same time, there are
no negotiations for bandwidth
 100% of bandwidth is available:
 10 Mbps increases to 20 Mbps of
potential throughput
 10 Mbps TX & 10 Mbps RX
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Ethernet Connections
 Computer NIC
 Pins 1,2 Transmit Data
 Pins 3,6 Receive Data
 Hub/Switch/Router
 Pins 1,2 Receive Data
 Pins 3,6 Transmit Data
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Ethernet Connections (2)
 Computer to Switch

Use straight-through cable
 Switch to Switch/Router

Use crossover cable
 Computer to Computer

Use crossover cable
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LAN Segmentation
 By segmenting a LAN
fewer devices are
sharing the same
bandwidth
 Improved performance
of a shared media LAN
 Each segment is
considered its own
collision domain
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Bridges
 Network devices connecting LAN segments
 Extend LAN when maximum connection
limit reached
 Example: the 30-node limit on an Ethernet bus
 Extend a LAN beyond the length limit
 Example: beyond 185 meters for thinnet
segment
 Segment LANs to reduce data traffic
bottlenecks
 Prevent unauthorized access to a LAN
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Segmenting with Bridges
 Bridges “learn” a
network’s
segmentation by
building address
tables that contain:
 Bridge interface
that will reach that
device
 Each device’s MAC
address
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Bridge Operation
 Three frame scenarios
 Destination on same segment as source
 Bridge drops frame, since no forwarding needed
 Destination on another segment known to bridge
 Bridge transmits frame to the known segment
only
 Destination segment not known to bridge
 Bridge transmits frame to all segments but
source
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Broadcast Domains
 Group of devices that can receive
layer2 broadcasts
 Ethernet address FF:FF:FF:FF:FF:FF
 Devices can communicate to each
other without going through a router
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Segmenting with Switches
 A switch is simply a multi-port bridge,
making forwarding decisions based on
MAC addresses
 Like a bridge, segmenting a LAN with
a switch creates more collision
domains
 Replacing hubs with switches
therefore decreases congestion and
increases available bandwidth
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Segmenting with Switches (2)
 A switch can microsegment a LAN
creating collision-free domains but
still be in the same broadcast domain.
 Switch creates a virtual circuits,
allowing many users to communicate
in parallel
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Switches VS Bridges
 Switches operate at higher speeds
 Switches are capable of creating virtual
LANs (VLANs) through microsegmentation
 Bridges switch use software; switches
typically switch using hardware (called the
“switch fabric”)
 Bridges use store-and-forward, Switches
can use cut-through switching which
switches the packet as soon as the
destination MAC is read
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Spanning Tree Protocol
 Defined by the IEEE 802.1d standard
 Bridges frames in networks with more than two
bridges
 Sets up a system of checks performed by
bridges
 Two motivations for using spanning tree
algorithm
 Ensure a frame does not enter infinite loop
 Causes congestion that may intensify to
broadcast storm
 Forward frames along the most efficient route
 Efficiency based on distance and utilization of
resources
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Spanning Tree Protocol (2)
 Create one-way path around network
(use bridge data)
 Establish maximum number of hops
for maximum route
 Enable bridges to send frames along
best route
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Spanning Tree Protocol (3)
 Example: Why STP?
 Larry sends a frame to
Bob
 Bob is powered off
 Bob’s address
unknown
 Frames forwarded by
each switch to all ports
 The frames will loop
forever!
Archie
Bob
Larry
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Virtual LANs
 Logical grouping of network devices
 Similar to splitting a switch into
separate logical switches
 Each VLAN forms a separate
broadcast domain
 Devices in different VLANs cannot
communicate without a router or a
layer 3 protocol (e.g. IP)
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Virtual LANs (2)
VLAN 1
VLAN2
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Trunking
 A VLAN spanning multiple switches
 Devices in a VLAN can be connected
to different switches
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Trunking (2)
VLAN1
VLAN1
Trunk
VLAN2
VLAN2
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Ethernet Addressing
 MAC Address: 6 bytes (48 bits)
 First 3 bytes
 Organizationally Unique Identifier (OUI)
 Each mfc has its own OUI
 Address Types
 Unicast: single device
 Multicast: Multiple devices
0100.5Exx.xxx
 Broadcast: All Devices FFF.FFF.FFF
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Address Resolution Protocol
 Used to translate IP addr to MAC addr
 Used between devices on the same
broadcast domain
 Each device maintains a cached table
of IP to MAC address mappings
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Address Resolution Protocol (2)
 ARP works as follows
 The inquiring device sends a broadcast message
(addr: FFF.FFF.FFF)
 The destination device responds with its MAC
address to the inquiring device
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Exp 1: Common Network Utils
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ping
traceroute (Windows: tracert)
nslookup
netstat
route (more details next class)
arp
telnet
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Exp 2: Access Switch Console
 Use the Cisco Catalyst 1900 switch
 Connect a serial cable to the switch
console port
 Use PC with terminal software to
access the console
 To use the command line interface,
type K
 Type ? To see available commands
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Exp 3: Ethereal Packet Sniffer
 Use Ethereal to capture and analyze
packets in the following scenarios
 Access a website with and input form
(e.g. www.google.com)
 Capture packets from your own PC and
other PCs in the same LAN
 When PCs are connected by a hub
 When PCS are connected by a switch
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Exp 4: ntop Traffic Monitor
 Start ntop on a PC
 Connected with a hub
 Connected with a switch
 Generate some network traffic
 View ntop reports and record your
observations
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Exp 5: RouterSim
 Use the CCNA Network Visualizer 5.0
to familiarize yourself with the Cisco
Catalyst 1900 switch
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Homework
1.
2.
3.
4.
5.
6.
Write (in your own words) a summary about each of the utilities used in
Experiment 1
Use ping to measure Round Trip Time (RTT) for 10 messages of size 64,
256, and 4096 bytes. Graph the message size versus RTT for two hosts on
a LAN (two workstations in the lab) and two nodes on a WAN (for instance,
your lab workstation and a host outside the OU campus). Discuss the
effects of distance, message size, and their relationship with bandwidth and
latency.
Use the traceroute utility on your lab workstation to find the route to
a.
b.
c.
a host in another city in Oklahoma
a host on the east or west coast of the United States
a host in Canada or Mexico
Next, using the traceroute utilities at the site www.traceroute.org, find the
routes between two hosts on different continents. Trace the route again
between these two hosts after at least an hour. Analyze your recorded
results.
Briefly discuss why ping would not necessarily provide an accurate estimate
of the round trip time for packets exchanged by two hosts on the Internet?
Use the Ethereal software to capture one traffic session generated while
using the ping and tracert commands. Report your results.
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