02 - fundamentals of lans
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02 - Fundamentals of LANs
By Muhammad Asghar Khan
Reference: CCENT/CCNA ICND1 Official Exam Certification Guide By Wendell Odom
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Agenda
Local Area Networks Physical Layer (L1)
Overview of Ethernet
History of Ethernet
Most Common Ethernet Standards
Comparing Ethernet Media Requirements
Ethernet UTP Cabling
Repeaters, Hubs, Bridges & Switches
Data Link Layer (L2)
CSMA/CD Protocol
Ethernet Addressing
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Agenda
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Unicast Ethernet Addresses Group Addresses
Ethernet Framing
Error Detection
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Local Area Networks
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LANs interconnect host devices over short distances
LANs can support high speed and a fairly largebandwidth
LAN traffic can be controlled with bridges and switchesor Hubs
Ethernet is the undisputed king of LAN standards today
Historically speaking, several competing LAN standardsexisted, including Token Ring, Fiber Distributed DataInterface (FDDI), and Asynchronous Transfer Mode
(ATM) Eventually, Ethernet won out over all the competing
LAN standards
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Local Area Networks
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Typical Small Modern LAN
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Overview of Ethernet
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The term Ethernet refers to a family of standardsthat define the Physical and Data Link layers of the
LAN networks
Most of the standards define a different variation of
Ethernet at the Physical Layer, with differences in
speed and types of cabling
Additionally, for the Data Link Layer, the IEEE
separates the functions into two sublayers: The 802.3 Media Access Control (MAC) sublayer
The 802.2 Logical Link Control (LLC) sublayer
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History of Ethernet
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The IEEE in the early 1980s formed two committees that worked
directly on Ethernetthe IEEE 802.3 committee and the IEEE 802.2committee
The 802.3 committee worked on Physical Layer standards as well asa subpart of the Data Link Layer called Media Access Control (MAC)
The IEEE assigned the other functions of the Data Link Layer to the
802.2 committee, calling this part of the data link layer the LogicalLink Control (LLC) sublayer
The two early Ethernet standards were 10BASE5 and 10BASE2.
You should not expect to need to implement 10BASE5 or 10BASE2Ethernet LANs today
10BASE5 and 10BASE2 had limitations on the total length of a cable.With 10BASE5, the limit was 500 m; with 10BASE2, it was 185 m
Repeaters was used with 10BASE5 and 10BASE2 standards
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Most Common Ethernet Standards
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The T and TX in the alternative names refer to the fact that each ofthese standards defines the use of UTP cabling, with the T referring to
the T in twisted pair.
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Most Common Ethernet Standards
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With 10BASE-T, the concept of cabling each deviceto a centralized connection point was introduced
Originally, 10BASE-T called this centralized
connection Ethernet hub
When building a LAN today, you could choose to
use either a hub or a switch as the centralized
Ethernet device to which all the computers connect
Even though modern Ethernet LANs typically use
switches
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Comparing Ethernet Media Requirements
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Ethernet UTP Cabling
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The three most common Ethernet standards usedtoday are:
10BASE-T (Ethernet)
100BASE-TX (Fast Ethernet, or FE), and
1000BASE-T (Gigabit Ethernet, or GE) These three standard use UTP cabling
The cable ends have some form of connectorattached (typically RJ-45 connectors)
RJ-45 connector has eight specific physical locationsinto which the eight wires in the cable can beinserted, called pin positions, or simply pins
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Ethernet UTP Cabling
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RJ-45 Connectors and Ports
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http://www.learn44.com/the-physical-layer-of-the-osi-reference-model-protocols-media-and-concepts
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Ethernet UTP Cabling
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Many Cisco switches have a interfaces that useeither Gigabit Interface Converters (GBIC) or Small-
Form Pluggables (SFP)
GBIC SFP
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Ethernet UTP Cabling
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UTP Cabling Pinouts for 10BASE-T and 100BASE-TX The wiring pinoutsthe choice of which color wire
goes into which pin positionmust conform to theEthernet standards
Two cooperating industry groups, theTelecommunications Industry Association (TIA) andthe Electronics Industry Alliance (EIA), definestandards for UTP cabling, color coding for wires, andstandard pinouts on the cables
Figure on next slide shows two pinout standards fromthe EIA/TIA, with the color coding and pair numberslisted
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Ethernet UTP Cabling
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Ethernet UTP Cabling
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A UTP cable needs two pairs of wires for 10BASE-Tand 100BASE-TX and four pairs of wires for
1000BASE-T
10BASE-T and 100BASE-TX Ethernet define that one
pair should be used to send data in one direction,with the other pair used to send data in the other
direction
Ethernet NICs should send data using the pair
connected to pins 1 and 2in other words, pair 3according to the T568A pinout standard
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Ethernet UTP Cabling
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Similarly, Ethernet NICs should expect to receive datausing the pair at pins 3 and 6pair 2 according to the
T568A standard
Hubs and switches receive on the pair at pins 1,2 (pair
3 per T568A), and they send on the pair at pins 3,6(pair 2 per T568A)
Straight-Through Cable Concept
Used when the devices on the ends of the cable use
opposite pins when they transmit data
Connects the wire at pin 1 on one end of the cable to
pin 1 at the other end of the cable;
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Ethernet UTP Cabling
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The wire at pin 2 needs to connect to pin 2 on the other
end of the cable;
Pin 3 on one end connects to pin 3 on the other; and so
on
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Ethernet UTP Cabling
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Crossover Ethernet Cable Concept A cable that swaps the wire pairs inside the cable is
called a crossover cable
Use when connecting two devices that both use the
same pins to transmit Many LANs use multiple switches, with a UTP cable
connecting the switches, because both switches send
on the pair at pins 3,6, and receive on the pair at pins
1,2, the cable must swap or cross the pairs
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Ethernet UTP Cabling
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Figure below shows the Crossover Ethernet Cable
In short, devices on opposite ends of a cable that use thesame pair of pins to transmit need a crossover cable
Devices that use an opposite pair of pins to transmit need astraight-through cable
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Ethernet UTP Cabling
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Table below lists the devices and the pin pairs theyuse, assuming that they use 10BASE-T and 100BASE-
TX
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Ethernet UTP Cabling
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Typical Uses for Straight-Through and CrossoverEthernet Cables
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Repeaters, Hubs, Bridges & Switches
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Repeaters
Repeaters was used with 10BASE5 and 10BASE2standards
Repeaters connect to multiple cable segments,
receive the electrical signal on one cable, interpretthe bits as 1s and 0s, and generate a brand-new,clean, strong signal out the other cable
A repeater does not simply amplify the signal,because amplifying the signal might also amplify anynoise picked up along the way
Repeaters propagate the
collisions
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Repeaters, Hubs, Bridges & Switches
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Hubs
Hubs were introduced to interconnect several hostdevices using one cable for each device
Hub forwards a data frame on all outbound ports,
except on the port through which the frame came in Hubs requires CSMA/CD logic to work properly.
However, CSMA/CD imposes half-duplex logic oneach device, meaning that only one device can sendat a time
Hubs also propagate the collisions, therefore; DataLink Layer (L2) bridges and switches were introduced
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Repeaters, Hubs, Bridges & Switches
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Bridges Bridges create one collision domain per port and can
forward data frames only on the outbound port that
reaches the destination of the frame, as opposed to
hubs, which send the frame out on all ports A bridge is slower than a switch because it uses
software instead of hard-ware application-specific
integrated circuits (ASICs)
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Repeaters, Hubs, Bridges & Switches
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Switches Create one collision domain per port and can forward
data frames only on the outbound port that reaches
the destination of the frame
Switches can buffer frames in memory, switches cancompletely eliminate collisions on switch ports that
connect to a single device
As a result, LAN switches with only one device cabled
to each port of the switch allow the use of full-duplex
operation. Full duplex means that an Ethernet card
can send and receive concurrently
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CSMA/CD Protocol
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Ethernet uses the carrier sense multiple accesscollision detect (CSMA/CD) protocol
Whenever several computer hosts share the
bandwidth on a common network medium, theres
a risk of frame collisions. CSMA/CD was developed
to mitigate this risk
The CSMA/CD algorithm works like this:
Step 1 A device with a frame to send listens until theEthernet is not busy
Step 2 When the Ethernet is not busy, the sender(s)
begin(s) sending the frame
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CSMA/CD Protocol
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Step 3 The sender(s) listen(s) to make sure that no
collision occurred
Step 4 If a collision occurs, the devices that had been
sending a frame each send a jamming signal to
ensure that all stations recognize the collision Step 5 After the jamming is complete, each sender
randomizes a timer and waits that long before trying
to resend the collided frame
Step 6 When each random timer expires, the processstarts over with Step 1
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Ethernet Addressing
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Ethernet LAN addressing identifies either individualdevices or groups of devices on a LAN
Each address is 6 bytes long, is usually written inhexadecimal, typically is written with periods
separating each set of four hex digits. For example,0000.0C12.3456 is a valid Ethernet address
Ethernet address are also known as HardwareAddresses, Physical Addresses or MAC Addresses
Unicast Ethernet Addresses
Identify a single LAN card
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Ethernet Addressing
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Each LAN card comes with a burned-in address (BIA)
that is burned into the ROM chip on the card
BIAs sometimes are called universally administered
addresses (UAA) because the IEEE universally (well, at
least worldwide) administers address assignment
Structure of Unicast Ethernet Addresses
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Ethernet Addressing
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Group Addresses Identify more than one LAN interface card. The IEEE
defines two general categories of group addresses for
Ethernet
Broadcast addresses: The most often used of the IEEEgroup MAC addresses, the broadcast address, has a
value of FFFF.FFFF.FFFF (hexadecimal notation). The
broadcast address implies that all devices on the LAN
should process the frame
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Ethernet Addressing
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Multicast addresses: Multicast addresses are used to
allow a subset of devices on a LAN to communicate.
When IP multicasts over an Ethernet, the multicast
MAC addresses used by IP follow this format:
0100.5exx.xxxx, where any value can be used
in the last half of the address
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Ethernet Addressing
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LAN MAC Address Terminology and Features
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Ethernet Addressing
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Ethernet Framing Framing defines how a string of binary numbers is
interpreted
The term framing refers to the definition of the fields
assumed to be in the data that is received
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Ethernet Addressing
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Ethernet Addressing
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Error Detection The Ethernet Frame Check Sequence (FCS) field in the
Ethernet trailer allows to detect an error
Ethernet defines that the errored frame should be
discarded, but Ethernet takes no action to cause theframe to be retransmitted.
Other protocols, notably TCP an notice the lost data
and cause error recovery to occur
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