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


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


Typical Small Modern LAN

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Overview of Ethernet


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


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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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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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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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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RJ-45 Connectors and Ports

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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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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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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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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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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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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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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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Table below lists the devices and the pin pairs theyuse, assuming that they use 10BASE-T and 100BASE-

TX

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Typical Uses for Straight-Through and CrossoverEthernet Cables

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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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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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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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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


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


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


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


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


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


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


LAN MAC Address Terminology and Features

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Ethernet Addressing


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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02 - Fundamentals of LANs