3 CCNA 200-120 Fundametalof Lan

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CCNA 200-120

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CCNA 200-120

CCNA 200-120

Fundamentals of Ethernet LANs


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Ethernet Local Area Networks (LANs)

Most every enterprise computer network can be separated into two general types of

technology: local-area networks (LAN) and wide-area networks (WAN).

LANs typically connect nearby devices: devices in the same room, in the same building, or

in a campus of buildings.

In contrast, WANs connect devices that are typically relatively far apart.

Together, LANs and WANs create a complete enterprise computer network, working

together to do the job of a computer network: delivering data from one device to another.

While many types of LANs have existed over the years, today’s networks use two general

types of LANs: Ethernet LANs and wireless LANs. Ethernet LANs happen to use cables

for the links between nodes, and because many types of cables use copper wires,

Ethernet LANs are often called wired LANs.

In comparison, wireless LANs do not use wires or cables, instead using radio waves for

the links between nodes


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Ethernet

The term Ethernet refers to a family of LAN standards that together define the

physical and data link layers of the world’s most popular wired LAN technology.

The standards, defined by the Institute of Electrical and Electronics Engineers

(IEEE), defines the cabling, the connectors on the ends of the cables, the protocol

rules, and everything else required to create an Ethernet LAN.

The Variety of Ethernet Physical Layer Standards

The term Ethernet refers to an entire family of standards. Some standards

define the specifics of how to send data over a particular type of cabling,

and at a particular speed. Other standards define protocols, or rules, that

the Ethernet nodes must follow to be a part of an Ethernet LAN. All these

Ethernet standards come from the IEEE and include the number 802.3 as

the beginning part of the standard name


CCNA 200-120

The Variety of Ethernet Physical Layer Standards

Today, Ethernet includes many standards for different kinds of optical and copper cabling, and

for speeds from 10 megabits per second (Mbps) up to 100 gigabits per second (100 Gbps).

The standards also differ as far as the types of cabling and the allowed length of the cabling.

To be ready to choose the products to purchase for a new Ethernet LAN, a network

engineer must know the names and features of the different Ethernet standards supported

in Ethernet products.

The IEEE defines Ethernet physical layer standards using a couple of naming conventions.

The formal name begins with “802.3” followed by some suffix letters.

The IEEE also uses more meaningful shortcut names that identify the speed, as well as a

clue about whether the cabling is UTP (with a suffix that includes “T”) or fiber (with a suffix

that includes “X”).


CCNA 200-120 Examples of Types of Ethernet


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Ethernet LAN Forwards a Data Link Frame over Many Types of Links


CCNA 200-120 Physical and Data Link Features of Ethernet

• Layer 1 – involves signals, bit streams that travel on the media, various topologies

• Layer 2 – MAC sub layer – concerned w/physical components (802.3)


CCNA 200-120

To address L1 limitation IEEE divides the OSI data link layer into two

separate sub layers. Recognized IEEE sub layers are:

Media Access Control (MAC) (transitions down to media)

Logical Link Control (LLC) (transitions up to the network layer)

IEEE 802.3

IEEE 802.2


CCNA 200-120 LLC – Logical Link Sub layer

• Logic Link Control – Connecting the Upper Layers

• IEEE 802.2 standard is represented here


CCNA 200-120 LLC – Logical Link Sub layer

LLC Provides versatility in services to network layer protocols that are above it,

while communicating effectively with the variety of technologies below it.

The LLC, as a sub layer, participates in the encapsulation process.


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Media Access Control (MAC)


CCNA 200-120

• Success of Ethernet is because of: simplicity/ease of

maintenance, ability to adapt new technologies, reliability, and

lower cost to install and upgrade


CCNA 200-120

To build Ethernet LAN , required NIC,UTP cables , connectors & switches

Network Interface Card (NIC)

Network Interface Card (NIC)

• Layer 2, Data Link Layer, device

• Connects the device (computer) to the LAN

• Responsible for the local Layer 2 address (later)

• Common Layer 2 NICs:

– Ethernet ( AKA Ethernet card )

– Token Ring (not used today )

• Common Bandwidth

– 10 Mbps, 10/100 Mbps, 10/100/1000 Mbps


CCNA 200-120 The MAC Address

Every Ethernet NIC card has a unique MAC address.

MAC addresses provide a way for computers to identify themselves.

They give hosts a permanent, unique name.

MAC addresses are sometimes referred to as burned-in addresses (BIAs) because they are burned into read-only memory (ROM) and are copied into random-access memory (RAM) when the NIC initializes

• MAC addresses are:

– 48 bits in length

– Expressed as 12 hexadecimal digits.

– The first 6 hexadecimal digits, which are administered by the IEEE, identify the manufacturer or vendor and thus comprise the Organizational Unique Identifier (OUI).

– The remaining 6 hexadecimal digits comprise the interface serial number, or another value administered by the specific vendor.


CCNA 200-120 The MAC Address

– Components of a MAC address:

• 1-bit broadcast: Identifies broadcast MAC address

• 1-bit local: Identifies local MAC address

• 22-bit OUI: Identifies the manufacturer of the NIC card

• 24-bit vendor-assigned end-station address: Uniquely identifies the Ethernet hardware

– MAC address notation:

• A MAC address is expressed as six groups of two hexadecimal digits, separated by

hyphens (-) or colons (:), in transmission order.

Vendor-Assigned OUI

24 bits 22 bits 1 1

Lo

ca

l

Bro

ad

ca

st

An Intel MAC address: 00-20-E0-6B-17-62

OUI Vendor-assigned


CCNA 200-120

What is the Address on my NIC?


CCNA 200-120

• MAC – used locally in the frame, layer 2 address

• IP – used to cross a WAN in a packet, layer 3 address


CCNA 200-120

• IP addresses NEVER change in a packet (layer 3)

• MAC addresses DO change within a frame (layer 2)

• If a device doesn’t know the MAC address for an IP address,

it will broadcast an ARP request for this information.


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Ethernet frame

The Ethernet data link protocol defines the Ethernet frame: an Ethernet header at

the front, the encapsulated data in the middle, and an Ethernet trailer at the end.

Ethernet actually defines a few alternate formats for the header, with the frame

format shown in Figure being commonly used today.

DH NH TH DATA DT

Ethernet frame

Header Trailer


CCNA 200-120 Ethernet Frames – more detail

Header Trailer

Preamble or Start Field

• When computers are connected to a physical medium, there must be a way

they can grab the attention of other computers to broadcast the message,

"Here comes a frame!"

• Various technologies have different ways of doing this process, but all

frames, regardless of technology, have a beginning signaling sequence of

bytes.

• Depending up frame format: Preamble = 7 bytes, Start or Start of Frame

Delimiter (SFD) = 1 byte


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Header Trailer

Destination MAC address

Identify intended recipient of this frame .

It can be :-

• Unicast address: A single Ethernet frame to be received by a single station.

– Unknown Unicast: This is from the perspective of a switch, when the

unicast address is not in its MAC Address Table

• Multicast address: A single Ethernet frame to be received by a group of

stations.

• Broadcast address: Special case of a multicast address, which is all 1’s.

This is an Ethernet frame to be received by all stations. It has a value of

FFFF.FFFF.FFFF


CCNA 200-120 Header Trailer

Source MAC address :-

Identify MAC address of sender ( sender NIC )for this frame

It always unicast address



Type Field

• Usually information indicating the layer 3 protocols in the data field, I.e. IP

Packet.

• Type field values of particular note for IEEE 802.3 frames include:

– 0x0800 (IPV4)

– 0x86DD (IPV6)

– 0x0806 ARP Message



Data and Pad

Hold the data from higher layers usually L3PDU (IPV4 OR IPV6 )

The sender add padding to meet minimum length requirement for

this field ( 46 bytes )



FCS ( Frame check sequence ):-

The methods of receiving NIC to determine whether the frame experienced

transmission error

CRC ( cyclic redundancy check ) algorithm work here


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Source MAC :- 00.00.00.00.00.01 Destination mac:- 00.00.00.00.00.03

00.00.00.00.00.01 00.00.00.00.00.02 00.00.00.00.00.03 00.00.00.00.00.04

Source MAC :- 00.00.00.00.00.01 Destination mac:- FF-FF-FF-FF-FF-FF

Source MAC :- 00.00.00.00.00.01 Destination mac:- multicast MAC

Communicating Within the LAN


CCNA 200-120 Building Physical Ethernet Networks with UTP

– Speed and throughput: 10 to 1000 Mb/s

– Average cost per node: Least expensive

– Media and connector size: Small

– Maximum cable length: Varies

RJ-45 connector


CCNA 200-120 Building Physical Ethernet Networks with UTP

Twisted pair cable comes in different grades, called categories, which support

different network speeds and technologies such as:

Widely used in todays Ethernet LAN


CCNA 200-120

Ethernet NIC pin layout

Before Building Physical Ethernet Networks with UTP engineer must known the pin number of

Ethernet NIC at PC end & Ethernet port at switch end

Building Physical Ethernet Networks with UTP

Ethernet NIC transmitters use the pair connected to pins 1 and 2; the NIC receivers use a pair of wires

at pin positions 3 and 6.

LAN switches, knowing those facts about what Ethernet NICs do, do the opposite:

Their receivers use the wire pair at pins 1 and 2, and their transmitters use the wire pair at pins 3 and 6.


CCNA 200-120

Be able to identify cable and know what it is used for:

• Straight thru – connecting unlike devices on different layers

• Cross over – connecting like devices on same layer

• Rollover – console connection from PC to switch/router


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Straight-through Cross-over Rollover


CCNA 200-120

UTP Implementation: Straight-Through vs. Crossover

Crossover Cable Straight-Through Cable


CCNA 200-120

Media-dependent interface-crossover (mdix)


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Rollover cable

• Connect a workstation serial port to a router or switch

console port, possibly using an adapter.


CCNA 200-120

Understanding the Challenges of Shared LANs

Ethernet LAN segment

A segment is a network connection made by a single unbroken network cable .

Ethernet cables & segment can span only limited physical distance , beyond which

transmissions will become Degraded because of line noise ,reduced signal strength .

cable type , data rate & modulation affect The maximum segment length

Each type of Ethernet specification has defined set of cable type, data rate ,& modulation

technique , Which in turn define a maximum segment length

Example:- 100baseT( Ethernet over twisted pair )

100 = 100 Mb/s

Base = baseband transmissions

T= UTP cat cable ( cat5 &above )


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Ethernet LAN segment

Each type of Ethernet specification has defined set of cable type, data rate ,&

modulation technique , Which in turn define a maximum segment length as

shown in figure


CCNA 200-120 Extending LAN Segments

Repeater :-

Is physical layers(L1) devices

Used to extend the network segment

Amplify the signal

Limited port

Number of repeater required

You can add devices to an Ethernet LAN to extend segments

Hub :-

also physical layers devices (L1)

Used to extend the network segment

Amplify the signal

Has Multiport

It extend , but not terminate ,an Ethernet LAN

Bandwidth are shared among all users


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Half-duplex Ethernet is defined in the original 802.3 Ethernet and uses only one wire pair

with a digital signal running in both directions on the wire.

Collisions are part of the operation of Ethernet , occurring when two stations attempt to

communicate at same time . Adding the hub to an Ethernet LAN can extend the segment

but hubs cannot improve collision issues


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Ethernet networking uses Carrier Sense Multiple Access with Collision Detect

(CSMA/CD) to overcome the problem of those collisions that occur when packets are

transmitted simultaneously from different nodes.


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Hub

Half-duplex

• Hubs operate only in Half-duplex.

• Half-duplex means that only one end can send at a time.

• With half-duplex NICs, a host can only transmit or receive, not both at the same time, or a collision will occur.

• When multiple devices are connected to a hub or series of hubs, only one device can transmit.

• Uses CSMA/CD.

• If the a carrier is detected, then the NIC will not transmit.

• Ethernet hubs and repeaters can only operate in half-duplex mode.


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Collision Domain: Shared Access

• Collision domain (Wikipedia): A group of

Ethernet or Fast Ethernet devices in a

CSMA/CD LAN that are connected by

repeaters/hubs and compete for access on the

network.

– Only one device in the collision domain

may transmit at any one time, and the

other devices in the domain listen to the

network in order to avoid data collisions.

– A collision domain is sometimes referred to

as an Ethernet segment.

• If you connect several computers to a single

medium that is only connected by repeaters and hubs (Layer 1 devices), you have a

shared-access situation, and you have a single

collision domain.

collision domain.


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Broadcast Domain

Broadcast domain ,is a network of devices , in which all devices see &

participate in broadcast sent from host or server . Within a broadcast domain,

each computer can communicate directly with another computer


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Full-duplex Ethernet uses two pairs of wires, instead of one wire pair like half duplex.

And full duplex uses a point-to-point connection between the transmitter of the

transmitting device and the receiver of the receiving device.

This means that with full duplex data transfer, you get a faster data transfer compared

to half-duplex. And because the transmitted data is sent on a different set of wires

then the received data, no collisions will occur


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• Full-duplex is allows simultaneous communication between a pair of stations or devices.

• Full-duplex allows devices to send and receive at the same time.

• Both ends of the link must be in full-duplex mode.

• Most switches operate at either full-duplex but can operate in half-duplex.

• If a hub is connected to a switch, the switch port must be in half-duplex.

• The collision domain will end at the switch port.


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CCNA 200-120

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CCNA 200-120 Ethernet LAN segment

Multiple Collision Domains

In traditional Ethernet segment ,the network deices compete for the same bandwidth , with

only one devices able to transmit data at a time .the network segment that shred the same

bandwidth are known as collision domain , because when two or more devices within that

segment try to communicate at the same time may occur

Using the devices working at L2 layers like switch & above can segment the network

but also reduce the number of devices participate in collision domain .

Collisions are isolated within one domain (does not effect other collision domain )


CCNA 200-120

Solving Network Challenges with Switched LAN Technology

Network Congestion

Internet cloud

The most common causes of network congestion are as follows:

Increasingly powerful computer & network technologies : today , cpu & peripherals are

much more powerful ,they can send more data at higher rates through the network

Increasing volume of network traffic

High-bandwidth application : software application are become richer in their functionality &

are required more & more bandwidth (like desktop publishing ,VoD, e-learning . etc)

Switch Switch Switch


CCNA 200-120 LAN Switch Features


CCNA 200-120 LAN Switch Features



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3 CCNA 200-120 Fundametalof Lan