1 10base-t ethernet lan. 2 lans u local area networks u limited geographical area –single office...

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10Base-T Ethernet LAN

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LANs Local Area Networks

Limited Geographical Area– Single office– Single building– University campus or industrial park

Generally, high speeds– Now, most operate at around 10 Mbps– 100 Mbps is emerging as the new “base speed”

Most Data Traffic is Local

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Standards Setting LANs are Subnets (single networks)

Subnet technology is Dominated by OSI Standards (true for LANs)

IEEE Creates most LAN Standards– Institute for Electrical and Electronic Engineers– Submits its standards to ISO and ITU-T for ratification

IEEE 802 Committee– LAN standards are set by the IEEE 802 Standards Committee. – 802.3 for Ethernet Standards– 802.5 for Token-Ring LAN Standards– 802.11 for Radio and Infrared Wireless LANs

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LANs and OSI Architecture

OSI is a 7-layer architecture

LAN transmission only uses Layers 1 and 2

Layer 1: Physical Layer– Connectors, Media, Electrical signaling

Layer 2: Data Link Layer– Packaging data into frames– Managing transmission over link (error handling, etc.)– Access control: when each station may transmit

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OSI Physical and Data Link Layers

F4-1

StationA

StationA

StationB

StationB

ConnectorPlug

ConnectorPlug

Transmission Medium(telephone wire, etc.)

Electrical Signal

Physical Layer (OSI Layer 1)Physical (plugs, media, etc.); Electrical (voltages, timing, etc.)

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OSI Physical and Data Link Layers

Data Link Layer (OSI Layer 2)

Frame 2Frame 2 Frame 1Frame 1

StationA

StationA

StationB

StationB

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Data Link Layer

For point-to-point transmission– A point-to-point connection is a data link– So is a transmission system shared by multiple devices,

only one of which can transmit at a time because of collisions

Transmission

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Data Link Layer

First function: Packaging of Data (1s and 0s)– PDU at Data Link Layer

is called a frame

Second Function: Access Control

– Only one station can transmit at any time

– If another transmitted, their signals would scramble one another

– Must control access to (transmission into) the transmission medium

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Data Link Layer

Logical Link Control LayerLogical Link Control Layer

Media Access Control (MAC) LayerMedia Access Control (MAC) Layer

802.310Base-T

802.310Base-T

802.310Base-5

802.310Base-5

802.3Other

PhysicalLayer

802.3Other

PhysicalLayer

802.5Physical

Layer4 Mbps

802.5Physical

Layer4 Mbps

802.5Physical

Layer16 Mbps

802.5Physical

Layer16 Mbps

OtherPhysical

Layer

OtherPhysical

Layer

OSIData Link

Layer(Layer 2)

OSIPhysical

Layer(Layer 1)

OSI Data Link Layer is subdivided into two layers Media access control Logical link control

OSI Data Link Layer is subdivided into two layers Media access control Logical link control

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Media Access Control (MAC) LayerMedia Access Control (MAC) Layer

OSIPhysical(Layer 1)

MediaAccessControl

MAC Layer

802.310Base-T

802.310Base-T

802.310Base-5

802.310Base-5

802.3Other

PhysicalLayer

802.3Other

PhysicalLayer

802.5Physical

Layer4 Mbps

802.5Physical

Layer4 Mbps

802.5Physical

Layer16 Mbps

802.5Physical

Layer16 Mbps

OtherPhysical

Layer

OtherPhysical

Layer

MAC layer implements media access control: When a station may transmit Controls the framing of data along the wire

MAC layer implements media access control: When a station may transmit Controls the framing of data along the wire

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OSIPhysical(Layer 1)

LLC

Logical Link Control Layer

802.2 Logical Link Control Layer802.2 Logical Link Control Layer

802.3 Media Access Control(MAC Layer)

802.3 Media Access Control(MAC Layer)

802.5MAC

4 Mbps

802.5MAC

4 Mbps

802.5MAC

16 Mbps

802.5MAC

16 Mbps

OtherMAC

OtherMAC

802.310Base-T

802.310Base-T

802.310Base-5

802.310Base-5

802.3Other

PhysicalLayer

802.3Other

PhysicalLayer

802.5Physical

Layer4 Mbps

802.5Physical

Layer4 Mbps

802.5Physical

Layer16 Mbps

802.5Physical

Layer16 Mbps

OtherPhysical

Layer

OtherPhysical

Layer

Provides Control Function Begin/end connections between stations Error correction (optional)

Provides Control Function Begin/end connections between stations Error correction (optional)

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Simple LAN Using Ethernet 10Base-T

RJ-45 jacks10Base-T Hub (Multiport Repeater)

Unshielded Twisted Pair (UTP) Wiring(4-Pair Bundle)

PC

RJ-45Jack

NIC

NetworkInterface

Card

10Base-TUTP Wiring

Bundles:4 Pairs

EIACategory3, 4, or 5

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Ethernet 10Base-T (802.3u)

Physical Layer Standard– 10 Mbps (10 in 10Base-T)– Baseband signaling: Injects voltage changes directly

into the wires (Base in 10Base-T)

Hubs (Multiport Repeaters)– Connect the stations together

10Base-T Hub

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NICs

Network Interface Cards

– Implement Physical Layer

Plug and Electrical Signaling

– Implements the Data Link Layer (data packaging, access control, etc.)

LLC (802.2)

MAC (802.3 MAC)

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Wiring

Unshielded Twisted Pair (UTP)– Twisted several times per foot to reduce interference, T in

10Base- and Unshielded No protection except for plastic coating Distance limitation: 100 meters (attenuation, distortion, noise and

interference, crosstalk) -- propagation

Categories of UTP Wiring– Category 5: The best. Good for 100 Mbps– Category 3 and 4: lower. May be OK for 100 Mbps

Wiring Plugs: RJ-45 Standard– Similar to home (RJ-11) jacks, but wider

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Attenuation

As signal travels, gets weaker– If too weak, cannot tell 1s and 0s

Distance

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Distortion

As signal travels, it become distorted– Changes shape– Successive bits may merge, making reception difficult

Distance

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Noise

Unwanted energy on line. Always present Noise floor is average level Noise spikes will cause problems

SignalStrength

Time

Signal

Noise SpikeNoise Floor

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Signal to Noise Ratio (SNR)

Ratio of Signal Power to Noise Power– If SNR is high, few noise errors

SNR

ErrorRate

100%

0%1

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Interference

Unwanted signal from outside sources– Often intermittent, difficult to diagnose

SignalStrength

Signal

Interference

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Physical Layer Transmission in 10Base-T

F4-4

10Base-T Hub

Step 1Station A

Transmits onUpstream

Pair(Wires 1&2)

Step 1Station A

Transmits onUpstream

Pair(Wires 1&2)

StationA

StationB

StationC

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Physical Layer Transmission in 10Base-T10Base-T Hub

Step 2Hub Repeats(Broadcasts)The Message

To AllStations

On DownstreamPairs

(Wires 3&6)

Step 2Hub Repeats(Broadcasts)The Message

To AllStations

On DownstreamPairs

(Wires 3&6)

StationA

StationB

StationC

Bus transmission means broadcasting

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Manchester Encoding

Inefficient– Transmits 10 million bits per second– Must change line state 20 million times per second– 20 Mbaud (baud is a change in the line state)– Technology limits the baud rate– Only 10 Mbps for 20 Mbaud– Bit rate is less than the baud rate (opposite in modems)

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MAC LayerCSMA/CD Media Access Control

Controls when stations may transmit– If two transmit at once, signals will be scrambled

Collision

X

Collisions will grow rapidly above 30% of line utilization. Keep traffic moderate, or throughput will be affected.

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CSMA/CD Media Access Control

CS: Carrier Sense– Each NIC always listens for traffic on the line– This lets it recognize messages sent to its address– This also lets it know if the line is free

CSMA: Carrier Sense Media Access– A station may transmit if it hears no traffic on the

network

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CSMA/CD Media Access Control in Ethernet

Station Ais Transmitting

Station Bmust wait

Station Ais Transmitting

Station Bmust wait

10Base-T Hub

StationA

StationB

StationC

MustWait

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CSMA/CD Media Access Control

CD: Collision Detection– If two stations transmit at once ….– Their signals collide, scrambling one another– Because each sender listens (senses the carrier), both

know that there has been a collision– Both stop and wait a random amount of time.

X

Collision

101010 001110

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802.3 MAC Layer Frame

PreamblePreamble

Start of Frame DelimiterStart of Frame Delimiter

Destination AddressDestination Address

Source AddressSource Address

LengthLength

DataData

PadPad

Frame Check SequenceFrame Check Sequence

In asynchronous transmission, each byte is sent separately, in a 10-bit frame.

In 802.3 MAC Layer frames, transmission is synchronous.

Many data bytes are sent in each frame of variable length.

Frames must be between 64 octets and 1518 octets long, divided into groups of bits called fields.

Field lengths are measured in “octets”, eight bits.

“Octet” is a synonym for “byte”

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Error Detection MAC Layer Process Provides error detection

– Determines that an error exists

Does not provide error correction– Merely discards the frame– No request for retransmission if there is an error

Logical Link Control Layer may do error correction– Receiving LLC process detects discarded frames– Receiving LLC process asks for retransmissions

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Logical Link Control (LLC) Layer

Next Higher Layer(Usually Internet)

Next Higher Layer(Usually Internet)

LLC LayerProcess

LLC LayerProcess

MAC LayerProcess

MAC LayerProcess

Station A

Next Higher Layer(Usually Internet)

Next Higher Layer(Usually Internet)

LLC LayerProcess

LLC LayerProcess

MAC LayerProcess

MAC LayerProcess

Station B

LLC Frame

MAC Frame

802.2 can askfor retransmission of

lost MAC frames.Optional.

LLC frame placedwithin MAC frame

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Ethernet 10Base-T Network with Two HubsHub 1 Hub 2

Station A Station B Station C

Station ATransmits

toHub 1 Hub 1 Transmits

Out All Ports,Including thePort to Hub 2

Hub 2 Broadcaststhe MessageOut All of Its

Ports

Station CReceives the

Message

100 mSegmentMaximum

100 mSegmentMaximum

100 mSegmentMaximum

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Ethernet 10Base-T LAN with Multiple Hubs

Hub 1

Hub 2 Hub 3

Hub 4

StationA

StationB

StationC

StationD

UTPWire

UTPWire UTP

Wire

UTPWire

UTP Wire

Daisy chain, no Loopsallowed!

Daisy chain, no Loopsallowed!

Maximum distancebetween farthest Stations is

4 Hubs/5 100 meter segments

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Speed and Distance

Transmission speed worsens problems

– Error rates increase because bit periods are smaller and are more likely to be damaged by brief noise spikes and interference

– High speeds create high-frequency components in the signal that attenuate more rapidly than lower-frequency components

– In general, as speed increases, maximum distance decreases, although improving technology can lessen the decrease

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Dealing with Propagation Effects

Use High-Quality, High-Cost Media– Use media designed for long-distance propagation– Optical fiber, coaxial cable– Too expensive for runs to many desktops, good for hub-

to-hub

Use Inexpensive Media to the Desktop– Improve the technology (allows 100 Mbps+ on UTP)– Accept distance limitations (100 meters for UTP)– More popular alternative to desktop because of low cost