Computernetze 1 (CN1)

Steffen/Stettler, 20.09.2013, 2-Ethernet.ppt 1

Computernetze 1 (CN1)

Prof. Dr. Andreas SteffenInstitute for Internet Technologies and Applications

2 Ethernet Grundlagen


Lesestoff im Ethernet Buch

• Kapitel 2 Ethernet, Seiten 31-832.1 Die Geschichte des Ethernet2.2 Der Physical Layer2.3 10Base52.4 10Base22.6 10BaseT2.7 10BaseF2.8 Das Manchester-Kodierungsverfahren2.9 Media Access Control (MAC)

• Kapitel 6 Ethernet Internals, Seiten 208-2146.2 Power over Ethernet

• SelbststudiumErarbeiten Sie als Vorbereitung für die Übung 2selbstständig das Thema “Media Access Control”mit Hilfe von Kapitels 2.9 des Ethernet Buchs unddes Kapitels 2.4 dieses Foliensatzes.


Wie es begann...

• 1972 stoss Robert Metcalfe auf ein Paper von Norman Abramson, welches das Aloha Random Access System der Universität Hawaii beschrieb.

• Ausgehend von Aloha erfand er am Xerox Palo Alto Research Center (PARC) das robuste CSMA/CD* Protokoll, mit dem mehrere Teilnehmer fast kollisionsfrei auf ein „shared-medium“ zugreifen können.

• 1976 stellte er sein Protokoll unter dem NamenEthernet an einer Konferenz vor.

• 1979 gründete er die Firma 3Com.* Carrier Sense Multiple Access with Collision Detection



2.1 Ethernet Standards


802.3 Ethernet

Ethernet und das OSI Modell

802.2


IEEE 802

• LAN standardization is done• by the IEEE (Institute of Electrical and Electronical Engineers)• The IEEE LAN/MAN standards committee 802 was founded in

February 1980

• OSI Data Link Layer (Layer 2)• was originally designed for point-to-point line communication• but LAN is multipoint line, shared media

• Therefore OSI Layer 2 had to be split into two sublayers• Logical Link Control (LLC)• Media Access Control (MAC)


• Verbindungsverwaltung• Connect request, indication, response, confirm, etc.• Synchronisation von gemeinsamen Zählern, etc.

• Fehlererkennung und evtl. Korrektur• Vorwärtsfehlerkorrektur

Erkennen+Rückmelden+Wiederholung (ARQ), etc.• Flusssteuerung / Flow Control

• Achtung: nur für das nächste Segment

• Erstellen von Rahmen / Frames• Ethernet, Token Ring, FDDI, ATM etc.• Layer 2 Addressierung dieser Frames

• Zugriffsverfahren • Wie teile ich mir ein gemeinsames Medium mit

anderen Kommunikationspartnern ?

LogicalLink

Control(LLC)

MediaAccess Control(MAC)

Aufgaben der Schicht 2


IEEE 802 Working GroupsIEEE

Standard Boards

IEEE 802LAN/MAN

Standard Committee

P802.3bm40 Gb/s and 100 Gb/s Operation over Fiber

Optic Cables

802.1Higher Layer

LAN Protocols Working Group

802.11Wireless LAN

Working Group

802.24Smart GridTechnical

Advisory Group

802.3Ethernet

Working Group

P802.3bq40GBASE-T

P802.3bkExtended Ethernet

Passive Optical Networks (EPON)

P802.3bj100 Gb/s

Backplane & Copper Cable


IEEE 802 Active Working Groups

• 802.1 Higher Layer LAN Protocols Working Group• 802.3 Ethernet Working Group• 802.11 Wireless LAN Working Group• 802.15 Wireless Personal Area Network Working Group• 802.16 Broadband Wireless Access Working Group

(WiMAX)• 802.18 Radio Regulatory Technical Advisory Group• 802.19 Wireless Coexistence Working Group• 802.21 Media Independent Handoff Working Group• 802.22 Wireless Regional Area Networks Working Group• 802.24 Smart Grid Technical Advisory Group


IEEE 802 Inactive and Disbanded Working Groups

• 802.2 Logical Link Control Working GroupInactive

• 802.4 Token Bus Working GroupDisbanded

• 802.5 Token Ring Working Group Inactive• 802.6 Metropolitan Area Network Working Group

Disbanded• 802.7 Broadband Technical Advisory Group

Disbanded• 802.8 Fiber Optic Technical Advisory Group

Disbanded• 802.9 Integrated Services LAN Working Group

Disbanded• 802.10 Security Working Group

Disbanded• 802.12 Demand Priority Working Group

Inactive• 802.14 Cable Modem Working Group

Disbanded• 802.17 Resilient Packet Ring Working Group

Inactive• 802.20 Mobile Broadband Wireless Access WG

Inactive• 802.23 Emergency Services Working Group

Disbanded



2.2 Ethernet Physical Layer (PHY)


Ethernet IEEE 802.3 Overview

FiberTwisted PairCoaxRarely used

10GBase-T

802.3an-2006

40/100Gbps

802.3ba-2010


Logical Link Control LLCMAC Control (optional)

Media Access Control MACPLS

AUI

PMA (MAU)MDI

Medium

Reconciliation Reconciliation Reconciliation

PCSPMAPMD

GMII

MDI

PLSAUIPMA

MII

MDI

PCSPMAPMD

MII

MDIMedium Medium Medium

Data Link Layer

PHY

1-10 Mbit/s

10 Mbit/s 100 Mbit/s

1000 Mbit/s

AUI...Attachment Unit Interface, PLS...Physical Line Signaling, MDI...Medium Dependent Interface,PCS...Physical Coding Sublayer, MII...Media Independent Interface, GMII...Gigabit Media Independent Interface, PMA...Physical Medium Attachment, MAU...Medium Attachment Unit, PMD...Physical Medium Dependent

Ethernet Technology Overview


PHY Sublayers

• Physical Line Signaling (PLS) serves as an abstraction layer between MAC and PHY and provides• Data encoding/decoding (Manchester)• Signalling of media states (busy, free, collision occurred etc.)• Attachment Unit Interface (AUI) to connect with PMA

• Several new coding techniques demand for a Media Independent Interface (MII) that serves as an interface between MAC and PHY• hides coding issues from the MAC layer • MII: often a mechanical connector for a wire; GMII is an interface

specification between MAC-chip and PHY-chip upon a circuit board

• one independent specification for all physical media• supports several data rates (10/100/1000 Mbits/s) • 4 bit (GMII: 8 bit) parallel transmission channels to the physical

layer• Today coding is done through a media-dependent Physical

Coding Sublayer (PCS) below the MII.


PHY Sublayers

• Physical Coding Sublayer (PCS) • encapsulates MAC-frame between special PCS delimiters• 4B/5B or 8B/10B encoding respectively• appends idle symbols

• Physical Medium Attachment (PMA)• interface between PCS and PMD• (de) serializes data for PMD (PCS)

• Physical Medium Dependent (PMD)• serial transmission of the codegroups• specification of the various connectors (MDI)


10Base5

• Introduced in 1980 as part of the original IEEE 802.3 standard.

• Transmits 10 Mbps over a single thick coaxial cable bus.• The primary benefit of 10Base5 was its length:

up to 500m without a repeater.• 10Base5 uses Manchester encoding.• The thick and sturdy cable was difficult to install and was

therefore called Thick Net or due to its color Yellow Cable.


10Base2 I

• Introduced in 1985.• Installation is easier then 10Base5 because of its lighter

size and greater flexibility. Therefore it was called Thin Net or Cheaper Net.

• 10Base2 also uses Manchester encoding.• Computers on the LAN are linked together by an

uninterrupted chain of coaxial cable lengths.• These lengths are attached by BNC connectors to a T-

shaped connector on the NIC.• Each 10Base2 segment may be up to 185 meters long and

may accommodate up to 30 stations.


10Base2 II

1. Termination of each end of the coax should be 50 Ohms.2. Minimum distance between taps is 0.5 meters.3. Each station must be connected within four centimeters of

the thin coaxial cable.4. Maximum segment length is 185 meters.5. Link segments between repeaters should have a total of

only two attachments, the repeaters themselves.


10Base-T I

• Introduced in 1990.• 10base-T uses cheap and easy to install Cat 3 Unshielded

Twisted Pair (UTP) copper cable rather than coaxial cable.• The UTP cable is plugged into a central connection device

that contains the shared bus => Hub.• Preferred Topologies: Star and Extended Star.• Originally 10Base-T was a half-duplex protocol, but full-

duplex features were added later.• 10Base-T also uses Manchester encoding.• Due to their high attenuation 10Base-T links can have

unrepeated lengths of up to 100 m.


10Base-T II

• UTP cable uses RJ-45 connectors with eight pins.• Cat 3 cable is adequate for use in 10Base-T networks,

althoughCat 5e or better is strongly recommended for any new cable installations.

• All four pairs of wires are used either with the straight-through T568-A or the cross-over T568-B cable pinout arrangement.Pin

2 1

43

6 5

7 8

Signal TD+ (Transmit Data) TD- (Transmit Data) RD+ (Receive Data) a (reserved for

POTS) b (reserved for POTS) RD- (Receive Data)

unused unused


Power-over-Ethernet (IEEE 802.3af PoE)

PSE Power Sourcing Equipment

PD Powered Device

1

2

3

6

1

2

3

6

Pair 3

Pair 2

PDPSE

350 mA13.0 W@PD

48 V DC

Alternative A

4

5

7

8

4

5

7

8

48 V DC

Pair 1

Pair 4

1

2

1

2Pair

3

3

6

3

6Pair 2

350 mA13.0 W@PD

Alternative B

PDPSE


Power-over-Ethernet (IEEE 802.3at PoE+)


PD Powered Device

1

2

3

6

1

2

3

6

Pair 3

Pair 2

PDPSE

600 mA25.5 W@PD

53 V DC

Alternative A

4

5

7

8

4

5

7

8

53 V DC

Pair 1

Pair 4

1

2

1

2Pair

3

3

6

3

6Pair 2

600 mA25.5 W@PD

Alternative B

PDPSE


Power-over-Ethernet (IEEE 802.3at PoE+)


PD Powered Device

1

2

3

6

1

2

3

6

Pair 3

Pair 2

PDPSE

600 mA25.5 W@PD

53 V DC

Alternative A

4

5

7

8

4

5

7

8

53 V DC

Pair 1

Pair 4

1

2

1

2Pair

3

3

6

3

6Pair 2

600 mA25.5 W@PD

Alternative B

PDPSE


Energy-Efficient-Ethernet (IEEE 802.3az EEE)

• In 2005 all Network Interface Controllers (NICs) in the USused 5.3 TWh (600 MW)

• EEE introduces a Low Power Idle (LPI) sleep signal

• Transmitter sends LPI in place of Idle for a period Ts to indicate that the link can go to sleep and then stops signaling.

• Periodically, the transmitter sends a refresh signal for a time Tw so that the link does not remain quiescent for too long.

• To resume the transmitter sends normal Idlesignals. After a time Tw the link is active.



2.3 EthernetFrame Synchronisation


0 1 0 1 1 1 0 0 1 0 1 1 0 0

10Mb/s-Ethernet: Manchester Code

"0" = fallende Flanke in Bitmitte (H->L)"1" = ansteigende Flanke in Bitmitte (L->H) T=Bitdauer


1 0 1 0 1 0 1 0 1 0 1 0 1 1

10Mb/s-Ethernet: Frame-Synchronisation

Bitmitte

Frame Start

• Präambel bestehend aus einer 1-0-1-0-… Sequenzermöglicht die Synchronisation auf die Bitmitte.

• Das erste Auftreten von 1-1 kündigt den Start der Nutzdaten an.


IEEE 802.3 Ethernet Frame Präambel

1010101010101010101010101010101010101010101010101010101010101011

7 BytesPräamb

el

1 Byte SFD

SFD Start-of-Frame Delimiter



2.4 Ethernet Media Access

Control (MAC)


Half-Duplex Transmission

• Historically Ethernet was a half-duplex technology. • Using half-duplex, a host could either transmit or receive at

one time, but not both.• Host checks the network to see whether data is being

transmitted before it transmits data.• If the network is already in use, the transmission is

delayed.• Only ONE host can transmit at a time.


Carrier Sense Multiple Access / Collision Detection

1. Listen to the medium

2. Sending if medium is free, else waiting for a random time and try again

3. The amplitude of the signal increases because a collision occurs.

4. The nodes stop transmitting for a random period of time, which is different for each device.


CSMA/CD Ablaufdiagramm


CSMA/CD Collision Handling

• Abortion of current transmission by all stations involved• Emission of a Jam-signal (32 bit)

• to make sure that every station can recognize the collision• collision is spread to a minimum length

• Generation of a random backoff timeout value• truncated binary exponential backoff algorithm (the more

often a collision occurs the larger is the range for the random number)

• After expiration of the timeout a retransmission is attempted

• Number of retransmission trials is limited to 16• after 16 collisions in a sequence a error is signaled to the

higher layer


Truncated Binary Exponential Backoff Algorithm

Runde 0: 0t pcollision = 1 (3 hosts)

pcollision = 1 (3 hosts)

pcollision = ¼ (2 hosts)

Runde 1:0 1

t

Runde 2: t0 1 2 3

slottim

e


Signalausbreitung auf Koaxialkabel

RaumT1 = Lmax/v

Lmax, v = 0.2 m/ns

T1

T1

Tmax = 2T1 = 2Lmax / v Lmax = vTmax / 2

Late Collision

Collision

Zeit

A B C


Collision Window und Kollisionsdomäne

• Worst-Case Betrachtung• Um eine Kollision zuverlässig detektieren zu können, muss die

minimale Dauer eines Ethernet Frames grösser als die doppelte einfache Signallaufzeit, d.h. dem Round Trip Delay (RTD) sein.

• Diese maximale Zeit Tmax nennt man Collision Window.• 10 Mbit/s und 100 Mbit/s Ethernet definieren eine minimale

Frame-Grösse von 512 Datenbits (64 Bytes). • Maximale Ausdehnung einer Kollisionsdomäne

• 10 BASE: Tmax = 512·100 ns = 51.2 μs Lmax 2000 m• 100 BASE: Tmax = 512·10 ns = 5.12 us Lmax 200 m• Werden diese Längen überschritten, können Late Collisions

auftreten.


Full-Duplex Transmission

• Allows transmission of a packet and the reception of a different packet at the same time.

• Host can transmit immediately without checking the network first.

• The connection is considered point-to-point and is collision free.

• Full-duplex Ethernet offers 100% of the bandwidth in both directions.

Requires a dedicated connection to a switched port.

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