2 Ethernet Grundlagen

Transkript

1 Computernetze 1 (CN1) 2 Ethernet Grundlagen Prof. Dr. Andreas Steffen Institute for Internet Technologies and Applications Steffen/Stettler, , 2-Ethernet.ppt 1

2 Lesestoff im Ethernet Buch Kapitel 2 Ethernet, Seiten Die Geschichte des Ethernet 2.2 Der Physical Layer Base Base BaseT BaseF 2.8 Das Manchester-Kodierungsverfahren 2.9 Media Access Control (MAC) Kapitel 6 Ethernet Internals, Seiten Power over Ethernet Selbststudium Erarbeiten Sie als Vorbereitung für die Übung 2 selbstständig das Thema Media Access Control mit Hilfe von Kapitels 2.9 des Ethernet Buchs und des Kapitels 2.4 dieses Foliensatzes. Steffen/Stettler, , 2-Ethernet.ppt 2

3 Wie es begann 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 stellte er sein Protokoll unter dem Namen Ethernet an einer Konferenz vor gründete er die Firma 3Com. * Carrier Sense Multiple Access with Collision Detection Steffen/Stettler, , 2-Ethernet.ppt 3

4 Computernetze 1 (CN1) 2.1 Ethernet Standards Steffen/Stettler, , 2-Ethernet.ppt 4

5 Ethernet und das OSI Modell Ethernet Steffen/Stettler, , 2-Ethernet.ppt 5

6 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) Steffen/Stettler, , 2-Ethernet.ppt 6

7 Aufgaben der Schicht 2 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? Steffen/Stettler, , 2-Ethernet.ppt 7

8 IEEE 802 Working Groups IEEE Standard Boards IEEE 802 LAN/MAN Standard Committee Higher Layer LAN Protocols Working Group Ethernet Working Group Wireless LAN Working Group Smart Grid Technical Advisory Group P802.3bj P802.3bk P802.3bm P802.3bq 100 Gb/s Backplane & Copper Cable Extended Ethernet Passive Optical Networks (EPON) 40 Gb/s and 100 Gb/s Operation over Fiber Optic Cables 40GBASE-T Steffen/Stettler, , 2-Ethernet.ppt 8

9 IEEE 802 Active Working Groups Higher Layer LAN Protocols Working Group Ethernet Working Group Wireless LAN Working Group Wireless Personal Area Network Working Group Broadband Wireless Access Working Group (WiMAX) Radio Regulatory Technical Advisory Group Wireless Coexistence Working Group Media Independent Handoff Working Group Wireless Regional Area Networks Working Group Smart Grid Technical Advisory Group Steffen/Stettler, , 2-Ethernet.ppt 9

10 IEEE 802 Inactive and Disbanded Working Groups Logical Link Control Working Group Inactive Token Bus Working Group Disbanded Token Ring Working Group Inactive Metropolitan Area Network Working Group Disbanded Broadband Technical Advisory Group Disbanded Fiber Optic Technical Advisory Group Disbanded Integrated Services LAN Working Group Disbanded Security Working Group Disbanded Demand Priority Working Group Inactive Cable Modem Working Group Disbanded Resilient Packet Ring Working Group Inactive Mobile Broadband Wireless Access WG Inactive Emergency Services Working Group Disbanded Steffen/Stettler, , 2-Ethernet.ppt 10

11 Computernetze 1 (CN1) 2.2 Ethernet Physical Layer (PHY) Steffen/Stettler, , 2-Ethernet.ppt 11

12 Ethernet IEEE Overview Fiber Twisted Pair Coax Rarely used 10GBase-T 802.3an /100Gbps 802.3ba-2010 Steffen/Stettler, , 2-Ethernet.ppt 12

13 Ethernet Technology Overview Logical Link Control LLC MAC Control (optional) Media Access Control MAC PLS Reconciliation Reconciliation Reconciliation AUI PMA (MAU) MDI MII PLS AUI PMA MDI MII PCS PMA PMD MDI GMII PCS PMA PMD MDI Medium Medium Medium Medium 1-10 Mbit/s 10 Mbit/s 100 Mbit/s 1000 Mbit/s Data Link Layer PHY 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 Steffen/Stettler, , 2-Ethernet.ppt 13

14 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. Steffen/Stettler, , 2-Ethernet.ppt 14

15 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) Steffen/Stettler, , 2-Ethernet.ppt 15

16 10Base5 Introduced in 1980 as part of the original IEEE 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. Steffen/Stettler, , 2-Ethernet.ppt 16

17 10Base2 I Introduced in 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. Steffen/Stettler, , 2-Ethernet.ppt 17

18 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. Steffen/Stettler, , 2-Ethernet.ppt 18

19 10Base-T I Introduced in base-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. Steffen/Stettler, , 2-Ethernet.ppt 19

20 10Base-T II UTP cable uses RJ-45 connectors with eight pins. Cat 3 cable is adequate for use in 10Base-T networks, although Cat 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 Signal TD+ (Transmit Data) TD- (Transmit Data) RD+ (Receive Data) a (reserved for POTS) b (reserved for POTS) RD- (Receive Data) unused unused Steffen/Stettler, , 2-Ethernet.ppt 20

21 Power-over-Ethernet (IEEE 802.3af PoE) Alternative A Alternative B 1 Pair Pair Pair V DC ma W@PD 48 V DC ma 13.0 W@PD 3 Pair Pair 4 7 PSE 6 PSE Power Sourcing Equipment PD Powered Device 6 PD PSE Pair PD Steffen/Stettler, , 2-Ethernet.ppt 21

22 Power-over-Ethernet (IEEE 802.3at PoE+) Alternative A Alternative B 1 Pair Pair Pair V DC ma W@PD 53 V DC ma 25.5 W@PD 3 Pair Pair 4 7 PSE 6 PSE Power Sourcing Equipment PD Powered Device 6 PD PSE Pair PD Steffen/Stettler, , 2-Ethernet.ppt 22

23 Power-over-Ethernet (IEEE 802.3at PoE+) Alternative A Alternative B 1 Pair Pair Pair V DC ma W@PD 53 V DC ma 25.5 W@PD 3 Pair Pair 4 7 PSE 6 PSE Power Sourcing Equipment PD Powered Device 6 PD PSE Pair PD Steffen/Stettler, , 2-Ethernet.ppt 23

24 Energy-Efficient-Ethernet (IEEE 802.3az EEE) In 2005 all Network Interface Controllers (NICs) in the US used 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 Idle signals. After a time Tw the link is active. Steffen/Stettler, , 2-Ethernet.ppt 24

25 Computernetze 1 (CN1) 2.3 Ethernet Frame Synchronisation Steffen/Stettler, , 2-Ethernet.ppt 25

26 10Mb/s-Ethernet: Manchester Code "0" = fallende Flanke in Bitmitte (H->L) "1" = ansteigende Flanke in Bitmitte (L->H) T=Bitdauer Steffen/Stettler, , 2-Ethernet.ppt 26

27 10Mb/s-Ethernet: Frame-Synchronisation Bitmitte Frame Start Präambel bestehend aus einer Sequenz ermöglicht die Synchronisation auf die Bitmitte. Das erste Auftreten von 1-1 kündigt den Start der Nutzdaten an. Steffen/Stettler, , 2-Ethernet.ppt 27

28 IEEE Ethernet Frame Präambel Bytes Präambel 1 Byte SFD SFD Start-of-Frame Delimiter Steffen/Stettler, , 2-Ethernet.ppt 28

29 Computernetze 1 (CN1) 2.4 Ethernet Media Access Control (MAC) Steffen/Stettler, , 2-Ethernet.ppt 29

30 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. Steffen/Stettler, , 2-Ethernet.ppt 30

31 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. Steffen/Stettler, , 2-Ethernet.ppt 31

32 CSMA/CD Ablaufdiagramm Steffen/Stettler, , 2-Ethernet.ppt 32

33 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 Steffen/Stettler, , 2-Ethernet.ppt 33

34 Truncated Binary Exponential Backoff Algorithm Runde 0: 0 t p collision = 1 (3 hosts) Runde 1: 0 1 t p collision = 1 (3 hosts) Runde 2: t p collision = ¼ (2 hosts) slot time Steffen/Stettler, , 2-Ethernet.ppt 34

35 Signalausbreitung auf Koaxialkabel A B C L max, v = 0.2 m/ns T 1 Collision T 1 = L max /v Raum T 1 Late Collision Zeit T max = 2T 1 = 2L max / v L max = vt max / 2 Steffen/Stettler, , 2-Ethernet.ppt 35

36 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 T max 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: T max = ns = 51.2 μs L max 2000 m 100 BASE: T max = ns = 5.12 us L max 200 m Werden diese Längen überschritten, können Late Collisions auftreten. Steffen/Stettler, , 2-Ethernet.ppt 36

37 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. X X Steffen/Stettler, , 2-Ethernet.ppt 37