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I ntroduction of Ethernet

I ntroduction of Ethernet. Metcalfe’s Ethernet sketch. Also refer to http://www.cs.nthu.edu.tw/~nfhuang/chap04.htm. Questions (1/2). OSI 7-layer? What is LAN? 如何連結電腦形成一個 LAN? 是什麼網路卡 ? 我們常用到那些網路卡 ? 網路卡的工作是什麼 ? 請想像 Ethernet MAC layer frame 應該要有那些欄位，才能讓對方得到足夠的資訊 。. Questions (2/2).

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I ntroduction of Ethernet

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  1. Introduction of Ethernet Metcalfe’s Ethernet sketch Also refer to http://www.cs.nthu.edu.tw/~nfhuang/chap04.htm

  2. Questions (1/2) • OSI 7-layer? • What is LAN? • 如何連結電腦形成一個LAN? • 是什麼網路卡?我們常用到那些網路卡? • 網路卡的工作是什麼? • 請想像Ethernet MAC layer frame應該要有那些欄位，才能讓對方得到足夠的資訊。

  3. Questions (2/2) • 同一LAN中的許多電腦同時透過Ethernet送出frames，會不會有問題？ • What is Multiple Access？ • 如何用 CSMA/CD 的方式讓各個電腦可以在 bus 上達到相互通訊的目的？ • CSMA/CD是好的方法嗎？有沒有其他方法？ • 如何讓Ethernet傳輸速度從最早10Mbps不斷增加再增加？ • Ethernet成功的原因？

  4. Outlines • Ethernet History • Ethernet Frame • Multiple Access in Ethernet • IEEE 802.3

  5. switch Part 1Ethernet History star bus: coaxial cable

  6. Ethernet History • “Dominant” wired LAN technology • Ethernet's history • Ethernet's success • First widely used LAN technology • Simpler, cheaper than token LANs and ATM • Cheap $20 for NIC • Kept up with speed race: 10 Mbps – 10 Gbps

  7. application transport network link physical MAC protocol and frame format 100BASE-T2 100BASE-FX 100BASE-TX 100BASE-BX 100BASE-SX 100BASE-T4 fiber physical layer copper (twister pair) physical layer What is Ethernet? • Link & Physical Layers • Different physical layer media: fiber, cable • Different speeds: 2 Mbps, 10 Mbps, 100 Mbps, 1Gbps, 10G bps • Common MAC protocol and frame format

  8. IEEE 802.3 Standards • Many different Ethernet specifications • Ethernet has no IEEE standard. • Ethernet is similar to IEEE 802.3 but not the same.

  9. Ethernet's Name • According to PHY techniques, Ethernethas several specifications and named them as n-signal-PHY • n：data rate • signal：Baseis baseband; broadis broadband • PHY：distance, media or coding methods • Examples: • 10Base5：10Mb/s、baseband、500m • 10Base-T：10Mb/s、baseband、Twisted pair

  10. Comparisons of Different Spec.

  11. 10Base2 Bus Topology • 一條纜線的範圍稱為「區段」（segment）。 • 利用MAC位址判斷是誰的資料。 • 若一個T型節點損毀，則整個網路無法運作。 • 使用終端子 （Terminator） 收吸收訊息的回音。 • A bit transmission time is 1/10M= 0.1msec.

  12. Thin Ethernet with Repeater • 3 segments connected by repeaters 距離是0.5公尺的倍數。 兩終端電阻間最長不可超185公尺。 任兩台電腦間電纜總長度不可超過925公尺。 任兩台電腦間不能超過4個 repeater。

  13. 10Base5 Bus Topology 兩終端電阻間最長不可超過500公尺。 任兩台電腦間電纜總長度不可超過1500公尺，因此這 5 段同軸電纜不可以連接成一直線狀。 最多5個segment。任何二個工作站之間的路徑上最多只能有 2 個訊號增益器。

  14. 1 5 data 2 Hub 3 4 10Base-T with Hub • 10Base-T • Centralized control by Hub • Still useBus Topology －Broadcast • Half duplex • One hub node crashes →Do not affect other nodes • Bus topology popular through mid 1990s • All nodes in same collision domain (can collide with each other) 不可超過250公尺。 不可超過100公尺。

  15. switch 10Base-T with Switch • Centralized control by Switch • Check the destination MAC address of Frame , forward this frame to a correct port • No forward frames to other ports • Full duplex • Parallel sending many signals • Today: star topology prevails • Active switch in center • Each “spoke” runs a (separate) Ethernet protocol (nodes do not collide with each other) star

  16. Fast Ethernet • 100BaseT Ethernet • 100Mbps: reduce the transmission delay of each bit • 100BaseX uses 2 UTP pairs • 100Base4T uses 4 UTP pairs • Compatible to 10BaseT wiring system, frameformat, max packet size • A bit transmission time is 1/100M=0.01msec.

  17. Gigabit Ethernet • IEEE 802.3ab 1000Base-T, IEEE 802.3z 1000Base-SX/LX • Compatible to Ethernet、Fast Ethernet • The same as old wiring system but speed is up to 1000Mbps • Use fiber of Category 5 UTP • Used as a backboneor the interconnection among servers

  18. 10 Gigabit Ethernet • IEEE 802.3ae • There are many differences between 802.3ae and old Ethernet specifications • Use Optical Fiber, Full-duplex • Have the same frameformat、max packet size • Applications • Server interconnect for clusters of servers • Links between switches and servers • Backbone • Very high-speed connections between buildings

  19. Part 2Ethernet Frame

  20. Ethernet: Unreliable, Connectionless • Connectionless: No handshaking between sending and receiving NICs • NIC: Network Interface Card • Unreliable: receiving NIC doesn’t send acks or nacks to sending NIC • Stream of datagrams passed to network layer can have gaps (missing datagrams) • Gaps will be filled if application is using TCP • Otherwise, application will see gaps • Ethernet’s MAC protocol: unslotted CSMA/CD

  21. Ethernet frame(1/3) • Define the frame in MAC layer • Preamble • 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 • Used to synchronize receiver, sender clock rates

  22. Ethernet frame (2/3) • Source MAC address • Source accepts the datafrom upper layer, encapsulates as a frame, then sends it to destination. • DestinationMAC address • If adapter receives frame with matching destination address, or with broadcast address (eg., ARP packet), it passes data in frame to network layer protocol. Otherwise, adapter discards frame. • LSB=0：physical MAC address • LSB=1：multicastaddress • All 1s in 6 bytes：broadcast address

  23. Ethernet frame (3/3) • Type • Indicates higher layer protocol • 0800H：IP, 0600H：XNS (Xerox protocol suit) • Mostly IP but others possible, e.g., Novell IPX, AppleTalk • Data • 46-1500 bytes • Frame Check Sequence(FCS) • Checked at receiver. If error is detected, the frame is dropped. • Use 32-bits CRC to check address, type and data.

  24. Manchester Encoding • Each bit has a transition • Allows clocks in sending and receiving nodes to synchronize to each other • No need for a centralized, global clock among nodes! for UTP -0.225V 0.7V -1.825V -0.7V for coaxial cable

  25. Round-trip Propagation Delay • The time delay to send a signal on one round-trip • In specification, the maximum distance between 2 hosts is 2800m. • Use 10Mbps Ethernet as an example • Round-trip propagation delay is about 46.38 ms. • Finally, round-trip propagation delay is set to be 51.2 ms.

  26. Thick Ethernet Topology • Max distance =500m3+1000m2+300m=2800m • 訊號傳遞速度為光速的0.65倍。 • 2800m2÷(31080.65)=28.72ms • Round trip delay還包括電纜線傳遞延遲及元件延遲等。 source: http://www.cs.nthu.edu.tw/~nfhuang/chap04.htm

  27. Part 3Multiple Access in Ethernet

  28. Sending an Ethernet Frame • Broadcast • If there are two computers send data at the same time, two frames will collide. • A collision occurs. • Receivers discards these frames. • CSMA/CD（Carrier Sense Multiple Access / Collision Detection）is used. • Best Effect

  29. CSMA/CD • Carrier Sense • 表示網路卡要對 carrier 進行監測，判斷網路線上是否有資料的傳送。 • Multiple Access • 指出這種機制的目的是要讓多個網路卡可以在同一個 carrier 上存取資料。 • Collision Detection • 表示 collision 是會發生的，必須靠 detection的機制來判斷是否有 collision。

  30. 1. NIC receives datagram from network layer and creates frame! 2. If NIC senseschannel idle, starts frame transmission. If NIC senseschannel busy, waits until channel idle and then transmits. 3. If NIC transmits entire frame without detecting another transmission, NIC is done with frame ! Ethernet CSMA/CD algorithm (1/2)

  31. Ethernet CSMA/CD algorithm (2/2) 4. If NIC detects another transmission while transmitting, aborts and sends jam signal. • Jam Signal: make sure all other transmitters are aware of collision; 48 bits 5. After aborting, NIC enters exponential backoff. • NIC chooses K at random from {0,1,2,…,2m-1} after the m th collision. NIC waits K·512 bit times, returns to Step 2.

  32. No Collision Occurs why? Ex: 10BaseT

  33. Collision Occurs • If host Bsends it frame before getting A’s frame, a collision occurs. • Host Afinds noise, treat it as a collision. • The worst case • A finds noise after a round-trip propagation delay time.

  34. How Long to Find Collision? • Round-trip propagation delay is 51.2 ms for 10Mbps Ethernet. • Host Amust listen at least 51.2 ms and know whether a collision occurs or not.

  35. CSMA/CD Random Time Delay (1/2) • Let two collided hosts retransmit frames at a different time. • Host generates a random time delay (r), r  { 0、1、... 、2k-1} • n：the number of collisions（n16） • k = min (n，10) • Retransmit the frame after r • Exponential backoff algorithm • The random time delay is round-trip propagation delayr • For 10BaseT, 51.2 msr

  36. CSMA/CD Random Time Delay (2/2) • Goal: adapt retransmission attempts to estimated current load • Heavy load: random wait will be longer

  37. Examples of Binary Exponential Backoff Algorithm • First collision • n=1, k=min(n,10)=1, r  { 0、1} • Probability=1/2 to send frame immediately • Probability=1/2 to send frame after 51.2 ms • Second collision • n=2, k=min(n,10)=2, r  { 0、1、2、3} • Four cases: 0ms、51.2 ms、51.2×2ms、51.2×3ms • After ten collisions, choose r from {0,1,2,3,4,…,1023}

  38. Data Size Limitation • To avoid that any host occupies Ethernet too long, the data size is smaller than 1500 bytes. • If the size of a frame is too small, adapters can not detect collision in time. • 10MHz  0.1ms/bit • Within 51.2ms, 51.2ms÷0.1ms/bit＝512bits =64 bytes can be sent. • Every frame must be larger than 64 bytes. • Data size must be larger than 46 bytes.

  39. Effect of CSMA/CD • Unpredicable when the frame can be received • Not fair • Bandwidth utilization degrades if heavy loading • Not suitable for real-time applications

  40. Properties of An Ethernet • Shared bus technology（broadcast） • Lowest level hardware does not support addressing. • Best-effort delivery mechanism • No central authority to grant access • Each transmission is limited in duration.

  41. Part 4IEEE 802.3

  42. IEEE 802 Series SMTP、FTP、HTTP SNMP、DNS UDP • 802.3 CDMA/CD • 802.4 Token-bus • 802.5 Token-ring • 802.11 Wireless LAN • 802.12 100VG-AnyLAN TCP IP 802.2 LLC (Logic Link Control) Ethernet 802.11 802.3 802.4 802.5 802.12 Fiber, UTP, wireless, cable

  43. IEEE 802.3 Protocol • Ethernet is compatible to 802.3 • Length: # of bytes in LLC

  44. Flow Chart of IEEE 802.3

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