Chapters 2 and 3 Extending Ethernet LANs: Repeaters, Hubs, Bridges and Switches

1. Chapters 2 and 3Extending Ethernet LANs: Repeaters, Hubs, Bridges and Switches Professor Rick Han University of Colorado at Boulder rhan@cs.colorado.edu

2. Announcements • Previous lecture will be online by Friday • Homework #1 is on the Web site, due Feb. 5 • Programming assignment #1 is now available on Web site, due Feb. 19 (3 weeks) • Next, Chapter 3, Ethernet, repeaters, switched Ethernet, Ethernet bridges and hubs Prof. Rick Han, University of Colorado at Boulder

3. Recap of Previous Lecture • Wireless Ethernet – 802.11 • MAC layer • DCF = CSMA/CA • PCF mode polls for delay-sensitive traffic • Physical layer • 802.11b: Direct Sequence Spread Spectrum multiplies data d(t) by faster chipping sequence c(t) to spread spectrum • 802.11a: OFDM • Token-based MAC protocols • Token Ring • FDDI Prof. Rick Han, University of Colorado at Boulder

4. Ethernet / 802.3 • CSMA/CD • Technologies: • 10Base2 = 10 Mbps using Baseband signalling over coaxial cable, longest segment <= 200 m • 10Base 5 = 10 Mbps Baseband over coax, max seg <=500 m • 10BaseT = 10 Mbps Baseband over Twisted pair of copper wires, <= 100 m • 100BaseT = “Fast” Ethernet, 100 Mbps Baseband over twisted pair, <= 100 m • Gigabit Ethernet over fiber and copper wire • In future, 10G Ethernet Prof. Rick Han, University of Colorado at Boulder

5. Preamble An Ethernet Frame • All of the Ethernet technologies use the same Ethernet frame: Dest Addr Src Addr Type Data CRC 8 bytes 6 6 2 4 46-1500 • Addressing: • Every Ethernet host has a unique address, burned into ROM by manufacturer • Each manufacturer is given a unique prefix • Receiver accepts frames: • When dest. address = its own address • When dest. address = all 1’s (broadcast address) • When it’s in promiscuous listening mode Prof. Rick Han, University of Colorado at Boulder

6. Preamble Preamble An Ethernet Frame (2) Dest Addr Src Addr Type Data CRC 8 bytes 6 6 2 4 46-1500 • Data size: • at least 46 bytes to allow collisions to be detected • maximum transmission unit (MTU) is 1500 bytes • Type field: • identifies the type of network layer protocol encapsulated within, e.g. IP, ARP, RARP, … Data Dest Addr Src Addr Type IP Packet CRC Prof. Rick Han, University of Colorado at Boulder

7. Preamble An Ethernet Frame (3) • How does Ethernet determine End-of-Frame? Dest Addr Src Addr Type Data CRC 8 bytes 6 6 2 4 46-1500 • Answer: • 10 Mbps Ethernet uses Manchester encoding, 100 Mbps Fast Ethernet uses 4B5B encoding. • Both encodings force transitions in bits/groups of bits (for clock recovery) • When bit transitions start, receiver detects preamble to signal start-of-frame • Receiver listens until there are no more bit transitions – this is an implicit end-of-frame Prof. Rick Han, University of Colorado at Boulder

8. Connecting Ethernet LANs • Companies/universities consist of many departments, each with own Ethernet LAN • Interconnect Ethernet LAN’s so computers across departments can communicate Ethernet Ethernet Prof. Rick Han, University of Colorado at Boulder

9. Interconnection Topologies Cascade or Daisy Chain Star Topology Prof. Rick Han, University of Colorado at Boulder

10. Attenuated Data Re-amplified data Ethernet Repeaters • Repeaters interconnect Ethernet LANs at the physical layer • Repeat an incoming waveform on all outgoing interfaces using analog amplifier • No memory in repeater • Plug-and-play, cheap, extends range of LAN Data Prof. Rick Han, University of Colorado at Boulder

11. Ethernet Repeaters (2) • Preserves the property of a broadcast medium that any transmission by any node is heard by other nodes • Collisions will be heard by all nodes Colliding signal Data Data Colliding signal Prof. Rick Han, University of Colorado at Boulder

12. Ethernet Repeaters (3) • Problem: Can’t extend repeaters indefinitely • CSMA/CD requires low prop. delay to work efficiently • If nodes too far apart, then prop. delay too long • Ethernet limits # of repeaters to <=4 between any two Ethernet nodes • 10Base5 has max segment size of 500 m, so 5 10Base5 LANs daisy-chained with 4 repeaters = 2.5 km length • Star topology enables interLAN communication yet largely avoids this problem • Problem: Noise in analog waveform is amplified each time repeater is traversed Prof. Rick Han, University of Colorado at Boulder

13. Ethernet Hubs • Essentially the same as physical layer repeaters, but with additional features • Monitoring • Fault isolation – has intelligence to detect and isolate a faulty hub or faulty Ethernet node (flooding the local Ethernet) • Multi-port • 5 port and 10 port 10BaseT Ethernet Hubs for $40 • 5 port Fast Ethernet Hub for $60 Prof. Rick Han, University of Colorado at Boulder

14. Multi-Tier Ethernet LANs • Connect the hubs together into tiers • Hub-hub backbone connections don’t require hosts • Hybrid of star and daisy-chain topologies Backbone Hub Department Hub Department Hub Hub Prof. Rick Han, University of Colorado at Boulder

15. An Actual Ethernet Hub Normal 8 ports: connect hosts here Uplink: connect to another hub Prof. Rick Han, University of Colorado at Boulder

16. Problems with Ethernet Hubs • All nodes are in the same CSMA/CD collision domain • => more collisions, exponential backoff, and reduced throughput Backbone Hub Hub Hub Hub Prof. Rick Han, University of Colorado at Boulder

17. Problems with Ethernet Hubs (2) • Heterogeneous LANs: what happens when one LAN is 10BaseT and another is 100BaseT? • Need digital buffering in hub • But hubs are just analog repeaters • Strictly speaking, can’t use hubs to connect LANs w/ diff. speeds, though see ads for 10/100 “Hubs” ? 100BaseT 10BaseT • Same problems as repeaters: limited distance and noise Prof. Rick Han, University of Colorado at Boulder

18. Frame to Z Frame to Z Ethernet Bridges • Interconnects 2 or more Ethernet LANs at Layer 2 • Forwards and filters complete digital frames, unlike hubs that repeat analog waveforms • When a frame arrives, bridge looks at destination MAC address to see on which outgoing interface to forward the frame Node Z Layer 2 Bridge Prof. Rick Han, University of Colorado at Boulder

19. Ethernet Bridges (2) • Bridge is transparent to Ethernet nodes. • Nodes aren’t aware that bridge is being used to connect one LAN to another LAN • Bridge itself has no address. • Conceptually, interconnection topologies are similar to hubs: cascade, star, multi-tier, … LAN 1 LAN 2 Layer 2 Bridge Prof. Rick Han, University of Colorado at Boulder

20. Frame to Z Bridges Maintain a Table for Forwarding and Filtering • Label the interfaces into/out of the bridge • When a frame arrives, store the source address and the originating interface from which the frame came (and current time) Z U Layer 2 Bridge Interface 1 Interface 2 Prof. Rick Han, University of Colorado at Boulder

21. Self-Learning Bridges Build A Table U V Y Z Layer 2 Bridge Simplified Table: Interface 1 Interface 2 Until Y sends, don’t know its location Prof. Rick Han, University of Colorado at Boulder

22. Frame Forwarding Rules U V Y Z Layer 2 Bridge Interface 1 Interface 2 • If dest. node is on same LAN interface as src. node, then don’t forward frame • If dest. node is on diff. LAN interface than src. node, then route frame to dest. LAN • If dest. node is not in table, then forward to all outgoing interfaces • Don’t know yet where dest. node is located, so forward the frame to all outgoing LAN’s Prof. Rick Han, University of Colorado at Boulder

23. Frame Forwarding Rules (2) U V Y Z Layer 2 Bridge Interface 1 Interface 2 Prof. Rick Han, University of Colorado at Boulder

24. Advantage of Bridges vs. Hubs • Can interconnect heterogeneous LANs • Installed infrastructure of 10BaseT can interconnect with new 100BaseT Fast Ethernet • Buffering of digital frames in bridges enables this • No theoretical limit to extending the geographical reach of a LAN • After determining outgoing LAN interface where frame is to be sent, transmit via CSMA/CD on that LAN • Collision domains are isolated, so don’t have to deal with propagation • Noise doesn’t accumulate as with analog amplifiers • Plug-and-play (as with hubs) Prof. Rick Han, University of Colorado at Boulder

25. Problems With Bridges • Bridges can interconnect LANs and have multiple paths between every node • Inadvertent Layer 2 Bridge Bridge • Purposely for robustness, in case highest tier fails Bridge • Problem: Frames can cycle forever in a loop and multiply to crash LAN! Bridge Prof. Rick Han, University of Colorado at Boulder

26. Problems With Bridges: Packet Multiplication Effect • Suppose all bridges have just booted • Suppose A wants to send to Z Bridge 4 • Bridge 1 sends A’s frame to LAN 5 & 4 • These two frames propagate to Bridge 3, where they multiply into 4 copies Bridge 1 LAN4 Z LAN1 A LAN3 LAN5 Bridge 2 LAN2 Bridge 3 • Exponentially multiplying copies! Prof. Rick Han, University of Colorado at Boulder

27. Problems With Bridges:Endless Looping • Suppose all bridges have just booted • Suppose A wants to send to Z Bridge 4 • Bridge 2 sends frame to LAN 2 • Bridge 3 sends frame to LAN 3 • Bridge 4 -> LAN 4 • Back to LAN 1 Bridge 1 LAN4 A Z LAN1 LAN3 Bridge 2 LAN2 • Frames can cycle forever! Bridge 3 Prof. Rick Han, University of Colorado at Boulder

28. Spanning Tree Solution To Looping and Packet Multiplication • Bridges communicate with each other to set up a spanning tree that has no loops Bridge 4 • Disconnect some interfaces, though physical link exists • Some frames may take long route though shorter direct physical route exists Bridge 1 LAN4 A Z LAN1 LAN3 Bridge 2 LAN2 Bridge 3 Prof. Rick Han, University of Colorado at Boulder

29. Spanning Tree Solution (2) • Some bridges may be eliminated completely! Original Spanning Tree LAN4 LAN4 LAN1 LAN3 LAN1 LAN3 LAN2 LAN2 Original Spanning Tree root LAN4 LAN4 LAN1 LAN3 LAN1 LAN3 LAN2 LAN2 Prof. Rick Han, University of Colorado at Boulder