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Ch 11 Extending LANs

Ch 11 Extending LANs. Fiber modems, Repeaters, Bridges, and Switches. Distance Limitation. Each LAN technology has a distance limitation LAN hardware is engineered to emit a fixed amount of power LAN use shared comm. medium for saving cost

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Ch 11 Extending LANs

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  1. Ch 11 Extending LANs Fiber modems, Repeaters, Bridges, and Switches

  2. Distance Limitation • Each LAN technology has a distance limitation • LAN hardware is engineered to emit a fixed amount of power • LAN use shared comm. medium for saving cost • Fair access mechanism, such as CSMA/CD, takes time proportional to the size of network • Frequent collision caused by mass population

  3. Extension Techniques • Use connection with lower delay than copper (fiber) • Repeaters or Hubs • Bridges or switches

  4. Fiber Optical Extensions • Optical fiber • Has lower delay, higher bandwidth • Without changing the original LAN • To connect computers with a remote LAN (several Km)

  5. Fiber Optical Extensions • Typically optical fiber • Can span buildings • Bridge has local traffic be local

  6. Repeaters • Connects two LAN segments • Copies signal from one segment and amplifies it to the other • Do not understand the frame format (work at Layer 1) • Propagates noise and collisions

  7. Repeaters • Operates in two directions simultaneously • Network will not operate correctly if more than 4 repeaters separate any pair of stations

  8. Repeaters and the OriginalEthernet Wiring Scheme Segments on floor 3 • Designed for office • Only two repeaters between any pair of stations R1 Vertical Segment Segments on floor 2 R2 Segments on floor 1 R3

  9. Hub Hubs • Physically • Multi-port repeater • Has connections from several computers • Logically • Operates on signals • Propagates each incoming signal to all connections • Does not understand packets (work at Layer 1)

  10. Hub Hub Hub Hubs • Multiple hubs • Can be interconnected in a daisy chain • Operate as one giant hub, called stacking • Forms 1-BC domain and 1-collision domains

  11. Bridges • Hardware device that connects two LAN segments • Forwards frames if necessary (work at Layer 2) • Does not forward noise or collisions • Isolate problems • Forms 1-BC domain and 2-collision domains collision domain Segment Bridge BC domain

  12. Frame Filtering

  13. Frame Filtering • Listen in promiscuous mode • Uses source MAC address to learn location of computers • Make list of computers on each segment • Learning is completely automated • Watch source address in incoming frames • Only forward if necessary (in the steady state) • Always forward broadcast / multicast

  14. Planning a Bridged Network • Two segments can be used simultaneous • Allows independent transmissions • Improve the performance of an existing LAN by dividing the LAN into 2 segments

  15. Bridging Across Long Distance • Satellite connection can span arbitrary distance • Each site has bridge HW to filter local traffic (low BW) • Supports buffering and flow control

  16. A Cycle of Bridges • Complex bridge connections may not be apparent • Adding one more bridge inadvertently introduces a cycle

  17. A Cycle of Bridges Segment a Segment b B1 B2 B3 Segment c Segment d B4 • Consider a broadcast frame issued by station on segment a • Computer on all segments receive a infinite number of copies

  18. Spanning Tree Algorithm • Used by all bridges to • Discover one another • Break cycle(s) • Known as Distributed Spanning Tree (DST) Segment a Segment b B1 B2 B3 Segment c Segment d B4

  19. Switching • Electronic device with multiple ports • Only forwards packets when necessary (work at Layer 2) • Permits separate pairs of computers to communicate at the same time • Higher cost than hub Switch Hub Category 3/5 UTP Share 10 Mbps 10 Mbps

  20. Switching • Maximal throughput = RN/2 • R: data rate • N: total number of port Collision BC domain Switch Hub Hub

  21. Exercise • 11.6, 11.7

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