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  Typically optical fiber  Can span buildings  Bridge has local traffic be local Fiber Optical Extensions

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 Original Ethernet Wiring Scheme  Designed for office  Only two repeaters between any pair of stations R1R1 Segments on floor 3 R2R2 Segments on floor 2 R3R3 Segments on floor 1 Vertical Segment

9 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) Hub

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

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 Bridge Segment BC domain collision 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  Consider a broadcast frame issued by station on segment a Computer on all segments receive a infinite number of copies Segment a Segment c Segment b Segment d B2 B3 B1 B4

18 Spanning Tree Algorithm  Used by all bridges to Discover one another Break cycle(s) Known as Distributed Spanning Tree (DST) Segment a Segment c Segment b Segment d B2 B3 B1 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 Hub Category 3/5 UTP Switch Share 10 Mbps

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

21 Exercise  11.6, 11.7