LAN extensions

• ethernet limitations
• extending Ethernet, repeaters
• bridges
• switches

Ethernet Limitations

• collision detection: minimum packet size 64 bytes says network diameter can be at most 512/2=256 bit periods
• for 10Mb/s, this is 25.6 us -- at 200,000 Km/s, this is 5,120 m.
• for 100Mb/s, this is 2.56 us -- at 200,000 Km/s, this is 512 m.
• another limitation is from the power provided by the transceivers
• thickwire ethernet is designed to reach up to 500 m
• 10BaseT ethernet is limited to 100 m between stations

Extending LANs

• one way to overcome ethernet distance limitations is to:
  • attach a fast modem to the ethernet
  • design it to send everything on one side to the other
• fiber modems could run faster than the ethernet
• might reach several kilometers
• not more, since the computer is still doing the collision detection and retransmission

Repeaters

• analog electronic amplifier
• any signal on one port is sent to the other port
• a repeater connects two Ethernet segments
• repeaters are not visible to the computers on the network
• repeaters extend the maximum distance
• a repeater will repeat noise and collisions as well as signals
• at most 4 repeaters are allowed between any pair of stations

Hubs

• a repeater with more than 2 ports is a hub
• CAT-5 wiring (for 10Base-T Ethernet) has the computer send on one pair of wires, and the hub on a different pair
• this means a regular CAT-5 wire cannot connect a hub to another hub
• instead, use a crossover cable, or the uplink port on the hub

Bridges and Switches

• repeaters have no knowledge of Ethernet frames
• a computer with two Ethernet interfaces can be programmed to transfer all the (correctly received) frames from one interface to the other
• this is a bridge
• this simple bridge breaks the collision domain and restores signals, but does not decrease the traffic on each segment
• a bridge that has hardware to automatically transfer the frames from one port to another is a switch

Learning Switches/Bridges

• a bridge or switch could keep track of the port P on which it receives frames from address A
• then, any frames for A can be sent only to port P, not to the other ports
• if the frame for A is received on port P, it need not even be forwarded
• this can dramatically improve congestion, and somewhat improve security
• such a system is a learning bridge or learning switch
• broadcast packets must still be sent to all ports

Spanning Tree

• if many bridges connect different segments, we might get a topology loop
• if a broadcast packet is sent on this network, it will loop around, consuming resources
• as more broadcast packets loop around, regular traffic is delayed or dropped, which is a broadcast storm
• avoiding broadcast storms:
  • don't have topological loops. If one bridge goes down, the network is partitioned
  • have the bridges compute a spanning tree, and only forward packets along that spanning tree

Bridging Remote Segments

• because a bridge only forwards the essential traffic, it is good to have a bridge at each end of a long-distance connection
• the long-distance connection is point-to-point, and may have long delay and low bandwidth
• the bridge buffers segments, sending only as fast as the link allows