Lower layer tech: ethernet

Lower layer tech: ethernet

1. Technologies that move data from point to point

   • Link layer

2. Original topology of a bus

   • Single cable with computers connected
   • Packets flow through ethernet of a standardised format

3. Basic Logic

   • Only one station can talk at a time
     • Every station can see what every other station is saying
     • Wait until no other station is talking, then send data
       • Susceptible to collisions

4. Collisions

   • Two or more stations talking at the same time
   • Detection
     • As station transmits, it listens to ether and checks it only contains the signals being sent
       • If there is other data (mismatch), then someone else is transmitting
   • Action
     • Whole ether is told that there is a collision
       • "Jam" network - set packet sent
       • Whole ether needs to know before packet has finished being sent
   • Recovery
     • First attempt
       • Choose random k from {0,1} and delay k x 512 bit periods if 1
     • Successive atempts
       • Increase intervals {0..2^n}
       • Give up after 10 attempts
   • Problem
     • Collisions dependant on what sort of traffic is being sent
       • Unpredictable latency - collisions cause delays
       • "Ethernet capture effect" - last station to send data before collision as best chance to resume sending after resolution

5. Sizes

   • 10Base5
     • 500m and 10Mbps
   • Longer cables = increase in minimum packet size
   • 10BaseT
     • Modern ethernet

6. Problem with cables

   • Heavy, expensive and difficult to install
   • Transceivers add cost and complexity
   • Wears out eventually
   • Performance issues

7. Bridges

   • Dumb bridge
     • Packets can still be seen from other stations
     • Sees collision in process
   • Learning bridge
     • Knows which addresses belong on which side of the bridge
       • Only sends frames belonging to other side
     • Avoids collisions and traffic cannot be seen by other stations
   • Bridge receives, buffers and transmits frames
     • Collisions not propagated

8. Ethernet switch

   • Set of learning bridges in a box
   • Full duplex
     • Traffic can go in and out without colliding
       • Each direction is in a seperate collision domain
   • Packets to unknown destinations sent by all ports, otherwise only traffic for devices plugged in are sent
   • Large internal buffers to deal with congestions
   • Cut-through switches
     • Faster
       • Conservative switches check the checksum then send - introduces latency
       • Cut-through just looks at the packet header and sends immediately after
         • Bad for broken frames - still propagates
   • No collisions

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