Ethernet

1
Title

• Ethernet

2
Background

• Ether-Net
• From aether, proposed medium for propagation of
  light in late 19th century
• Developed by Robert Metcalfe at Xerox
• Shared transmission medium
• Early Ethernet used coaxial cable bus
• Point-to-point links with Ethernet hubs/switches
  more prevalent today
• Bandwidths of 10Mbps/100Mbps/1000Mbps

3
Shared Media

• Medium Access Protocols
• Time sliced / Fixed Assignment
• Each node gets a fixed time slot to transmit
• This is essentially circuit switching!
• E.g. CDMA (Cellular networks)
• Reservation or token based
• Need to acquire token before transmitting
• Take turns transmitting
• E.g. Token Ring network
• Contention based
• All nodes try to access channel whenever there is
  need to transmit
• Should be able to sense if channel is busy
  (carrier sensing)
• Should be able to recover from collision
  (collision detection)

4
The Ethernet frame
Sourcewww.xilinx.com/esp/consumer/home_networking
/pdf_files/ethernet/complete.pdf
5
CSMA/CD

• Ethernet uses CSMA/CD
• Carrier Sensing Medium Access w. Collision
  Detection

Packet?
Sense Carrier
Detect Collision
Send
Report to host
Jam channel bCalcBackoff() wait(b) attempts
Image Source http//pages.cs.wisc.edu/pb/640/eth
ernet.ppt
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Collision Detection

• Adapter senses collision through voltage levels
• Two nodes A and B at opposite ends of bus
• For A to detect collision, A must keep
  transmitting for time 2T
• Why?

A transmits to B
Time to travel T
A
B
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Collision Detection

• IEEE 802.3 specifications
• T 51.2 microseconds
• Max distance between hosts 2500m
• Hence, at 10Mbps, must transmit 512 bits (64B)
• 14B header 46B data 4B checksum
• Padding to ensure data is 46B
• When collision detected
• 32 bit jamming signals sent
• Ensure all hosts know about collision

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Exponential Backoff

• After collision, wait some time, try again
• Delay computed using exponential backoff alg.
• delay K 51.2 microseconds where
• K random number in 0,...,2n 1
• n number of collisions detected
• Why random?
• Why exponential range of K?

9
Physical Layer Details

• Ethernet Technologies
• 10Base5 10Mbps, baseband, 500m, coaxial cable
• 10Base2 10Mbps, baseband, 200m, coaxial cable
• 10BaseT 10Mbps, baseband, twisted pair
• Twisted pair
• twisted wires to cancel EM interference
• two cables transmit and receive
• 10BaseT with multiple hubs
• Use crossover cable to connect two hubs
• Use coaxial cable to connect multiple hubs

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The good, the bad and the ugly

• Good
• Cheap
• Easy to administer and setup
• Bad
• Performance suffers as number of hosts increase
  more collisions
• Ugly
• Ethernet capture effect a single node may
  capture the channel
• Caused by zero-ing of counter after successful
  transmission

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• Questions?