Ethernet

1
Ethernet

• dominant LAN technology
• cheap 20 for 100Mbs!
• first widely used LAN technology
• Simpler, cheaper than token LANs and ATM
• Kept up with speed race 10, 100, 1000 Mbps
• Uses Manchester encoding or 4B/5B Encoding
• Exports a connectionless, unreliable service
  interface

Metcalfes Ethernet sketch
2
Ethernet Service Interface

• Connectionless No handshaking between sending
  and receiving adapter.
• Unreliable Does not implement any reliable
  delivery protocol Receiving adapter doesnt send
  acks or nacks to sending adapter
• stream of datagrams passed to network layer can
  have gaps.
• reliability must be implemented by upper layer
  protocols if desired
• otherwise, application will see the gaps

3
Ethernet Frame Structure
46-1500 bytes

• Preamble
• 7 bytes with pattern 10101010 followed by one
  byte with pattern 10101011 (start of frame flag)
• used to synchronize receiver, sender clock rates
  to have the receiver detect the beginning of
  the frame
• Addresses 6 bytes
• Type Mux/Demux key
• indicates the higher layer protocol
• CRC Uses CRC-32 as EDC
• No Length Field!
• End of frame is detected by the lack of current
  on the link LL PL are strictly tied to each
  other

4
Ethernet CSMA/CD algorithm
New Frame
after the nth collision, adapter chooses a K at
random from 0,1,2,,2n-1. Adapter waits K512
bit times (random wait)
Backoff (2n RTT)
Ready to send
CS (LBT)
Busy
Not Busy

• Jam Signal make sure all other transmitters are
  aware of collision 48 bits

TxF CS (LWT)
Collision
Tx JAM Signal
CS Carrier Sense TxF Transmit Frame LBT
Listen Before Talking LWT Listen While Talking
No Collision
Tx Done
5
Collision Detection (CD)
B

• For A to detect a collision, we have to assure
  that TxT gt RTT. How?
• Small RTT (short link), long packet, or slow link
• Suppose distance lt 2.5km
• Speed of Light C 2105 km/sec
• Then RTT lt (22.5)/C 25us
• 25us_at_10Mbps 250 bits, which is about 30 bytes
• Thus the required minimum size, 46 bytes, for the
  payload of an Ethernet frame is sufficient to
  detect collision for 10 Mbps shared-medium
• Also means that shared medium Ethernets of speeds
  100Mbps or more are not practical. Thats why
  they use Point-to-Point dedicated physical layer

A
LBT
LBT
Time
B starts transmission here
TxT
B detects collision, sends JAM signal
A detects collision here