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Ethernet

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.

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Ethernet

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  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, unreliableservice interface Metcalfe’s Ethernet sketch

  2. Ethernet Service Interface • Connectionless: No handshaking between sending and receiving adapter. • Unreliable: Does not implement any reliable delivery protocol: Receiving adapter doesn’t 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. 16 64 48 48 32 Ethernet Frame Structure • 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 46-1500 bytes

  4. Jam Signal: make sure all other transmitters are aware of collision; 48 bits; Ethernet CSMA/CD algorithm New Frame after the n^th collision, adapter chooses a K at random from {0,1,2,…,2^n-1}. Adapter waits K*512 bit times (random wait) Backoff (2n *RTT) Ready to send CS (LBT) Busy Not Busy 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 A • For A to detect a collision, we have to assure that TxT > RTT. How? • Small RTT (short link), long packet, or slow link • Suppose distance <= 2.5km • Speed of Light C = 2*10^5 km/sec • Then RTT < (2*2.5)/C = 25us • 25us@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. That’s why they use Point-to-Point dedicated physical layer LBT LBT Time B starts transmission here TxT B detects collision, sends JAM signal A detects collision here

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