Ethernet

• History
• Encoding and Framing
• Headers and Trailers
• CSMA/CD
• Exponential backoff
• Packet Sizes
• Performance
• Variations

History

• Aloha Net
• Cable instead of radio
• Carrier Sense
• Collision Detection
• 48-bit addresses encoded in hardware:
• 48-bit addresses encoded in hardware:
  • 24-bit part assigned to vendor
  • 24-bit part chosen by vendor
• All-1s broadcast address

Encoding and Framing

• Manchester encoding:
  • (-1,1) represents 0
  • (1,-1) represents 1
  • 20 MHz transmission for 10Mb/s ethernet (0.5 b/Hz)
• Framing:
  • Frame begins with preamble (64 bits of 10101010101...)
  • Frame ends with 32-bit CRC of everything except preamble
  • After end of frame, voltage returns to zero

Headers and Trailers

• Ethernet Header:
  • 48-bit destination address
  • 48-bit source address
  • 16-bit protocol type field (IP = 0x800)
  • standard (big-endian) byte order for protocol type field
• Padding:
  • Minimum packet length 60 bytes
  • Pad if needed to achieve minimum packet length
  • Higher-Level protocol must recover actual data length
  • IEEE 802.3 uses a length field instead of the protocol type field.

CSMA/CD

• Carrier Sense Multiple Access with Collision Detection
• To transmit:
  • Wait for line to be free
  • Transmit and listen
  • If collision:
    • jam the wire
    • do exponential backoff
    • try again
• minimum packet size and maximum ethernet diameter guarantee that, if there is a collision, all transmitters see it.

Exponential Backoff

• if this packet already collided n times,
• choose a random number w \in 0 ... 2^n
• given time T = transmission time for minimum sized packet (a constant)
• wait for w * T before trying to send again

Packet Sizes

• minimum packet size 60 bytes = 480 bits
• (some people claim 64 bytes = 512 bits)
• at 10Mb/s, 480 bits = 48u s
• at 0.6 c = 1.8 * 10^8 m/s, 48u s = 8640 m
• ethernet radius suitably below that\footnote{preamble and destination address might be same, source address should be different} should correctly detect collision.
• maximum packet size 1500 bytes = 12000 bits
• maximum time to wait for a packet to clear the ethernet is
• 12,000 b / 10,000,000 Mb/s = 1.2 ms

Performance

• 100% of bandwidth when a single host is sending
• Full-duplex mode: a wire in each direction, only one sender on each wire
• theoretically, 30%-60% of bandwidth when many hosts are sending
• in practice, an ethernet loaded more than 30% becomes very slow
• Throughput efficiency:
  • 1500 data bytes, 14-byte header, 8-byte preamble, 4-byte CRC, user data is 1500/1526 = 98% efficient
  • 1 data byte, 59-byte padding, 14-byte header, 8-byte preamble, 4-byte CRC, user data is 1/86 = 1% efficient.

Switched Ethernet

• use switches instead of hubs to connect hosts (or hubs) to each other
• switches forward packets only on link to destination (once route has been learned)
• ==> reduces collisions, allows more traffic

Fast Ethernet

• 100 Mb/s Ethernet
• Ethernet protocol (packet format, backoff)
• Maximum ethernet diameter?
• 100Base-T:
  • three pairs of wires
  • ternary encoding (3 voltage values)
  • 3^3 = 27 symbols, of which 16 used for transmission
  • 25 MHz transmission
  • additional pair of wires transmits collision signal

Fast Ethernet

• 100Base-TX:
  • two pairs of wires
  • full duplex (I think): one pair of wires in each direction (no collision)
  • 4B/5B at 125 MHz
• 100Base-FX:
  • two fibers
  • full duplex: one fiber in each direction (no collision)
  • up to 2000m. long