Ethernet Notes
CPSC 330 - Fall 2003

history

• developed by Bob Metcalfe of Xerox in 1973
• chose "ether" as a way of describing how the physical link carries bits to all stations, in the same way that the idea of "luminiferous ether" was once thought to propagate electromagnetic waves through space
• originally coax cable running at 2.94 Mbps (Manchester encoding) and up to 256 stations
• became DEC-Intel-Xerox (DIX) standard in 1980
• later became IEEE 802.3 standard

original implementations

• Mbps / signalling method / max segment length (x 100 m)
• 10Base5 - thick coax bus structure, 500 m max, 100 stations/segment
• 10Base2 - thin coax bus structure, 200 m max, 30 stations/segment
• 10BaseT - twisted pair hub structure, 100 m max, 1024 stations/segment
• 10BaseF - fiber optic hub structure, 2000 m max, 1024 stations/segment

  key point: multiple physical-link implementations for same data-link protocol

recent implementations

• Fast Ethernet - 100 Mbps (different signal encoding)
• Gigabit Ethernet - 1 Gbps
• 10 Gigabit Ethernet - 10 Gbps

network type

• frames (64 to 1518 bytes in size)
• connectionless
• broadcast

addressing

• 48-bit addresses, guaranteed to be globally unique
• IEEE handles address assignment - first three octets indicate vendor; vendor assigns last three octets e.g., Sun Microsystems = 08-00-20-xx-xx-xx
• address fixed for station (e.g., held in ROM on Ethernet adapter card) so adapter card always has same LAN address, no matter which LAN it is connected into
• called MAC addresses (media access control)
• each station checks the destination addresss of each frame it senses on channel, accepts it only if the destination address matches, then checks for errors (frame size and CRC)
• originally, address field had high 0 for unicast, high 1 for multicast using predefined group address, and all 1s for broadcast
• also local and global addresses

CSMA/CD - carrier sense multiple access with collision detection

• multiple access
  • shared wire
  • shared wireless
  • satellite

• decentralized control (see textbook, Figure 10.7, p. 463)
  1. transmit if no other station is currently transmitting
  2. listen during transmission to detect collision
  3. if collision, then tramsmit jamming signal and wait a random amount of time before attempting to retansmit

     ("listen before speaking; and, if someone else begins talking at the same time, stop talking")

• implications of contention approach
  • long propagation delays make collisions more likely
  • frame collisions increase with load
  • collisions mean that ethernet systems never reach the maximum possible transmission rate

• collision detection
  • using bus - stations sense bus voltage too high
    (note that frames need to be long enough so that stations can detect collisions before the frame ends => min frame length and max bus length / number of bus segments; otherwise CD scheme fails)
  • using hub - hub senses multiple transmissions and sends a collision presence signal

• exponential backoff - after each collision double the mean delay value (after 16 collisions, report error)

frame format

(see textbook, Figure 10.8, p. 463)

  # of bytes
           .---------------------.
        8  | preamble            | provides clock reference 0xaaaaaaaa
           >---------------------<                          0xaaaaaaab
        6  | destination address |
           >---------------------<
        6  | source address      |
           >---------------------<
        2  | type or length      | DIX type, e.g., 0x0800 => IP packet;
           >---------------------< IEEE length, SNAP (subnet access protocol)
           |                     |    provides type field in data area
  46-1500  | data                |
           |                     |
           >---------------------<
        4  | CRC                 |
           `---------------------'

popularity

• Ethernet is most popular LAN technology (token ring, FDDI, ATM are more complex and more expensive)
• its popularity has lead to commodity products and thus commodity prices
• always kept up with speeds of competitors

switched Ethernet

• broadcast and CSMA/CD are not used
• more efficient
  • switch supports multiple simultaneous transfers with buffers at each input and output port (store-and-forward)
  • switch checks for errors and discards mangled packets
  • flow control by discarding packets or using pause and continue control frames
• collision domain - the network area within which frames that have collided are propagated (repeaters and hubs propagate collisions, but switches, bridges, and routers do not)
• allows subdividing stations into virtual LANs - see below

virtual LAN

• administratively-configured broadcast domain independent of underlying physical links
• broadcast domain - set of all stations that will receive broadcast frames originating from any station within the set. (Broadcast domains can be bounded by software configurations in a switched intranetwork environment; in an internetworking environment, they are typically bounded by routers because routers do not forward all frames/packets.)
• Cisco discussion of VLANs
• while most VLANs are configured at layer 2, some are configured at layer 3 using IP addresses

wireless 802.11

• compatible with Ethernet above data link layer
• base stations connected by Ethernet

• can't do CSMA/CD

      ((((((A))))))   x       A and C can transmit without sensing collision
           ((((((B))))))      B will get signal interference
            x   ((((((C))))))
  

• instead MACA (multiple access with collision avoidance)
  1. A transmits request-to-send message with length of intended data frame
  2. if no collision at B, then B sends clear-to-send message with length
  3. C hears clear-to-send and using length knows how long to wait before next attempting to transmit

• after studying this protocol for wireless, added acknowledgement message and CSMA for request-to-send (called MACAW)

traffic studies

• traffic is not Poisson but self-similar (i.e., fractal-like)
• burstiness is a property of all time scales (spikes, ridges, swells)
• traffic does not smooth out as number of traffic sources increase, but in fact the burstiness increases (differs from telephone traffic)
• means that simple Poisson-based analytic models are incorrect
• see Will Leland, Murad Taqqu, Walter Willinger, and Daniel Wilson, "On the Self-Similar Nature of Ethernet Traffic (Extended Version)," IEEE/ACM Transactions on Networking, Vol. 2, No. 1, pp. 1-15, February 1994. (earlier version in SIGCOMM 93, Sept. 1993.)

Ethernet animations

• animated gifs for bus, hub, and switch
• applets from Darmstadt
• see 6_1 and 6_5 (Purdue netbook)

Ethernet figures

• ethernet flow
• frame format
• frame types
• wiring options

See also

• how ethernet works

[CPSC 330 homepage]

mark@cs.clemson.edu