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Ethernet/IEEE 802.3
An Introduction


Prof. David Bernstein
James Madison University

Computer Science Department
bernstdh@jmu.edu

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History
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  • Xerox PARC (1970s):
    • Metcalfe and Boggs developed the first system
    • The network had 256 stations spread over 1 kilometer (running at 3Mbps)
  • IEEE 802 Committee (1980s):
    • Standard 802.3 describing CSMA/CD was published in 1985
Data/Signal
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  • Baseband Transmission:
    • The signal is not shifted into another frequency band
  • Manchester Encoding:
    • Each bit contains a transition
    • A 1 is a transition from up to down and a 0 is a transition from down to up
Different Standards
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  • 10BASE-5: Bus network (with taps), coaxial cable, 10Mbps, 500 meter maximum segment length
  • 10BASE-2: Bus network (with taps), thin coaxial cable, 10Mbps, 200 meter maximum segment length
  • 10BASE-T: Star network, twisted pair cable, 10Mbps
  • 100BASE-TX: Star network, CAT-5 twisted pair cable (two pairs per connection), 100Mbps, 100 meter maximum segment length
  • 100BASE-FX: Star network, multimode fiber-optic cable (one strand in each direction), 100Mbps, 412 meter maximum segment length
  • 1000BASE-SX (1000BASE-LX): Star network, fiber-optic cable and short (long) wavelength, 1Gbps, 500 (5000) meter maximum segment length
  • 1000BASE-T (1000BASE-LX): Star network, CAT-5 twisted pair cable, 1Gbps, 100 meter maximum segment length
Frames (in IEEE 802.3)
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  • Preamble:
    • Seven octets of 10101010 used to synchronize clocks (using the transitions in the bits)
    • One octet of 10101011 used as an alert
  • Destination Media Access Control (MAC) Address:
    • Six octets
  • Source MAC Address:
    • Six octets
  • Length:
    • Two octets indicating the length of the data field
    • To remain compatible with the original standard, if the value is greater than 1500, these octets indicate the network-layer protocol being used (e.g., IP, Novell IPX, AppleTalk, DECnet)
  • Data:
    • 46-1500 octets
  • Frame Check Sequence (FCS):
    • Four octets used for a cyclic redundancy check (CRC)
Properties
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  • Connectionless:
    • No handshaking
  • Unreliable:
    • CRC can be used to find errors but the receiver just discards the frame
Collisions
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  • Why They Might Arise:
    • It takes time for a signal to travel through the medium
    • Devices at different locations might, as a result, transmit at the same time
  • How Detection is Ensured:
    • A device must continue to transmit for the "round trip time" (the maximum time required for a round trip)
Collisions (cont.)
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  • When They Can Arise:
    • Bus networks
    • Star networks using a hub (a physical layer device that simply retransmits individual bits to all edges making the star behave like a "virtual"/"logical" bus)
    • Half-Duplex Connections on Star Networks
  • When They Do Not Arise:
    • Full-duplex (a pair/fiber for each direction) connections on star networks using a "Fast Ethernet Switch" (a physical layer device that filters and forwards frames)
Collisions (cont.) - The Process
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  1. Carrier Sense (CS): Listen to the network to determine that no other devices are transmitting.
  2. Start transmitting. [Note: More than one device can be transmitting at the same time, even if they don't start at exactly the same time. This is called Multiple Access (MA).]
  3. Collision Detection (CD): If two devices are transmitting, the signals interfere and this is detected by the devices (by comparing the transmitted/expected signal with the actual signal).
  4. A device that detects a collision sends out a jamming signal to inform other devices not to transmit.
  5. Each device that caused the collision "backs off" for a random amount of time and re-transmits.
Collisions (cont.)
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  • An Observation:
    • One frame can be involved in multiple collisions (over time)
  • Exponential Backoff at Collision \(n\):
    • Let \(m = \min(n, 10)\)
    • Choose a value \(K\) "at random" from the set \({0, 1, 2, \ldots, (2^m-1)}\)
    • Wait \(K \cdot 512\) "bit times"
  • Rationale for Exponential Backoff:
    • When \(n\) is large (often because there are a large number of devices), the set of possible backoff times should be larger and more dispersed
Collisions (cont.)
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  • Efficiency/Utilization:
    • The long-run fraction of time during which frames are being transmitted without collisions (when there are a large number of devices each with a large number of frames to send)
  • An Approximation (without derivation):
    • \(U = \frac{1}{1 + 5 d_{\text{prop}}/d_{\text{max}}}\)
    • where \(d_{\text{prop}}\) denotes the propogation delay and \(d_{\text{max}}\) is the transmission delay for the maximum size frame (about 1.2 msecs for a 10Mbps Ethernet)
Persistence (or, informally, Tenacity)
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  • The Issue:
    • What should a device do when it determines another device is transmitting?
  • Possible Algorithms:
    • Nonpersistent: wait a random amount of time and try again
    • \(1\)-Persistent: Listen until the medium isn't busy
Persistence (cont.)
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  • A Compromise:
    • A \(p\)-persistent system tries to reduce both collisions and idle time
  • The Algorithm:
    • 1. If idle, transmit with probability \(p\) and delay one time unit with probability \(1-p\)
    • 2. If busy, continue to listen until idle and then go to 1.
    • 3. If delayed go to 1.
Problems that Arise
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  • High Collison Rates:
    • Incorrect topology
    • High utilization
  • Bad Frame Check Sequence (FCS):
    • Hardware faults
  • Short/Long Frames:
    • Hardware faults
    • Line noise
There's Always More to Learn
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