Introduction To Ethernet



The MAC address (MAC = Media Access Control)
  • Ethernet MAC address
    • - The "physical" address of a network adapter
    • - Unique to a device
  • 48 bits / 6 bytes long
    • - Displayed in hexadecimal



Duplex
  • Half-duplex
    • A device cannot send and receive simultaneously
    • All LAN hubs are half-duplex devices
    • Switch interfaces can be configured as half-duplex, but usually only when connecting to another half-duplex device
    • e.g. Walkie-talkie
  • Full-duplex
    • Data can be sent and received at the same time
    • A properly configured switch interface will be set to full-duplex
    • e.g. Telephone call

CSMA/CD
  • CS - Carrier Sense
    • Is there a carrier? Is anyone communicating?
  • MA - Multiple Access
    • More than one device on the network
  • CD - Collission Detect
    • Collision - Two stations talking at once
    • Identify when data gets garbled
  • Half-duplex Ethernet - not used any longer
CSMA/CD operation
  • Listen for an opening
    • Don't transmit if the network is already busy
  • Send a frame of data
    • You send data whenever you can
    • There's no queue or prioritization
  • If a collision occurs
    • Transmit a jam signal to let everyone know a collision has occurred
    • Wait a random amount of time, then retry
CSMA/CA
  • CA - Collision Avoidance
    • Common on wireless networks
  • Collision detection isn't possible
    • A sending station can't "hear" other stations
  • Common to see RTS/CTS
    • I'm ready! You're clear!
  • Solves the "hidden node" problem
    • Station A can hear the access point
    • Station B can hear the access point
    • Station A can't hear station B

*****
The MAC address
Duplex
CSMA/CD
CSMA/CD operation
CSMA/CA
*****

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