Introduction to Ethernet

Question 1

The MAC address

Answer 1

• Ethernet Media Access Control address
  
  • –The “physical” address of a network adapter
  • –Unique to a device
• 48 bits / 6 bytes long
  
  • –Displayed in hexadecimal

Question 2

Duplex

Answer 2

• 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

• Full-duplex
• Data can be sent and received at the same time
• A properly configured switch interface will be set to
full-duplex

Question 3

CSMA/CD

Answer 3

• CS - Carrier Sense
  
  • Is there a carrier? Is anyone communicating?
• MA - Multiple Access
  
  • More than one device on the network
• CD - Collision Detect
  
  • Collision - Two stations talking at once
  • Identify when data gets garbled

• Half-duplex Ethernet - not used any longer

Question 4

CSMA/CD operation

Answer 4

• 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

Question 5

CSMA/CA

Answer 5

• 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