Introduction to Ethernet

Question 1

What are the components of a simple home LAN?

Answer 1

Internet - Router - Switch (Access Point) - Devices

Question 2

What are the components of the Ethernet frame?

Answer 2

Preamble - SFD (Start Frame Delimiter) - Destination MAC - Source MAC - Type - Payload - FCS (Frame Check Sequence)

Question 3

Describe Preamble.

Answer 3

Preamble, 7 bytes. 56 alternating ones and zeros used for synchronization.

Question 4

Describe SFD.

Answer 4

SFD, 1 byte. Start Frame Delimiter designates the end of the preamble.

Question 5

Describe Destination MAC Address.

Answer 5

Destination MAC Address, 6 bytes. Ethernet MAC address of the destination device.

Question 6

Describe Source MAC Address.

Answer 6

Source MAC Address, 6 bytes. Ethernet MAC address of the source device.

Question 7

Describe EtherType.

Answer 7

EtherType, 2 bytes. Describes the data contained in the payload.

Question 8

Describe Payload.

Answer 8

Payload, 46-1500 bytes. Layer 3 and higher data.

Question 9

Describe FCS.

Answer 9

Frame Check Sequence - Looks at all the data and comes up with a checksum.

Question 10

What is a MAC address?

Answer 10

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 11

What is half-duplex?

Answer 11

Device cannot send and receive data simultaneously. One or the other.

Question 12

What is full-duplex?

Answer 12

Data can be sent and received at the same time.

Question 13

What is CSMA/CD?

Answer 13

Carrier Sense Multiple Access Collision Detect

• Is there a carrier?
• Is there a signal available?
• More than one device on the network
• Two stations talking at once
• Usually used with half-duplex

Main purpose is to listen for an opening (don’t transmit if network is busy), send a frame of data whenever possible, and if a collision of data occurs then it will transmit a jam signal, wait a random amount of time, then retry.

Question 14

What is CSMA/CA?

Answer 14

Carrier Sense Multiple Access Collision Avoidance

• This is common on wireless networks
• Collision detection isn’t possible
• Uses RTS (ready to send) and CTS (clear to send) to communicate