Ethernet (Cambridge International A-Level Computer Science)

Ethernet

What is Ethernet?

• Ethernet is a protocol that controls

  • Wiring
  • Data transmission
  • Data encapsulation

• A wired networking standard used in a Local Area Networks (LANs)

• Data is transmitted in frames

• Uses CSMA/CD to help detect and avoid data collisions

Ethernet frames

• Data is transmitted across the network in structured units called frames

• Each frame typically includes:

  • Preamble – A sequence of bits used to synchronise communication between devices
  • Destination MAC Address – The unique ID of the device the data is being sent to
  • Source MAC Address – The unique ID of the device sending the data
  • EtherType / Length – Indicates the type of data or the size of the payload
  • Payload – The actual data being sent (e.g. a web page, file, or message)
  • FCS (Frame Check Sequence) – Used for error detection to check if data arrived correctly

CSMA/CD

What is CSMA/CD?

• CSMA/CD is a protocol used to detect and prevent collisions in a bus topology

• Stands for Carrier Sense Multiple Access with Collision Detection

• Devices check if the channel is free before sending data

• If the channel is busy, the device waits

• If the channel is free, the device transmits the data

• Since all devices share the same cable, two can send data at the same time, causing a collision

• When a collision happens:

  • Devices detect it
  • A jamming signal is sent to alert other devices
  • Both devices stop transmitting
  • Each device waits for a random amount of time, then tries again

Worked Example Ethernet uses Carrier Sense Multiple Access/Collision Detection (CSMA/CD).

Describe CSMA/CD. [4]

Final answer: Answer

1 mark per point to max 4

• A workstation / node (wishing to transmit) listens to the communication channel [1 mark]

  • the data is only sent when the channel is free [1 mark]

• As there is more than one computer connected to the same transmission medium [1 mark]

  • two workstations can start to transmit at the same time, causing a collision [1 mark]

• If a collision happens, the workstations send a jamming signal to abort transmission [1 mark]

  • and each waits a random amount of time before attempting to resend [1 mark]