Why does a minimum frame size exist?

The minimum Ethernet frame size exists because of the collision detection mechanism in classic Ethernet.

Briefly: a frame must be long enough for the sender to have time to notice a collision.

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#Where this limitation comes from

Early Ethernet networks used the CSMA/CD mechanism (Carrier Sense Multiple Access with Collision Detection):

• several devices shared one common medium
• they could start transmitting at the same time
• this caused collisions

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#The problem with a short frame

If a frame is too small:

• the device may finish transmitting
• before the collision signal makes it back to the device

In that case:

• the sender considers the transmission successful
• but the frame is actually corrupted
• data is lost without anyone noticing

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#Why a minimum size is needed

The minimum frame size guarantees that:

• the frame is transmitted for long enough
• a collision (if one occurs) has time to be detected
• the sender can:
  • stop the transmission
  • retry it later

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#Why exactly 64 bytes

For Ethernet:

• the minimum frame size is 64 bytes
• this is calculated from:
  • the maximum segment length
  • the signal propagation speed
  • the collision detection time

If there is less payload data:

• Ethernet adds padding
• to reach 64 bytes

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#What about now?

Important for an interview:

• On modern networks:
  • switches are used
  • full-duplex is used
  • there are no collisions

But:

• the minimum frame size has stayed in the standard
• for compatibility and consistency

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#What to remember

• the minimum frame size exists because of collisions
• it is tied to the CSMA/CD mechanism
• it equals 64 bytes
• padding is used when there is not enough data

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#Short version for an interview

The minimum Ethernet frame size exists so that a transmitting device has time to detect a collision on a shared transmission medium; that is why the standard sets a minimum of 64 bytes.