Why is the minimum size of Ethernet Frame 64 bytes?

Ethernet

When Ethernet was first created, the first cable type made for communication was 10Base-5.

In other words, it is a coaxial cable with a speed of 10 Mbps.

collision

In addition, since Ethernet was half-duplex in the past, it was a structure in which collisions occurred.

CSMA/CD method is used to prevent collision.

What if you deliver the first frame and then deliver the second frame, and the first frame goes fine but crashes?

The sender who sent the frame assumes that a collision occurred while the second frame is on the way, and only retransmits the second frame.

In other words. There will be a problem that the first sent frame is not received at the receiving end.

So how do we solve this problem?

First of all, the sending side should monitor whether a collision occurs until the frame I sent arrives at its destination.

So when you deliver a frame, you have to keep sending it until it arrives at its destination successfully.

In 10Base-5, the maximum transmission length of a single segment is 500m, and it can be extended up to 2.5Km, and it takes 2.8usec (micro second) per 500m.

2.5km requires 5 * 2.8usec = 14usec and since the signal has to go back and forth, it needs a transmission time of 28usec.

Also, in 10Base-5, up to 4 repeaters can be installed according to the 5-4-3 rule.

Each repeater takes 3usec of transmission delay, so 4 repeaters require about 12usec and 24usec is required because the signal has to go back and forth.

The sum of the maximum transmission delays above is about 51.2usec to fit 28usec + 24usec = 52usec bytes.

Since it is 10Mbps, it takes 0.1usec to transmit 1bit.

While detecting whether or not a collision occurs, frames must be continuously transmitted so that if a collision occurs, the frame can be discarded and retransmitted. Therefore, frames must be continuously transmitted until the collision signal comes and goes.

Since the total round-trip time of the signal is 51.2usec, we need at least 512bits to send a frame during this time.

That means you need 512bit / 8 = 64 bytes.

Therefore, the minimum frame size of Ethernet is 64 bytes.