why the minimum ethernet packet size is 64 bytes

Posted on February 26, 2010. Filed under: CCNA |


The following is the answer for ATul Singh about Ethernet Frame:

The smallest packet size is 64 bytes.
14 bytes (MAC Header) + 46 bytes (Data) + 4 bytes (CRC) = 64 bytes

The smallest packet size is defined in RFC document (RFC894), I don’t know why IEEE like the number 64. As far as I concern, physics sets a low bound for the packet size. let’s review what we have learned in operation of CSMA/CD :

“If two NICs simultaneously try transmit, then both could see no carrier signal in the medium, thus decide to transmit and a collision will occur.  Collision Detection (CD) resolved this problem. When there is data waiting to be sent, each transmitting NIC will observe if a collision occurs (excess current i.e. >24mA for coaxial Ethernet). If  collision is detected, it stops transmission and sends a 32-bit jam signal instead. The receiving NIC will discard the corrupted frame due to the jam signal.”

Remember, a packet is a electromagnetic wave. For small sized packet, the wave is narrower, for large sized packet, the wave is wider.

In the following pictures, the squres represent ethernet cable, the curves represent ethernet packets (or electromagnetic waves). Suppose the node at point A and node at point C send two packets simultaneously, the node at point B will receive a corrupted data.

picture 3 is the normal situation where the packet size is large or the eletromagnetic wave is wider. When the eletromagnetic waves overlap (or packets collide) at node B, they also overlap at Node A and Node C. Therefore, both node A and node C sensed excess current and terminated the packet transmission and sent jam signals instead. Node B receives jam, discarded the corrupted packet.

In picture 1 and 2, since the packet size is too small, problem arises.

As picture 1 shows, at time 0, node A send a packet towards node B and C, while node C sends a packet towards node B and A at the same time.

As picture 2 shows, at time t, the eletromagnetic waves overlap (or packets collide) at node B, but they are not overlap at Node A or Node C. As a result, neither node A nor node C sensed excessive current, so no jam signals sent. In another words, a collision is not discovered and node B happily accepted the corrupted data!

IEEE might do their research and calculation, and find out that 32 bit packets may be too small for large LANs, 64 bit packet is large enough for even the largest LANs, so they picked the number 64. Just my guess.

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One Response to “why the minimum ethernet packet size is 64 bytes”

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dont guess if you are not sure, rather do some research


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