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Ethernet frame: Minimum size 64 bytes but 26+46=72 bytes

LetMePass — Tue, 10 Oct 2023 08:50:49 GMT

Hello dear Cisco community,

I'm starting this thread because I'm still puzzled as to why the minimum size 64 bytes because 26 (header+trailer)+46 (minimum payload)=72 bytes

I assume the preamble and SFD are substracted but why?

Best Regards,

Chris

Re: Ethernet frame: Minimum size 64 bytes but 26+46=72 bytes

M02@rt37 — Tue, 10 Oct 2023 11:13:45 GMT

Hello @LetMePass,

The minimum Ethernet frame size of 64 bytes is primarily due to historical reasons and the way Ethernet operates.

An Ethernet frame consists of several parts:

-Preamble: A 7-byte (56-bit) pattern of alternating 1s and 0s, followed by a "start of frame delimiter" (SFD) which is one byte (8 bits).

-Destination MAC Address: 6 bytes.

-Source MAC Address: 6 bytes.

-EtherType or Length Field: 2 bytes in modern Ethernet frames (it can also be called the "Type" field). In older Ethernet frames, it can be 2 or 4 bytes.

-Data Payload: This is where your actual data goes. Its size varies depending on what you're transmitting. For a minimum-sized Ethernet frame, you'd have 46 bytes of payload.

-Frame Check Sequence (FCS): A 4-byte (32-bit) cyclic redundancy check (CRC) value used for error detection.

When you add these components together, you get:

Preamble (8 bytes) + Destination MAC (6 bytes) + Source MAC (6 bytes) + EtherType/Length (2 bytes) + Data Payload (46 bytes) + FCS (4 bytes) = 72 bytes.

So, you're correct that the total size, including all components, is 72 bytes. However, when we refer to the "minimum Ethernet frame size" of 64 bytes, we're talking about the payload size (46 bytes) plus the Ethernet header (the Destination MAC, Source MAC, and EtherType/Length fields, which total 14 bytes). The preamble, SFD, and FCS are not counted when discussing the minimum frame size, as they are considered overhead.

Historically, Ethernet frames needed to be a certain minimum length to ensure that collisions on shared media (like coaxial cables in older Ethernet implementations) were detected reliably. Short frames could be entirely transmitted before a collision was detected, which could lead to data corruption. By setting a minimum frame size of 64 bytes, Ethernet ensured that collisions would be detected within a reasonable time frame.

Re: Ethernet frame: Minimum size 64 bytes but 26+46=72 bytes

Blue_Bird — Tue, 10 Oct 2023 13:17:05 GMT

The minimum size of an Ethernet frame is 64 bytes, consisting of a 6-byte Destination MAC address, a 6-byte Source MAC Address, a 2-byte Length/Type field, a 46-byte Payload, and a 4-byte Frame Check Sequence.
The Payload may vary in size up to 1500 bytes, with 46 bytes being the lower limit in order to ensure that the transmission media is occupied by the sender for the entire time it is sending the ethernet frame in order to prevent a collision.
If the actual data is smaller than 46 bytes, it is padded by trailing zeroes.

Best regards
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