10-10-2023 01:49 AM - modifié 10-10-2023 01:50 AM
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
Résolu ! Accéder à la solution.
le 10-10-2023 04:13 AM
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.
10-10-2023 06:06 AM - modifié 10-10-2023 06:17 AM
Best regards
******* If This Helps, Please Rate *******
le 10-10-2023 04:13 AM
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.
10-10-2023 06:06 AM - modifié 10-10-2023 06:17 AM
Best regards
******* If This Helps, Please Rate *******
Découvrez et enregistrez vos notes préférées. Revenez pour trouver les réponses d'experts, des guides étape par étape, des sujets récents et bien plus encore.
Êtes-vous nouveau ici? Commencez par ces conseils. Comment utiliser la communauté Guide pour les nouveaux membres
Parcourez les liens directs de la Communauté et profitez de contenus personnalisés en français