Ethernet Frame and MAC Address

Packet Data Units encapsulated from all 7 layers in OSI

The encapsulated packet from the OSI layer is seen as below in Ethernet Frame

1. Preamble

• Length: 7 bytes 9 ( 56 bits ).
• It's a series of alternating 1's and 0's = 1010101.
• synchronizes the receiver block to make them ready to receive data

2. SFD

• SFD - Start Frame Delimiter.
• Length: 1 byte ( 8 bits ).
• Binary form = 10101011.
• Separates preamble with the rest of the frame.

3. Destination and Source

• Consist of the destination and source ‘MAC address.
• MAC = Media Access Control.
• = 6 bytes (48-bit) address of the physical device

4. Type / Length

• 2 byte (16-bit) field
• A value of 1500 or less in this field indicates the LENGTH of the encapsulated packet (in bytes).
• A value of 1536 or greater in this field indicates the TYPE of the encapsulated packet usually IPv4 or IPv6, and the length is determined via other methods.

5. CRC

• A cyclic redundancy check (CRC) is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data.
• Happens at the Data link layer.
• AKA FCS ( Frame Check Sequence ).
• 4 bytes ( 32 bits ) in length.

6. MAC Address

• A 6-byte (48-bit) physical address is assigned to the device when it is made.
• A.K.A. ‘Burned-In Address’ (BIA).
• Is globally unique
• The first 3 bytes are the OUI (Organizationally Unique Identifier), which is assigned to the company making the device
• The last 3 bytes are unique to the device itself