Question

Explain each part of an Ethernet Packet in detail (about two sentences)

Answer 1

Preamble:

• Ethernet packet begins with 7 bits preamble. These 7bits are mix of 0's and 1's.
• It is used for bit synchronization at sender host and receiver host
• It useful to safeguard actual data bits as loss of preamble bits due to any delays won't lead to loss of actual data.
• PREamble alerts the receiver the receiver about incoming frame.

Start of frame delimiter(SFD) :

• It is 8 bits field and its value is always 10101011.
• SFD is useful to identify actual beginning of  packet. Then the receiver that receive packets understands that after pattern 10101011.it is destination address.

Destination Address:

• It is 48 bits i.e, 6 byte field.
• It contains MAC address of destination host.

Source Address:

• It is 48 bits i.e, 6 byte field.
• It contains MAC address of source host.

Length:

• 16 bits field
• Indicates total length of ethernet frame.

Data /Payload :

• It is the fiels where actual data is present.
• Minimum 46 B (padding is done if data is less than 46 B) and Maximum 1500B can be sent in a ethernet frame.

Cyclic Redundancy Check (CRC) :

• It is 4 byte field
• Sender generates 32 bit hash code and places in this field. If checksum computed at destination host is not same as the sender checksum then it means that data is corrupted.