The minimum size of an Ethernet frame is 64 bytes. The Ethernet protocol is responsible for Network Interface Card (NIC) to Network Interface Card (NIC) communication of hosts on the same network. It is a protocol of the Layer 1 (Physical Layer) and Layer 2 (Datalink Layer) of the OSI Reference Model.
The Ethernet Protocol is used for NIC-to-NIC communication where the Sender and the Receiver are hosts on the same network.
For example, when a host on the network has to send a message to the receiver, it formulates an ethernet frame using the Media Access Control (MAC) addresses (The MAC address is the permanent unique address provided to the Network Interface Card (NIC) of a computer system by the manufacturer. It uniquely identifies a computer system over a network) of the sender and the receiver, and the message as the payload along with other data necessary for communication.
This frame(message) is then sent over the transmission media to the receiver. In Ethernet, the transmission medium is generally a twisted-pair cable. The receiver receives the frame, verifies the Destination MAC address as its own, and if the destination MAC address of the ethernet frame matches its own MAC address, proceeds to accept the packet.
The Ethernet Frame:
The Ethernet Frame
The Ethernet frame consists of the following parts:
- Preamble: 7 bytes or 56 bits of alternating zeroes and ones starting with one marking the beginning of an ethernet frame.
- Start Frame Delimiter (SFD): 1 byte or 8 bits of alternating zeroes and ones starting with one with the last bit as one making the binary value 10101011. It notifies the receiving device that the next bit is of the destination MAC address.
| 10101010 | 10101010 | 10101010 | 10101010 | 10101010 | 10101010 | 10101010 |
10101011 |
|---|---|---|---|---|---|---|---|
|
Preamble |
Start Frame Delimiter | ||||||
An example of preamble and Start Frame Delimiter (SFD)
- Destination MAC Address: The 6-byte MAC address of the NIC of the receiver.
- Source MAC Address: The 6-byte MAC address of the NIC of the sender.
|
2C:54:91:88:C9:E3 |
00:00:5E:00:53:AF |
|---|---|
| Destination MAC Address | Source MAC Address |
An example of Destination and Source MAC Addresses
- Length/Type: A 2-byte field containing the information about the protocol of the upper (Network Layer) layer.
|
0x86dd IPv6 |
|---|
| Length/Type of field of IPv6 |
- Payload: The actual data to be sent from the sender to the receiver along with padding bits wherever necessary. The size of this field ranges from 46 to 1500 bytes.
|
10101101……..1011011 |
00000……….00000 |
|---|---|
| Data (32 Bytes) | Padding (14 Bytes) |
|
Payload |
- Frame Check Sequence (FCS): This 4-byte field is used to ensure that the received Ethernet frame is intact.
|
00:20:2D:3A |
|---|
| The Frame Check Sequence (FCS) |
The Preamble and the Start Frame Delimiter are not taken into consideration when calculating the Ethernet frame size. The overall size of the Ethernet frame must be a minimum of 64 bytes 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. With the fixed size of the Destination MAC Address, the Source MAC Address, the Length/Type field, and the Frame Check Sequence adding up to 18 bytes, the minimum size of the Payload must be 64-18=46 bytes. If the actual data to be sent is less than 46 bytes, it must be padded to make the Payload size 46 bytes.
Applications of Ethernet:
Ethernet has become the de facto networking standard for Local Area Network (LAN) and Wide Area Network (WAN) connections due to its speed, security, and dependability. It is used in LAN and WAN connections of organizations, schools, hospitals, etc. It can also be used to generate a non-lag high-speed connection between hosts for online gaming. For any network, aiming to be mostly local and inexpensive, Ethernet is the best bet.
Advantages of Ethernet:
- It is a cheap option for creating a computer network as compared to other modes of network connectivity.
- It is a fast, secure, and reliable connection.
- Network maintenance and administration are easier.
Disadvantages of Ethernet:
- The size of the network is restricted in terms of distance as the ethernet cables cannot be stretched infinitely.
- It cannot be used in networks with a fluctuating host count.
- All the devices which want to connect to the ethernet network must have a physical ethernet port.
Conclusion
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.