What is the Ethernet Frame Format?
Quick Definition: Ethernet frame format is the standardized structure used for encapsulating and transmitting data over Ethernet networks. It includes components such as preamble, source and destination MAC addresses, EtherType or Length field, payload, and CRC, ensuring uniformity for reliable data communication across network nodes.
Ethernet frames serve as the foundational units of data exchanged within IT communications networks. The Ethernet frame format guarantees that all data transmitted between network nodes adheres to a specific and coherent structure, promoting swift and dependable transmission.
If you ever plan to take the Network+ Exam or CCNA, understanding the Ethernet frame format is critical for passing the exam. Luckily, it is not as complicated as it sounds. Ethernet frame format is simply a standard by which data is passed, and we’ll explain that format now—starting with a broad definition and its overall purpose.
What is an Ethernet Frame?
At a basic level, an ethernet frame can be thought of as a postcard for IT communication. Standard postcards are always the same dimensions: 4” by 6”. Every postcard includes the following information: the recipient's address, the sender’s address, a message area, a stamp, and a postmark. These elements collectively ensure the postcard is successfully sent, and make up the standard format of what makes a postcard a postcard.
Now, let’s map that logic to ethernet frames. An Ethernet frame is a piece of data that is streamed between two nodes. For the recipient to understand the sender’s request, it has to be in a standard format—just like a postcard.
An ethernet frame encapsulates the data it wants to send in the ethernet frame. The ethernet frame will then have standard metadata associated with it, like the sender’s and recipient’s MAC address, error-checking fields, and more. Then, the recipient de-encapsulates the frame and analyzes the content.
While postcards have a standard size of 4” by 6”, an Ethernet frame has a standard range of 64 bytes to around 1521 bytes.
So, in short, an ethernet frame is simply a postcard sent between devices on a network.
As you may have observed, Ethernet frame transmission does not involve IP addresses. Instead, they rely on MAC addresses for purposes of transmitting. A MAC address is a hexadecimal number on a physical device. Since we are referring to the physical characteristics of a network, it means the Ethernet frame format is an OSI Level 2 (Data Link Layer) concept.
What are the Components of an Ethernet Frame?
The Ethernet frame format mandates seven key components of all ethernet frames. In addition to listing them, the size of the metadata will be listed next to its name. Let’s go through them one by one:
Preamble (7 Bytes)
A sequence of alternating 1s and 0s at the beginning of the frame. It helps synchronize the receiving device's clock with the incoming data. This critical piece of data ensures the recipient can accurately interpret the data. As a loose analogy, think of it as showing the recipient the “rhythm” of the message so they are both on the same beat.
Start Frame Delimiter (SFD) (1 Byte)
This marks the end of the preamble and indicates the start of the frame. It's a specific bit sequence (10101011) that signals the beginning of the frame payload.
Destination MAC Address (6 Bytes)
A 6-byte (48-bit) field specifying the MAC address of the device to which the frame is being sent.
Source MAC Address (6 Bytes)
This is another 6-byte field that indicates the MAC address of the device sending the frame.
EtherType/Length (2 Bytes)
A 2-byte field specifying either the type of protocol in the payload (EtherType) or the length of the payload data.
Payload (Variable)
The payload refers to the actual data being transmitted. It can vary in size and includes the information to be delivered to the recipient. The payload is between 46 and 1500 bytes in length.
Frame Check Sequence (FCS) (4 Bytes)
A 4-byte (32-bit) field for error checking. It helps the receiving device identify any errors in the frame during transmission.
What About VLANs?
One thing to note is that VLAN tagging is often added to the ethernet frame if the network is broken up into subnetworks. This identifies which sub-LAN the frame should be routed to.
What is the Structure of the Ethernet Frame Format?
The previous section provided a brief overview of each component of an Ethernet frame, but it is worth looking at each one of them more in-depth.
With that in mind, the illustration below is a good visual diagram of what an Ethernet frame actually looks like. However, instead of thinking of them as words, remember they are actually a string of 0 and 1’s. (i.e., voltage differentials).
Source: StockExchange.
What are Preamble and Start Frame Delimiter (SFD) in an Ethernet Frame Format?
The preamble is a 7-byte piece of data that signals an Ethernet frame has started. It can be thought of as a “digital handshake” that ensures the recipient is on the same page about what will be sent. Without a preamble, the recipient would not understand the data, leading to failure.
The SFD is tightly coupled with the preamble. The SFD is typically 10101011, and specifies the end of the preamble and the beginning of the actual data pertinent to transmission.
What are Destination and Source MAC Addresses in an Ethernet Frame Format?
The destination MAC address is the physical address of the device the frame is going to. The source MAC address is the frame’s source. A MAC Address is a Layer 2, 48-bit value that is unique to every device in the world. Remember, when you hear “physical device,” think Layer 2!
What is the Type/Length Field in an Ethernet Frame Format?
The EtherType, or Type/Length, field in the Ethernet frame is a 2-byte (16-bit) identifier that specifies the protocol of the payload. Examples of protocols include IPv4, IPv6, ARP, and more.
In some Ethernet standards, the Length field, also a 2-byte (16-bit) field, specifies the size of the payload in bytes. Understanding the length allows the recipient device to efficiently parse and process the frame.
What is a Data Field in an Ethernet Frame Format?
The data field is the actual contents of the message, carrying the payload of the Ethernet frame. It must be less than or equal to 1500 bytes in standard Ethernet frames, but it can contain virtually any type of information.
It's essential to note that the data in the payload adheres to the protocol specified in the EtherType field of the frame. The data field is considered the most important part of the frame, containing meaningful information, while other components, such as MAC addresses and frame control information, can be seen as metadata facilitating the transmission process.
What is CRC (Cyclic Redundancy Check) in an Ethernet Frame Format?
The CRC is used to handle any errors or corruption within the frame itself. It is synonymous with FCS (Frame Check Sequence) and can be used interchangeably. The CRC is a checksum value calculated from the contents of the frame, including the destination MAC address. The whole process occurs in four steps. Let’s go over them briefly.
Calculate CRC value: Before sending an Ethernet frame, the sending device calculates a CRC value based on the frame's content.
Append CRC value to frame: This CRC value is then appended to the end of the frame.
Recalculate CRC value: When the frame reaches its destination, the receiving device recalculates the CRC based on the received frame (excluding the CRC field).
Verify the CRC value: If the recalculated CRC matches the received CRC, it indicates the frame has likely been transmitted without errors. If there's a mismatch, it suggests errors may have occurred during transmission.
Final Thoughts
Ethernet frame format is a fundamental concept in networking. Its raison d'etre is to ensure data is received reliably and consistently through a standardized format. The format is universally accepted by all nodes, and consists of seven parts: preamble, SFD, destination MAC address, source MAC address, length/type, payload, and CRC. Remember, a VLAN tag can be added to the Ethernet frame if the network has designated sub-LANs.
As a network engineer, or anyone preparing for the Network+, having a firm grasp of Ethernet format will prove vital in troubleshooting and scanning networks. Make sure this critical knowledge is in your skill repertoire!
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