Updated March 28, 2023
Introduction to Ethernet Frame Format
Ethernet frame format is a continuous pattern of alternative occurring 0’s and 1’s which indicates when the frame is starting and creates synchronization among the sender and receiver. Ethernet frame exist in data link layer of the OSI model (which has 7 layers in it) and is a PDU (aka protocol data unit). PDU’s are considered as a single unit of information which are transmitted among the nodes of the computing devices. Every PDU is composed of protocol specific control information and user data.
What is Ethernet Frame Format?
Ethernet is one of the most commonly and widely used networking topology which is preferred for LAN. As a whole it talks about how the wiring and signals are defined among the physical layers in the OSI model, among the 7 OSI layers if defines the frame format and protocols for data link layer.There is a very strong mechanism build inside every data communication which seems very obvious from outside but a complete engineering goes within. So, whenever a computing device tries to communicate with another lay on similar network, the transmitted data gets associated with an Ethernet header and an Ethernet trailer.
These headers and trailers are mandatory for process to work accordingly. Once these headers and trailers get linked to data which tries to move within the network it creates an Ethernet frame. These headers and trailers are very crucial in a way because it captures details related to data which is transmitted – like who sent the data and who to send the data, apart from these the Ethernet data frame also captures error detection functionalities. Every Ethernet frame varies from 64 to 1500 bytes size which depends upon the physical characteristic of Ethernet media.
Ethernet Frame Format
On the process of defining the Ethernet frame format, we have collected some ore-requisite definitions of associated elements which are mandatory to know.
We all know that from time to time enhancements are comings in any technology, in networking these enhancements are known as standards. The Ethernet frame rate talks of 802.3 standards, which are an essential OSI compliant and helps in defining the physical and MAC layer.
An Ethernet frame consists of 3 main division’s.
They are as follows:
- Ethernet header (it consist of Preamble, SFD, Destination, Source, and Type)
- Encapsulated data (it consist of Data & Pad)
- Ethernet trailer (it consist of FCS)
We will start with the header section as it contain the most contents:
- Preamble being the first section and is 7 bytes long which is a combination of strings. These bytes are store within themselves alternate patterns of 1’s and 0’s. During the communication these preamble bytes play a crucial role, they make identify the receiving end (or devices) of initiation of Ethernet frame. It happens to lock the incoming bit stream as the receiving device get to know with these ‘10101010’ as Ethernet frame.
- SFD aka start frame delimiter and is a 1 byte long section which contains string of 1 byte. The SFD byte holds similar byte structure as to preamble but the last bit becomes 1 instead of 0 (for preamble). It is the role of SFD byte to make aware the receiving device about the destination MAC address for the Ethernet frame. SFD – 10101011
- Destination holds the MAC address of the destination device and is a 6 byte long structure. Generally a MAC address is 6 byte of 48 byte. It is the role of destination MAC address to make aware the receiving device determine whether or not the incoming frame is for it or not. If the message is not for any device it will discard the frame.
- Source is again a 6 byte long and contains the MAC address of the source device. It performs the identification of source device by receiving device.
- Type or length field is a 2 byte long and stores the protocol information about the network layer. This 2 byte or 16 bit long filed holds value between 0 to 65534. Also it is important to note that there are two variants under the IP protocols – IPv4 & IPv6, if the type filed holds a value of ox800 then it belongs to IPv4 and if it has 0x86dd then it is a IPv6 protocol.
- Data is a place where actual data are placed, these are also known as Payloads, the Ethernet frame is designed in such a way that both the IP header and data will be used. The maximum byte size can be upto 1500 and minimum 46 bytes. This range of packing (i.e. 46 to 1500) decides the data capacity in one packet. If data is less than minimum (i.e. 46) than padding is added otherwise for more than maximum limit, extra data is packed.
- FCS aka frame check sequence or CRC (cyclic redundancy check) is a 4 byte long field, which checks whether the received frame is attached or not. This section generates a 4 byte long outcome which is placed in the FCS field. Once these are received by the destination devices, they take the same fields form the frame and runs through them with same algorithms.
Need of Ethernet Frame Format
- It is an industry standard, there is no replacement so far in computer networks.
- A predefined structure which we discussed above with predefined advantages.
- Device compatibility across the globe.
- Wide operating range.
- Constitute primarily and work accordance to 7 layer OSI model.
Conclusion
Talking of LAN and Ethernet which have been an industry standards for technology and communication medium and is also globally recognized. These crucial factors give a huge support to this segment of computer networks. These can be considered at very fundamental or root level of communication. This is how the data communication takes place.
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