What is an Ethernet Frame?

John Lister

An Ethernet frame is a way of arranging sections of data for transfer over a computer network. The frame is the one of the key elements of the Ethernet system, one of the most popular types of local network. It is made up of three elements: a pair of addresses, the data itself, and an error checking field.

Cat 5 cable with RJ45 plug.
Cat 5 cable with RJ45 plug.

Strictly speaking, Ethernet is the name for a collection of technologies and systems rather than the name for a network itself. Ethernet is commonly used for local networks, where the connected computers are in the same physical location such as a household or office building. The name Ethernet is also used for a specific type of physical connection, namely cat5 cable with an RJ45 plug. This is better known as an Ethernet cable and plugs into virtually all computers and other network devices.

Ethernet cables plugged into an Internet switch.
Ethernet cables plugged into an Internet switch.

An Ethernet-based network uses data packets to transmit information. This means that the files or commands being transferred are broken up into small collections of data. Each of these collections is then bundled together and sent as one piece, with the “packets” reassembled at the other end. The idea is that if there is a problem with transmitting one particular packet, the rest of the data can continue to be transmitted while the computers try to solve the problem.

The packet contains three types of data, arranged in an Ethernet frame. The first type of data in each packet consists of the address fields, which identify the particular computers which are sending and receiving the data. This ensures it doesn’t wind up at the wrong machine on a network.

The second type is the actual data which is being transmitted. Each packet contains between 64 and 1,500 bytes of data. This means that even a file as small as 1 Mb can be split into a thousand or more packets.

The third type of data in an Ethernet frame is the error checking field. This involves creating a code number which varies depending on the length and contents of the data contained in the packet. When the recipient computer gets the packet, it will use the same system to create its own code number from the data it receives. If the two code numbers do not match, the machine will know some of the data is either missing or incorrect and will request that it is sent again.

One way to understand how the different elements of an Ethernet frame work in a data packet is to imagine a physical packet sent through the mail. The address fields are equivalent to the delivery and sender addresses written on the packet. The data itself is equivalent to the contents of the packet. The error checking field is equivalent to a list of contents placed inside the packet so that the recipient can check everything is there.

A network cable tester is a necessary tool for troubleshooting issues with an Ethernet frame.
A network cable tester is a necessary tool for troubleshooting issues with an Ethernet frame.

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