The Ethernet Frame Preamble

What’s in a network frame preamble and how does it work?

I’m gonna give you the simple answer to those questions right now!

So I talked about the portions of a network frame in another video, but I left the Pre-amble out of that video (for several reasons, but) primarily because when you study textbooks on computer networking, they will always tell you the Pre-amble is NOT part of the frame, but when you actually sit down to take a certification test, the testing material and questions will oftentimes make the assumption the pre-amble IS part of the frame.

Now, it’s good to KNOW what the pre-amble of a frame looks like and the 2 biggest reasons it gets placed at the START of each frame.

I want to show you and tell you what those 2 reasons are for using the Pre-amble FIRST, then show you what a basic ethernet frame pre-amble looks like…

An ethernet pre-amble is used at the beginning of a frame on a computer network for 2 primary reasons:

  1. To give the computer or the device RECEIVING the incoming frame a heads-up that “Hey! There’s a frame coming right after this! Get ready!”
  2. To provide a 5MHz clock timing (with the 1’s and 0’s in the pre-amble) to allow the receiving computer or device to lock the incoming bit stream.

Now those may SOUND fairly simple, but if you don’t have a pre-amble, a receiving machine may actually miss or misunderstand the frame that follows the pre-amble and not read it or understand it correctly on its network interface card.

Feel free to leave a comment below if you’ve already taken a certification exam and you encountered a question on a certification exam about the pre-amble or if you have any additional questions about it.

So those are the 2 reasons the pre-amble is used and NEEDed. Let’s take a look at what a pre-amble actually LOOKS like and how it’s made up…

7 Byte Preamble, 1 Byte SFD

An ethernet pre-amble is made up of 7 bytes of alternating 1’s and 0’s and it actually looks like this:

10101010 10101010 10101010 10101010 10101010 10101010 10101010

It is immediately followed by the Start Frame Delimiter (SFD) which is 1 byte long and ends in 11. The Start Frame Delimiter is the real “heads-up” to the receiving device that “Hey! Important frame information is coming immediately after me, so GET READY receiving device!”

The SFD looks similar to 1 of the bytes in the actual pre-amble, but it looks like this:


So technically, the frame pre-amble overall looks like this:

10101010 10101010 10101010 10101010 10101010 10101010 10101010 10101011

Again, you will read in networking textbooks that the pre-amble including the SFD is NOT actually part of the frame, but on most certification tests (really most networking tests in general) the questions you may get will 9 times out of 10 assume the pre-amble and SFD are indeed part of the frame.

Easiest thing to do with it is know it, know what it does and why it’s needed, and you should be able to get any related questions correct about it (as well as learn to recognize it when you’re looking at a frame capture).

I’ve put together a series of videos that go further in depth about ethernet frames and how to use them and recognize them on a computer network as well as a Free Subnetting Cheat Code that will help you score up to 30% higher on any certification exam that can be learned in 1-minute or less.

Also, if you want to join a community of networkers just like you that are learning about the technologies and getting started, I have a “secret society” where I have up-to-date networking know-how to help you navigate and get started right down this sometimes tricky career/business path in life.

If you like this post, please let me know by liking it below, subscribe to my youtube channel and share this video with your fellow networkers, and comment below with “helpful” if this post and video helped YOU!

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