What is a network frame preamble? And how does it work? Why do we need it? Well I'm
going to give you the simple answer to those questions right now. For the best startup
and newbie networking concepts ever taught subscribe to my channel and hit the bell to
be notified when I post a new video once a week. So I talked about the portions of a network frame
in another video but I left the preamble out of that video and for several reasons,
but primarily because when you study textbooks on computer networking they will always tell you the
preamble 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 preamble IS part of
the frame. So for all intents and purposes you can look at that any way you want to,
but if you're asked a question and you're taking a test I would suggest that you consider the
preamble and the start of frame delimiter as part of the frame and we'll just leave it at that. Now,
it's good to know what the preamble of a frame looks like and the two biggest reasons it gets
placed at the start of each frame. I want to show you and tell you what those two reasons are for
using the preamble first, then show you what a basic Ethernet frame preamble looks like. An
Ethernet preamble is used at the beginning of a frame on a computer network for two primary
reasons: Number one the first reason is that is to give the computer or the device receiving the
incoming frame kind of a heads-up that "Hey! There's a frame coming right after this! So
get ready!" And secondly the second reason is that it is to provide a 5 MegaHertz clock timing with
the 1's and the 0's in the preamble to allow the receiving computer or device to lock the
incoming bit stream. So it's kind of to get the timing down on what is actually involved in the
frame before the destination MAC address actually shows up on that network cable on that interface
on you know whatever portion of that frame is coming in. Now those may sound fairly simple but
if you don't have a preamble a receiving machine may actually miss or misunderstand the frame all
together and it may have misunderstand everything that follows the preamble and not actually read
it or understand it correctly on its own network interface card. So that's the reason behind that.
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 preamble or if you have any additional
questions about it and I'll be more than happy to respond as quickly as I can. So those are the 2
reasons the preamble is used and needed. Let's take a look at what a preamble actually looks
like and how it's made up. An Ethernet preamble is made up of 7 Bytes of alternating 1's and 0's and
it actually looks like this. It is immediately followed by the Start Frame Delimiter better
known as the SFD (the acronym for it which is the SFD) is 1 Byte long and it ends in 11. The Start
Frame Delimiter or Start of 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 one of the Bytes in the actual preamble, but it looks like this because it ends
in 1 1 instead of 1 0. So technically the frame preamble overall looks like this...It's 8 Bytes
overall including the SFD. 7 Bytes of 1's and 0's, 1's and 0's, then you had the last byte
which is the SFD. And again 1 1. Again, and I cannot emphasize this enough, you will read in
networking textbooks that the preamble 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 preamble and SFD ARE indeed part of the frame. Easiest thing to
do with it is to 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 Submitting
Cheat Code little subnetting mini-course that will help you score up to 30% higher on just about any
certification exam that can be learned. In it you can learn this in one 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 or business path in life. If you
liked this video, please let me know by liking it below, subscribe to my channel and share this
video with your fellow networkers and comment below with "Helpful" if this video helped you!