Here's a common network configuration
for a SOHO, a Small Office/Home Office.
You have an internet connection.
That internet connection usually has a router.
Sometimes, the router and the switch and the access point
are inside the same device.
But I've broken them out to show them differently
on this diagram.
And you might have some devices that
are connected over the wireless access point and others that
might be directly connected to the switch using a wired
Ethernet cable.
If you're sending information from one of these devices,
you might send it out over the wireless network to the access
point.
That would then transfer it to the switch,
which sends it to a router, which sends it out
to the internet.
The real challenge for the network administrator
is to determine what the route would be for this traffic
and determine what type of equipment
you would need to be able to provide this connectivity.
On an enterprise network, we add a bit more complexity,
and certainly hundreds or even thousands of individual users.
They might all be in a large office
building with multiple floors.
And in this design, I have core switches
that are on the first floor.
And each one of those switches is connecting the switches
on the second, third, and fourth floors.
You can see that those switches are then
connected to end users' web desks on that particular floor.
To be able to send traffic from one place to the other,
they would then be set up to a particular floor,
out to the user, and then sent back again
in reversing the direction.
We could even have a campus of these buildings all
connected together with these core switches
in each individual building.
And you can see that the complexity of designing
an Ethernet network can increase rapidly,
depending on the number of devices, number of locations,
and number of infrastructure components
that you would add to the network.
To understand more about how traffic
gets from one side of the network to the other,
it'd be useful to examine what's inside of an Ethernet frame.
I have a representation of an Ethernet frame
here at the bottom.
And let's step through each one of the fields
that you would find in this frame.
Let's start with the preamble.
The preamble is 7 bytes long.
And it's a series of 56 alternating 1's and 0's.
This preamble is a notification to a device
that everything after the preamble
is going to be an Ethernet frame.
Next, we have the beginning of the frame.
It has a specific name called the Start Frame Delimiter.
It's 1 byte long.
And this designates that the preamble is over
and we're now ready to begin the main part of the Ethernet
frame.
The first real Ethernet data we'll see in this frame
is the 6-byte destination MAC address.
This is the destination for the data that's contained
within this Ethernet frame.
Right after that destination MAC address
is the source MAC address, also a 6-byte frame.
And both of those together determine
where the traffic is going and where it originated from.
Right after the source MAC address is an EtherType field.
This is a small, 2-byte field that
describes what comes next inside of this Ethernet frame.
We can put all kinds of different data inside
of Ethernet.
That type field will help the receiving device
determine how the data inside of this frame should be handled.
Next is the payload.
This is, obviously, not to scale because the payload
can be a variable amount of data between 46 and 1,500 bytes
if you're not using jumbo frames.
We would put our Layer 3 data inside of that payload.
So if you were looking for the IP traffic,
it would be contained within that payload field.
And lastly is the FCS, or the Frame Check Sequence.
It is a cyclic redundancy check that
allows us to make sure that everything that we've received
in that Ethernet frame has not been corrupted as it was
transmitted across the network.
Although you don't have to rebuild an Ethernet frame
as part of your Network+ studies,
it's useful to know that inside of this Ethernet frame are
things like the destination MAC address and source MAC address,
which are very useful for determining how switching is
used on our network.
We also have this payload, which is where the IP traffic is.
And that's the bit of data that's
used by routers to be able to send traffic
between different IP subnets.
Let's get more information on these MAC addresses.
Inside of this Ethernet frame is a destination MAC address
and a source MAC address.
That term "MAC" stands for Media Access Control.
It is the physical address of your Ethernet adapter.
And every device has a unique MAC address.
Ideally, there would not be another MAC address
in the world that matches the MAC address on your Ethernet
card.
The MAC address is 6 bytes long, or 48 bits long.
And it's common to show the MAC address in hexadecimal.
For example, this is a very common MAC address,
8c:2d:aa:4b:98:a7.
All of that together is an Ethernet MAC address.
It's not only common to display it in hexadecimal form,
but it's also common to add delimiters
make it a bit easier to read.
The first 3 bytes of the MAC address
are what we call an Organizationally Unique
Identifier, or OUI.
You can think of this as the manufacturer
ID for this Ethernet adapter.
And every manufacturer has an OUI
that's been assigned to them.
And in some cases, manufacturers may have multiple OUIs
so that when you see the first 3 bytes of a MAC address,
you know exactly who made that adapter.
The last 3 bytes of the MAC address
are a network interface controller-specific value.
You can think of this as the serial number for this Ethernet
adapter.
Although your Ethernet adapters may have the same 3 bytes
because they're all made by the same manufacturer,
you'll find that all of your MAC addresses
will have different last 3 bytes because those
are a different serial number for every individual device.
Another concept we work with quite a bit in Ethernet
is one of duplex.
Duplex describes how we're able to send
data between two devices, either one at a time
or at the same time.
Half-duplex is a device that cannot send and receive
information at the same time.
You're either sending data or receiving data,
and not doing both of those at the same time using
half-duplex.
If you're connected to an Ethernet hub,
then you're using a half-duplex device.
And there's no way to perform full-duplex communication
with a hub.
Switch interfaces can be configured either
as half-duplex or full-duplex.
But you will very commonly find most switch connections are
automatically configured to always communicate over
full-duplex links.
With full-duplex, we're able to send and receive
data at the same time over the same Ethernet interface.
This is the most common configuration for switches
because it provides us with the highest amount of throughput.
The half-duplex operation of a hub
is not one we commonly see these days.
But it's useful for understanding how half-duplex
and full-duplex might operate.
Here we have a hub, and we have multiple users
connected to that hub.
Sam is going to be sending information to the SGC server.
But Sam doesn't know where that server is on the network.
All Sam knows is that she's going
to set that traffic to a hub.
Since a hub doesn't have any intelligence or knowledge
of where any other device is located on the network,
the only way that it can communicate to the SGC server
is to send that traffic out every other interface
on the switch.
Obviously, every device connected to this hub
is going to receive a copy of that traffic.
But only the SGC server will use that traffic
because the destination MAC address
is specific to the SGC server.
I mentioned that with half-duplex, you
can either send or receive at the same time.
But there may be times where two separate devices begin
sending traffic simultaneously.
And when that occurs, we have a collision.
Let's take this example, again, of Sam
sending information to the SGC server.
But at the same time, the SGC server
is going to send traffic.
And somewhere on that network, those signals
will conflict with each other and create a collision.
All of the devices on the network
will hear this collision occurring.
They will wait a random amount of time
and then try to resend that traffic over the network,
hopefully preventing a collision from occurring again.
This basic functionality of half-duplex Ethernet
is described as CSMA/CD.
Let's break this apart.
We'll start with the CS.
This means Carrier Sense.
Whenever you connect to a network,
the Ethernet adapter needs to identify
that it is connected to a network with other devices.
And it senses a carrier that it's
able to use to communicate across that network.
Next is the MA.
The MA stands for Multiple Access.
And that designates that you can have
many different devices communicating
on the same Ethernet network.
The CD at the end stands for Collision Detect.
And we've already talked about how collisions
occur on the network and how all of the devices
can detect when a collision happens.
It's important that all of the devices
are able to detect that collision so that they can all
back off, wait a random amount of time,
and then try transmitting their data.
CSMA/CD and this idea of detecting collisions
is, obviously, only required if you're
on a half-duplex Ethernet connection
because with full-duplex Ethernet,
there's no collisions.
From a single workstation's perspective,
CSMA/CD means that it will listen to the network
to see if there's any information currently being
transferred.
And if there isn't, it can then send some traffic
over that Ethernet network.
Notice that there's no prioritization.
There's no queue.
There's no check-in process.
A device simply listens to see if everything's clear
and then tries to send traffic over that Ethernet connection.
This, obviously, means that two devices
could communicate at the same time, creating a collision.
Those devices will then send a signal called a jam
across the network, letting everyone know
that a collision has occurred.
And it, effectively, clears out the network
and lets everyone know to wait a random amount of time
before trying to retransmit that data.
Let's now look at a full-duplex network.
And to be able to communicate with full-duplex,
we'll get rid of our hub and we'll
replace that hub with switches.
When Sam sends some traffic over the network,
Sam's MAC address will be the source of that traffic.
And it's going to be sending information to the SGC server.
And Sam has already determined what the MAC address
is of that server.
Sam is going to send that frame to switch A. Switch A will look
through this frame at the destination MAC address,
determine that the best way to get to that destination MAC
address is to send that frame to switch B.
Switch B will perform the same analysis of this frame.
It will look at the destination MAC address.
It will identify that the destination in this frame
matches the destination of the SGC server
that's currently connected.
And it will send that traffic down to the SGC server.
You'll notice that there was no flooding of frame
across the network.
We had a very directed frame that
went from Sam's workstation directly to the SGC server.
In fact, because this is a full-duplex connection,
we can have Sam and the SGC server
sending information at the same time over the same switches
and the same wires of Ethernet.
And they're able to do that without having any collision
because with full-duplex, you can send and receive data
simultaneously.