>> Even though the OSI model is a top-down model,
meaning when a sending computers
sends a request to a receiving computer,
the process starts at the application layer and
travels downward through the other six layers.
It's easiest to understand
>> networking from the bottom up
>> because the simpler devices
are at the bottom of the OSI model.
We're actually going to start in the reverse order.
We'll start with Layer 1 and discuss the physical layer.
The physical layer is made up of "Dumb devices."
Meaning those devices don't know where
data is going and they don't care.
They don't direct traffic or airwaves.
It just simply physical connectivity to the network.
When we talk about that, think about cable.
Cable doesn't know where data is going,
and cable doesn't know what type of data it is.
Cable just provides a pathway conduit for the signal.
Connectors like your RJ45 or BNC connectors,
just provide that connectivity to the network.
I'll also mention hubs here,
because at one point in time,
hubs were not even powered,
they were just racks that you plugged into and
that metal rack provided a pathway for signals to travel.
Hubs, even the ones we use today,
powered hubs, just amplify the signal.
There is no direction or error detection,
just a pathway for signals to travel.
As we go up the OSI model, we gain intelligence.
The next layer up is Layer 2,
which is the data link layer.
The data link layer is the only layer of
the OSI model that has two sublayers.
Those sublayers are made up of the first,
which is LLC, and that stands for logical link control.
That layer is responsible for error detection.
We won't say much about LLC that's
not relevant to the net plus exam.
What we do want to focus on is the second sublayer,
which is the MAC sublayer.
MAC stands for media access control.
We have MAC addressing and we also have media access,
which is which system gets to communicate and when?
With MAC addresses, the first thing we want
to look at is the MAC address itself.
A MAC address is
a 48-bit address expressed in a hexadecimal.
I have this circled for you
here on our example on the left.
Hexadecimal separated by dashes or hyphens,
they have a 48-bit address.
The first 24-bits are specific
to the manufacturer of the device.
If we all had three com network cards
or something from the same manufacturer,
it wouldn't be unheard of if
the first 24-bits are the same
for my MAC address as yours.
Now of course, manufacturers are
given a wide range of addresses,
but it wouldn't be impossible.
The last 24-bits are unique to
the host and unique to the network interface card.
Even if we do have the same first
24-bits our host address will be different.
This is called the physical address or hardware address,
but it's a MAC address.
How it's referred to will really
depend on the operating system.
Ultimately, what we're talking about is
the hardware address and the network card.
The screenshot that I have here on the left was
used by a command called the IP config,
which shows not just IP configuration,
but also shows MAC address.
MAC addresses are very different from
IP addresses. Definitely know that.
IF config is a similar command that Unix
uses and folks that use Unix will use.
Ultimately, these show the MAC address.
I do want to take one more minute to stress
the importance of a MAC address.
When traffic is on the network
an individual NIC network interface controller
is going to examine the packet
and look for its own MAC address.
If the packet is destined for my MAC address,
my network card pulls out off the network.
If it's not, it leaves it here.
What's critical for a host receiving
data is the systems MAC address.
What that also means is that as ascending host,
I'm going to have to learn the client's MAC address.
The way that happens is with a protocol called
ARP, Address Resolution Protocol.
What ARP does, it's broadcast space.
Essentially it sounds at a broadcast that says,
"Hey, is anybody 192,
168, 11" and that device,
we'll come back and say, "That's me,
here's my MAC address."
What you can see on the left is that I add
the information to what we refer to as our ARP cache.
Anytime you hear the term cache,
cache is always a place where we store
things that we think we're going to need again.
Once my computer learns the MAC address,
the specific IP address,
it will store that in cache.
The next time we need to go to 192,168,
11, I don't have to broadcast out,
it's already there in my cache.
Cache is very helpful.
Later we'll talk about some security issues
that might be associated with cache.
But cache also really
does speed things up and make things quicker.
Now, I'll also mention when we
talk about media access control,
who gets time on the cable.
Quite honestly, we can't even say
cable because there's wireless access.
Air traffic across the airwaves.
One of the main types of
networking technology that we
use today is called Ethernet.
It falls in IEEE standard 802.3.
If you want to remember that
Ethernet has three E's in it,
so 802.3 is Ethernet.
Ethernet uses a media access method called CSMA CD,
that stands for Carrier Sense Multiple Access
with Collision Detection. That's a mouthful.
Carrier Sense Multiple Access with
Collision Detection but it
actually is exactly what it sounds like.
If a network card has data to transmit,
it senses the cable carrier sense.
The trick with that is the two systems could be
sensing the cable at the exact same time.
Multiple access, they both throw their data out there.
But if two hosts put their data on
the cable at the same time, we have a collision.
The network card is able to detect that collision back
off and perform an algorithm
to determine whether they can retransmit.
CSMA CD,
Carrier Sense Multiple Access with Collision Detection,
expect collisions in an Ethernet environment.
Another media access method that we don't see
very much today is token passing.
If you're familiar with the old token ring networks
that were around in the 90s and early 2000s,
basically, there was
a 24-bit control frame on the network.
It would move from host to host to host.
If a system wanted to communicate,
it would capture the token,
then put its message out there.
There was only one token
and you couldn't transmit without it.
We actually had no collisions
in a token passing environment.
That was one of its benefits.
But token ring, which was the technology that
use token passing, was proprietary.
It was from IBM.
It was expensive, it was difficult to work with,
and we were bound to IPM as in vendors.
Ethernet really won out there.
There's also another media access method
that wireless communication uses.
That's the 802.11.
802.11 is CSMA CA,
Carrier Sense Multiple Access with Collision Avoidance.
Essentially what wireless systems do is
it's still senses whether or
not anybody is communicating.
Multiple systems can sense that at the same time.
But instead of sending that data out,
they send an intent message that essentially says,
"Hey, I'm getting ready to send.
Is that cool with everybody?"
If there are no other hosts coming
back saying I'm sending two,
then the wireless device transmits this information.
We don't have collisions in
a token environment or in a wireless environment.
We deal in Ethernet and whatever a big challenge is,
is going to be, how we're
going to address those collisions.
This is just a quick summary of the different types of
media access methods in the IEEE over to the left.
I think it's a good summary to have.
Remember, 802.3 is Ethernet,
CSMA CD, we expect collisions.
802.5 is token ring.
Token passing there will be no collisions.
One thing I'll mention, you can
remember that token ring is
802.5 because there are five letters in token,
then we have wireless 802.11,
no collisions here because we transmit
our intent in 802.12.
I didn't mention this isn't something that I would
anticipate using or seen on the exam.
I just have it here because this
was used at one point in time.
It's pulling where there is a specific survey that
pulls network devices to
determine if they want to communicate.
That's not really an environment
designed for speed or performance,
it is really fallen by the wayside.