- [Voiceover] Half-Duplex also uses the CSMA/CD protocol
that I just discussed, to help prevent collisions
and to permit re transmitting if one occurs.
Now if a hub is attached to a switch as shown in
the graphic, it must operate in Half-Duplex mode
because the end stations must be able to detect collisions.
Now theoretically all hosts connected to the switch
in this figure can communicate at the same time
because they can run full duplex.
Now just keep in mind that the switch port
connected to the hub, as well as the host connecting
to the hub must run at Half-Duplex using CSMA/CD.
The reason you don't need to worry about collisions
on the switch is because now it's like a freeway,
with multiple lanes instead of the single lane
road provided by Half-Duplex.
Now let's talk about that interface on the switch
that the hub is plugged into, which is interfacef01.
Let's say you get a call that everything in
your network is going slow, or someone is complaining
that the network is going slow, and you
need to find out what's going on.
Let's go out to the interface and
take a look at some statistics.
To tell if you're having a duplex issue,
you want to look at two things:
the input errors and the late collisions.
If both of these are incrementing,
then this shows that you have a duplex issue
with the device connecting into this port.
This will make your network go really slow.
If you had a broadcast storm for example,
you would have two statistics updating,
and that is the "no buffer" and the "ignored",
so I don't have any room in my buffer,
because I have received more frames than I can process
and we're ignoring any new frames that come in.
So to recap, for duplex issues on an interface,
we can see that the input errors and late collisions
are incrementing, and if we had a broadcast storm we have
the "no buffers" and "ignored" statistics incrementing.
Now let's get into how Ethernet addressing works.
It uses the Media Access Control or MAC address
that's burned into each and every
Ethernet network interface card.
The graphic shows us that it's a 48 bit MAC
address and how the bits are divided.
The MAC or hardware address is a 48 bit
6 byte address written in hexadecimal format.
Now the first 24 bits are comprised of what's
called the Organizational Unique Identifier or OUI.
It's assigned by the IEEE to an organization.
It's comprised of 24 bits or 3 bytes,
and in turns assigns a globally administered
address also made up of 24 bits or 3 bytes,
that's supposedly unique to each and every adapter
and organization manufacturers.
Now the data link layer is responsible for
combining bits into bytes and bytes into frames.
In this graphic, we can see an Ethernet frame
that's used at the data link layer to encapsulate
packets handed down from the network layer
for transmission on a type of media
access, in this case, Ethernet.
Now the functions of Ethernet Stations is to
pass data frames between each other using a
group of bits known as a MAC frame format.
This provides error detections from a CRC,
or cyclic redundancy check in
the frames check sequence field,
and remember this is error detection, not error correction,
and lastly, we can see that we have our destination MAC,
our source MAC and this is referred
to as the ether-type field,
and what this does, and this important for you,
is it finds who owns this packet,
and in this case we'd have 0x800 because we're
saying IP owns this packet, which is IPv4.
If it was IPv6, it would be 0x866,
so what's nice about this Ethernet frame is we can use
it with any routed protocol at the network layer.