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In taking a look at
where we are today,

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it often is helpful and
useful to take a look at where

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we've come from as well.

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For example, in the
old days when I first

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started working with networking,
we used coaxial cables.

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So we had these coax cables
with terminations on each end.

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And we'd use these
little T connectors

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to connect to each of the
computers with their network

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adapter cards to
this coax cable.

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And what this represented
was one common network

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or one common bus that
everything was connected

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to regarding the network.

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And it was a lot of fun.

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But there were some
problems with this,

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including the fact that if
we had a break in the cable,

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for example right there, the
whole network would go down.

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Well, it wasn't too many years
after that that they came out

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with hubs.

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And this is an example of a hub.

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And here's a graphical
representation

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of the icon for a hub.

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And we think of a hub like
a multi-port repeater.

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It simply takes the bits that
go into one of the ports.

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And it simply repeats them
out all of the other ports.

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So if PC1 on this
port right here

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is sending bits
into the network,

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this hub, acting as a repeater
with a whole bunch of ports,

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simply takes those bits
and spits them out,

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repeats them on
all the other ports

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that are actively connected.

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It's considered a physical
layer or a layer 1 device.

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And I remember being
so darn excited

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about a hub, because
even though it acted

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like a logical bus, where one
device sends and everybody

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else hears, the cool thing
was if we had a failure--

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maybe this cable broke or
that cable had a problem--

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it would only affect
those individual devices

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on those ports.

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It wouldn't affect the
rest of the network.

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So with the hub, it was
physically wired like a star.

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So we have our hub in the middle
and then all of the devices

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connecting off of that hub.

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So it's physically
wired to the star.

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But it logically acts
like a common single bus,

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because every other
device on the network

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gets the opportunity to
see all the bits being

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sent by any device.

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And one of the benefits
of having standardization

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regarding networking
technologies, including

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technologies like
ethernet, is that we

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have standards on MTU,
which is an acronym

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for the maximum
transmission unit,

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effectively, how
big a frame can be

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that's going to be
transmitted on the network.

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If all the PCs on this
network, if they're all

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playing by the same set
of rules and standards,

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including MTU and voltage
levels and so forth,

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it allows these
devices to communicate.

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And we can buy a standard hub
from pretty much any vendor

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at the time that made hubs.

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So even though we have
better technology today,

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let's continue our discussion
with a larger network that's

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still using hubs.

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Because these hubs, every
time they receive bits

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on one port repeated
to every other port,

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if a device on the network
issues a broadcast, everybody

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connected to these hubs is
going to see that broadcast.

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And that concept is
a broadcast domain.

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So if all of these devices are
in the same broadcast domain,

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that means or implies that
if any one device sends

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a broadcast, everybody else
in that same broadcast domain

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will be able to
see that broadcast.

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Now one of the
challenges that comes up

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is what if computer 1 right
here and computer 2 both

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decide that they want to try
to transmit and send bits

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at the same exact time.

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And with these hubs, we have
one giant collision domain

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that doesn't support
multiple people talking

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at the same time.

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So one of the technologies
that was developed

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to address the problem
of two devices talking

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at the same time and
running over each other

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is the technology
called CSMA/CD.

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And that's an acronym for
carrier sensing, which

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effectively means each computer
before it sends on the network

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is supposed to
listen and make sure

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that no one else is talking.

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Seems pretty courteous.

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Multiple access,
the MA, that means

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there's multiple devices that
may be accessing the network.

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And then the CD portion stands
for collision detection,

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which means that if
two devices are sending

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on the network at the
same time, they'll

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see that they're walking
on top of each other.

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Then they can back
off, stop, listen.

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And they can try again
when they think it's clear.

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So whether we were running
on coax cable or using hubs,

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CSMA/CD was the technology
that allowed it all

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to happen to that same network
where those devices were

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in the same collision domain,
which effectively means

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they have the potential
or opportunity

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to collide with other
devices who are also

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trying to send on that network.

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And CSMA/CD wasn't
the only technology

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that was out there to facilitate
multiple devices connected

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to the same network.

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There was also CSMA, carrier
sensing multiple access

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with collision avoidance.

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And the concept is
a device before it

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speaks would send a
little teeny frame.

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And if that frame
made it OK, it knew

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it had a big enough window so
it could send the full message

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and get it sent without a
collision on the network.

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And one of the realities
in current wired networks

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is that we no longer need to use
carrier sensing multiple axis

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collision detection or collision
avoidance on physical networks,

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because we've swapped out, or
we should have swapped out,

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our devices.

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Instead of using
hubs, we're going

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to be using layer 2 switches,
which are more intelligent

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and have some
additional benefits.

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And we'll put those
in a separate Nugget.

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Before we close this
Nugget, I'd like

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to share with you one
other concept regarding

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the types of traffic
that we might

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send from a networking device.

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The first one is a broadcast.

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So let's imagine that computer
1 is sending a broadcast

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into the network that's
intended to reach

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and be communicated to
every other device that's

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in that same broadcast domain.

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However, if we have
a server that's

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generating multicast
traffic, multicast

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is intended to go to a group
of computers, not all of them,

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just a portion.

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So if the intended audience
is, for example, computer 2

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and the wireless device who
are part of a multicast group,

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that's an example of
multicast traffic,

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traffic that's intended
for a group of computers.

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And then last but not least,
we have unicast traffic.

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Unicast traffic
is traffic that's

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intended for one other party.

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A user at this branch
office right here,

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let's say that's Lois.

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Let's imagine that she's
connected to a printer.

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When she sends that
print job to the printer,

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that traffic is intended
for just that one

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device, not a group, not a
broadcast, but just one device.

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That would be an example
of unicast traffic.

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I hope this has been
informative for you.

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And I'd like to thank
you for viewing.