In taking a look at where we are today,
it often is helpful and useful to take a look at where
we've come from as well.
For example, in the old days when I first
started working with networking, we used coaxial cables.
So we had these coax cables with terminations on each end.
And we'd use these little T connectors
to connect to each of the computers with their network
adapter cards to this coax cable.
And what this represented was one common network
or one common bus that everything was connected
to regarding the network.
And it was a lot of fun.
But there were some problems with this,
including the fact that if we had a break in the cable,
for example right there, the whole network would go down.
Well, it wasn't too many years after that that they came out
with hubs.
And this is an example of a hub.
And here's a graphical representation
of the icon for a hub.
And we think of a hub like a multi-port repeater.
It simply takes the bits that go into one of the ports.
And it simply repeats them out all of the other ports.
So if PC1 on this port right here
is sending bits into the network,
this hub, acting as a repeater with a whole bunch of ports,
simply takes those bits and spits them out,
repeats them on all the other ports
that are actively connected.
It's considered a physical layer or a layer 1 device.
And I remember being so darn excited
about a hub, because even though it acted
like a logical bus, where one device sends and everybody
else hears, the cool thing was if we had a failure--
maybe this cable broke or that cable had a problem--
it would only affect those individual devices
on those ports.
It wouldn't affect the rest of the network.
So with the hub, it was physically wired like a star.
So we have our hub in the middle and then all of the devices
connecting off of that hub.
So it's physically wired to the star.
But it logically acts like a common single bus,
because every other device on the network
gets the opportunity to see all the bits being
sent by any device.
And one of the benefits of having standardization
regarding networking technologies, including
technologies like ethernet, is that we
have standards on MTU, which is an acronym
for the maximum transmission unit,
effectively, how big a frame can be
that's going to be transmitted on the network.
If all the PCs on this network, if they're all
playing by the same set of rules and standards,
including MTU and voltage levels and so forth,
it allows these devices to communicate.
And we can buy a standard hub from pretty much any vendor
at the time that made hubs.
So even though we have better technology today,
let's continue our discussion with a larger network that's
still using hubs.
Because these hubs, every time they receive bits
on one port repeated to every other port,
if a device on the network issues a broadcast, everybody
connected to these hubs is going to see that broadcast.
And that concept is a broadcast domain.
So if all of these devices are in the same broadcast domain,
that means or implies that if any one device sends
a broadcast, everybody else in that same broadcast domain
will be able to see that broadcast.
Now one of the challenges that comes up
is what if computer 1 right here and computer 2 both
decide that they want to try to transmit and send bits
at the same exact time.
And with these hubs, we have one giant collision domain
that doesn't support multiple people talking
at the same time.
So one of the technologies that was developed
to address the problem of two devices talking
at the same time and running over each other
is the technology called CSMA/CD.
And that's an acronym for carrier sensing, which
effectively means each computer before it sends on the network
is supposed to listen and make sure
that no one else is talking.
Seems pretty courteous.
Multiple access, the MA, that means
there's multiple devices that may be accessing the network.
And then the CD portion stands for collision detection,
which means that if two devices are sending
on the network at the same time, they'll
see that they're walking on top of each other.
Then they can back off, stop, listen.
And they can try again when they think it's clear.
So whether we were running on coax cable or using hubs,
CSMA/CD was the technology that allowed it all
to happen to that same network where those devices were
in the same collision domain, which effectively means
they have the potential or opportunity
to collide with other devices who are also
trying to send on that network.
And CSMA/CD wasn't the only technology
that was out there to facilitate multiple devices connected
to the same network.
There was also CSMA, carrier sensing multiple access
with collision avoidance.
And the concept is a device before it
speaks would send a little teeny frame.
And if that frame made it OK, it knew
it had a big enough window so it could send the full message
and get it sent without a collision on the network.
And one of the realities in current wired networks
is that we no longer need to use carrier sensing multiple axis
collision detection or collision avoidance on physical networks,
because we've swapped out, or we should have swapped out,
our devices.
Instead of using hubs, we're going
to be using layer 2 switches, which are more intelligent
and have some additional benefits.
And we'll put those in a separate Nugget.
Before we close this Nugget, I'd like
to share with you one other concept regarding
the types of traffic that we might
send from a networking device.
The first one is a broadcast.
So let's imagine that computer 1 is sending a broadcast
into the network that's intended to reach
and be communicated to every other device that's
in that same broadcast domain.
However, if we have a server that's
generating multicast traffic, multicast
is intended to go to a group of computers, not all of them,
just a portion.
So if the intended audience is, for example, computer 2
and the wireless device who are part of a multicast group,
that's an example of multicast traffic,
traffic that's intended for a group of computers.
And then last but not least, we have unicast traffic.
Unicast traffic is traffic that's
intended for one other party.
A user at this branch office right here,
let's say that's Lois.
Let's imagine that she's connected to a printer.
When she sends that print job to the printer,
that traffic is intended for just that one
device, not a group, not a broadcast, but just one device.
That would be an example of unicast traffic.
I hope this has been informative for you.
And I'd like to thank you for viewing.