Broadcast Domains and Collision Domains – CompTIA Network+ N10-006 – 1.8
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Broadcast Domains and Collision Domains – CompTIA Network+ N10-006 – 1.8

Two networking terms
that you may have heard are broadcast domain
and collision domain. And although they have
domain in their name these really are two
very different things. A collision domain is something
we used to be very aware of. These days it’s more of
a historical footnote in networking. This was a term
that we used back in ethernet networks
that were not communicating with switches. In those scenarios, we were
in a half duplex communication where one person
could communicate or they could receive,
but you could not do both at the same time. With today’s full duplex
communications and connections to our switches we don’t tend
to see collisions any longer. The word collision was
also a bit of a bad word to use to describe
what was happening on an ethernet network when
two devices communicated at the same time. Yes, the communication
didn’t work properly, because the collision
of information occurred on that single bus. But, it isn’t
necessarily a bad thing. This was a normal part
of ethernet communication you expected to see
collisions occur, which are little bit different
than the negative connotation of the word collision. The reason these
collisions occurred is that the entire network
was one big ethernet segment and everybody on
that segment was communicating at half duplex. That meant that while you
were sending information, you could not
receive information. Why you were
receiving information, you couldn’t send information. It was like everybody being on
one big conference call, only one person can talk at a time. And if more than
one person starts talking on the conference call,
they may not even realize it. Then you have two
people communicating. You don’t hear
what either of them said and they’re going to have
to repeat that information. So on these half duplex
ethernet networks only one device can
communicate at a time. And it determined whether
it could communicate using something called CSMA,
which stands for Carrier Sense Multiple Access. The devices we’re listening
for a carrier and there were multiple devices
on the network that could communicate at one time. The other part of the ethernet
communication mechanism was the Collision
Detection or the CD. Devices when they were
sending information needed some way to tell if that
information was being collided with someone else who is trying
to talk with at the same time. There was a difference on
the wire and voltage when that occurred and both stations
then realized both of us are communicating
simultaneously. We need to now clear the wire by
sending something called a jam signal and then everybody waited
for a random amount of time. And then could resend
that information and try to get it
onto the network before anybody else
was communicating. In the early days of
ethernet, we communicated over coax networks and
there was a single wire that everybody connected to. And that single wire
or single ethernet bus was one big collision domain. Everyone could
hear everyone else when they were communicating
and if more than one device was to send simultaneously,
we would have a collision. When we upgraded to
twisted pair we still had a single collision domain,
because we were using hubs. A hub is effectively
a multipoint repeater, which means we have all
these devices plugged in. And when traffic comes
into one interface, it’s automatically sent to every
other interface on the hub. It was as if we collapse that
bus all into one single device and it all now exists
inside of this hub. There was no separation
of the signal. When one person
communicated everybody else on this collision domain
could hear that communication. One way to separate
out a collision domain would be to separate the
network by using a bridge. These days a bridge would
take the form of a switch. So if we added a switch into
the middle of the network one side of the network would
then be one collision domain and the other side would be
the other collision domain. By adding the switch into
the middle of the network, we were minimizing the number
of devices on a single network segment. Therefore, minimizing the
number of collisions that would be occurring on
that network segment. These days every single
device is connecting directly to a switch and we’re generally
connecting up at full duplex. So there’s no possibility
that more than one station could ever be involved
with a collision. So because of that, you can
think that every device would be on its own collision domain. And even though we call
this a collision domain because we’re running
at full duplex, we’ve effectively
removed the chance of ever having a collision. A broadcast domain is the size
of the network that’s impacted when a broadcast is sent. This is something that
is a necessary evil. We need these broadcast
to be able to perform our probes over the network
or for operating systems to advertise a service. But we have to be
concerned about this, because every device
on the network is listening for broadcast. It receives the
broadcast that are sent and it has to at
least examine what’s inside of those broadcast,
whether it pertains to that station or not. Unlike a collision
domain, a broadcast domain passes right through a switch. If you send one
broadcast into a switch, the switch will
send that broadcast to every other interface
on that switch. A broadcast domain
only stops at a router. That is the device that
will not allow a broadcast to pass through it. So if we’re going to
examine where our broadcast domains might be,
you’ll notice they stop every time there is a router. These are important
things to consider, because the more devices
you put on a subnet, the more devices you connect
to a switch, the more broadcast you’re going to have. And eventually, you’ll
find these broadcast create some overhead for every
single device on the network. So you want to try to minimize
the impact of broadcasts. If we place a router in
the middle of the network, we can separate out
these broadcast domains. So on the left side, we
have a switch and a network. If they broadcast is
set from this network, it stops at the router. So this is one broadcast domain. If broadcasts are
set on this network, they would communicate to all
the devices on this network. But again, would be
stopped by this router. Which means we have two separate
broadcast domains, both of them separated by this router
right in the middle.


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