we have been talking about the data link clear so far.
One of the confusions that people have
with the data link clear and I will clean that up right now is let's say you have a switch.
Let's call this which one, as always,
and you have to PC is connected to the switch
PC to, and we're not gonna worry about the poor stay connected on
now. Earlier, I said that the data link lair
deals with transmission over the local data link, which means
the link between two devices to directly connected devices.
Now, people take that to mean that this link between PC one
and switch one is one data link.
No, that is incorrect. The switch itself
So when you draw the switch out in a diagram
that is considered a physical diagram
in a logical diagram, I would draw the same topology
as such PC one and PC to, And it is assumed that there is a switch in the middle because you really can't buy a wire at best Buy that looks like that.
So ah, switch is one data link. Everybody connected to that switch all PCs or devices that are connected to that switch are on the same data link.
So this link between Switch one and PC one No, that's not a data link. The link between PC one and PC to is the same data link,
the whole function off a switch in the process that I described earlier on how Off Switch operates is called transparent bridging.
So switch is nothing more than a bridge between PC one and PC to and from the point of view of these PCs, it is transparent. To them. It doesn't exist.
So in a logical topology you will draw out the PCs or the end devices connected to the switch, but not the switch itself.
Now, having said that, we can move on to the concept off broadcast and collision domains. So first broadcast of
what is the broadcast on me?
a broadcast domain is the area off a network
where a broadcast will spread out, too,
if sent by one device.
So let's say you have a switch with
PC is connected to it, and one PC sends out a broadcast frame. Remember the broadcast frame being one with all F's in the destination portion of the frame.
How far will that broadcast? Send by one PC Spread out, too.
So by default, all switches
aural Cisco switches are one broadcast domain,
so if you have a switch, it is one whole broadcast domain by default.
Routers, on the other hand, brake up broadcast. Oh means
so. A broadcast cannot transfers a router.
our denoting a router
and you have a switch
attached and some PC. Let's call it P. C. One
sends out a broadcast frame.
That frame will reach the router, and the router will drop it by default.
This broadcast cannot transfer verse
so routers break up. Broadcast. Oh means a switch is one big broadcast domain in itself. And, of course, the hub always flood, so hub is one big broadcast don't mean to
now collision domains
commit Collision Domains are areas of the network
where if two devices transmit at the same time
and they are running half duplex Internet
and if they're running hard to put Internet and both devices transmit at the same time the frames that those devices send can collide.
So a collision domain is an area of the network where collisions can occur.
each port on a router and each port on a switch is its own collusion domain.
So once again, each port on a switch and each port on a row our is its own collision domain.
Ah, hub is one big collision domain.
So obvious. One big collision domain. One big broadcast domain.
A switch is one big broadcast. Oh mean
and each port on a switch and a router
is its own collision domain.
Now, collisions can only happen when you're running half duplex Internet
Before I draw the diagram
for broadcasting collision domains, I have to explain what eternity ISS
Eternity is a layer to protocol that runs at 10 megabits per second
So the speed is 10 megabits per second, and it runs at half Duplex, which something called C. S. M. A. C D.
Which stands for carrier sense multiple access with collision detection.
So first, let me take care off the duplex part. If it
Internet runs at half duplex,
have duplex means that bi directional communication is possible, but only in one direction at a time, and as an example, you can think of a walkie talkie
with the walkie talkie. Only one person can be talking at a time. Both people can't talk at the same time.
Full duplex in opposition means bidirectional communication is possible. You can transmit both ways and simultaneously, which is at the same time, and you can think of that as a phone conversation.
And then you have simplex, which is only unit directional communication but
not part of the CNN.
The C S M S C D. Part or carrier sense multiple access with collision detection part
when you're running half duplex eternity and two devices transmit at the same time. When it's not allowed,
the frames can collide.
And when this collision happened, see, SMS CD is constantly listening in on the while.
And when C. S. M S E. D, which is built into half duplex Ethernet,
here's a collision. It sends out a jamming signal to all the devices,
and it asks all the devices to stop transmitting for a randomized time period. So let's say if they have four devices and one of them gets told, do
stopped transmitting for 10 milliseconds and another one gets told to stop transmitting for 50 and 1/3 1
for 12 and 1/4 1 400 milliseconds When they try to retransmit, the chances are lesson that they will re transmit at the same time. And hence the collision avoidance comes in. Hence the name
carrier sense multiple access carrier sense. I'm listening to the wire. Multiple access. More than two people can be on the media or on a switch at the same time
with collision detection now have said that each port on a switch is its own collision toe.
is that true? Now it is No, because
by default running full duplex mode.
So in full duplex Internet bi directional communication can happen at the same time. You don't need C S M a. C D two devices can't transmit at the same time. And collisions won't happen,
however, since each port on a switch is its own collision domain.
well, if a company does not have the money to spend and they do something like this, they put ah huh on one of the switch ports. And then they put a bunch of PCs
hubs by default or half to place.
Then you're gonna have bunch of collisions when these PC strategy transmit through the hull,
so each port is its own collision domain.
However, if you connect a hug to one of the port, of course the device is connected to the hub
will have collisions.
such a diagram or such a question being asked on the CCN exam, so I'm going to go through it real quick.
So let's say we have four PCs attached to the switch and then you have a router,
and then you have, ah, hub on this end and you have to PCs attached to this hub.
We'll make them PC for
Now the question would be how many broadcasts and collision domains are in this apology
or this network diagram Topology being synonymous with network diagram?
since I said, each port on a switch is its own collision domain, and each port on a router H Ethernet port on a router is its own collision domain. We have one collision domain here, one
then the four ports that are connected to the PCs 2345 And then this side, since it as a hub is one big collision domain.
collision domains. How about broadcast? Oh, means, well, let's say PC to sends a broadcast out.
How far will that broadcast Spread
PC 13 and four will get that broadcast, and the router will get hit with that broadcast. But the router will not allow it to transfers.
So you have one collision broadcast domain on this side. You're one broadcast domain on
my left hand side of the router
and then one broadcast domain on the right hand side of the road.
in this topology, you have
by default. Remember, I keep saying by default, and that's for a reason.