In this video, I’m gonna show you how data flows through devices in a network.
So how does data flow for example, through a hub or a switch or router?
Is the flooded out of all ports or is it only send out of an individual port?
And when I talk about data, I’m implying that on a switch a frame is forwarded out
of a port and on a router I’m talking about a packet is forwarded out of a port.
So in networking, it’s important to understand those terms and it’s also very
important to understand how traffic flows through devices in the network.
If you have a router where does the traffic go?
If you have switch, out of which port does the traffic go?
So let’s look at how data flows in a network.
We firstly gonna start off with an introduction to the different types
of communication that takes place in the data network.
We gonna look at unicast traffic vs. multicast traffic vs. broadcast traffic.
I’m then gonna explain the term CSMA/CD which means
Carrier Sense Multiple Access Collision Detection
I’m then gonna spend quite a bit of time
looking at the differences between hubs bridges, switches and routers.
And I’m gonna show you how packet flows through those different devices.
I’ll be referring to the OSI model throughout this video.
I’m assuming at this point that you understand what the OSI model is
and the different layers of the OSI model.
If you’re unsure review the OSI video that’s part of this course.
So in this video I’m not gonna give you the details of where a device
in the OSI model resides, I’m gonna refer to terms
such as a router is a layer 3 device, a switch is a layer 2 device.
If you aren't sure about what that means please review the OSI video.
So let’s start off with the basic types of communication
that you'll encounter in the data network.
This is one of those fundamental building blocks that you need to understand.
So the first type of communication is unicast.
Unicast is essentially where one device speaks to one other device.
It's one to one communication, in other words, device A is communicating with device B
the second type of communication is a broadcast, a broadcast is where one device
sends traffic to all devices in the same subnet or in the entire network.
There are different types of broadcast and we'll talk about some of them later.
But as an example a broadcast to the local subnet means that all devices
in the same subnet have the broadcasting device receive the broadcast.
A broadcast to all hosts or networks means that the broadcast
goes everywhere in the entire network.
However in a lot of real world implementation, broadcast are limited
and blocked by layer 3 devices such as switches and routers.
So a broadcast would typically not go everyway in a network.
Broadcast can cause a lot of problems and are therefore contained or limited
in data networks, but essentially the idea of a broadcast is that all devices
on the local segment or local subnet will receive the broadcast sent by single device.
Now broadcast can be useful as an example if 2 windows machines
are configured with the same name, you'll receive a warning about that because
a broadcast is sent by those devices on the local network.
That’s how the devices learned about each other and realized that multiple
devices have the same name. So broadcast do have the uses but do cause a lot of
problems in IPv4 networks and therefore in IPv6 broadcast support has been removed.
In an IPv6 network broadcast are not supported. Now, what about multicast?
Multicast have some of the advantages of broadcast, in that a single device
can speak to many devices but it gets rid of some of the issue related to broadcast.
Broadcast send traffic to people that don’t necessarily want the traffic
where as with the multicast only devices that subscribe to the multicast receive it.
So devices have to explicitly ask for multicast traffic before they receive it.
So they won’t receive the multicast if they for example
unsubscribe from that multicast.
So to use some analogies, a broadcast is like a spammer were sending mail or email
or traffic to everyone even to people who don’t want it.
A multicast on the other hand is kind of a like an opt in or subscription service
to a mailing list, so you may subscribe to an email mailing system giving information
about Cisco CCNA, you can always opt out of that mailing list which means
you no longer receive the emails, well in theory at least.
So the idea with the broadcast is you will receive the traffic
even though you don’t want it.
The idea with the multicast is you can optionally receive the traffic or
optionally opt out of receiving the traffic.
Now another analogy which is useful in a classroom base environment is
if I were standing at the front of the class as an instructor I would be talking
and everyone in the class would be listening to what I’m saying, that’s a broadcast.
So typically that happens in the morning when the class first starts.
I’m speaking everyone in the room is listening, however typically especially
after lunch an example, the classroom turns into a multicast where some people
have unsubscribe from what I’m saying and that maybe happening right now
which is unfortunate but hopefully not.
Hopefully you're still listening, so I might be speaking in front of a class of 10
people but only 5 of those people are listening to what I’m saying
The other 5 people have unsubscribed from what I’m saying
and are no longer listening to me. That’s an example of a multicast.
so I’m hoping that this video won’t turn into
multicast where you unsubscribe from what I’m saying.