So in a situation like this, the phone has to indicate to the switch that its traffic is important.
So it needs a mechanism or a way to tell switch one that its traffic is more important than the PC traffic.
Now, there are two main ways to do this.
We either mark at layer two or mark at layer three.
So in the same way that FedEx or DHL will mark a parcel, in other words, they put a sticker or a barcode
on the parcel to indicate that the parcel is of great importance and should be delivered.
The next day, the phone will mark the packets that it sends to the switch, either at layer two or
at layer three, to indicate that the packets are important.
Assuming that switch one accepts those markings.
It will send the packets to switch to with the relevant markings.
Markings are important in packets because when they arrive at a switch such as switch to it needs to
make a decision.
Which class does the packet belong to?
In other words, is a platinum?
Is it gold?
Is it silver?
Is it bronze?
Or to use a airline analogy, is it first class or business class or economy class?
Packets within a class will be treated the same way, so the switch needs to determine which class the
packets go into.
So again, platinum, gold, silver and so forth, and then it will treat those packets accordingly.
So how does a phone or a switch indicate to another switch or rather the level of importance of packets?
Now, to use the airline analogy, again, this is very similar to a ticket that you get.
So does your ticket indicate that you're a first class passenger or business class or economy class?
Think of the markings as a ticket.
The ticket is an indicator of the service that you'll receive.
So when the phone sends a voice packets to the switch, it's going to mock them at a layer two using
an eight or 2.1 Q Marking.
I'll show you the details of the markings when I do a Wireshark capture in a moment.
But for now, understand that when you speak into a phone.
So let's assume these are your voice waves.
A phone trance codes that into a digital signal of zeros and ones they are put into UDP packets that
are encapsulated in IP, which are in turn encapsulated in Ethernet and or sent to the switch.
So at layer two, we have an ADA two, two, one, Q header.
The reason why is that Ethernet doesn't have these additional markings.
As an example, if you're using Ethernet to it has no way to indicate the importance of a packet.
So when we capture the frame, you'll see in a moment that we have a destination address.
We'll have a source address.
And then we'll have an attitude or one Q header and then the actual voice within the packet.
So the attitude of one Q header has a priority filter to this is three bits in size also called Cos
or cos of service or cos it's three bits in size.
So the binary values go from 000 all the way to 111.