What is quality of service?
Quality of service is providing a better quality to certain applications at the detriment of other applications.
In the same way that on the roads certain vehicles are prioritized over others, such as ambulances
or buses.
We are prioritizing certain traffic types, in other words, making sure that they go to the front of
the queue as an example versus other traffic types.
Certain traffic types such as voice or video are delay sensitive.
They can't handle a lot of delay, so they'll be prioritized over traffic types that can handle delay
such as FTP.
It's also called managed unfairness.
We are introducing unfairness into the network, prioritizing certain traffic types over other traffic
types.
As an example, a very important phone call from the CEO would be prioritized over someone watching
YouTube videos for the CCNA exam.
You need to be aware of three traffic types and their traffic characteristics.
Firstly, we have data.
Data traffic is generally bursty and greedy.
That's not always the case, but generally data applications such as FTP will grab as much bandwidth
as they can.
If you give an FTP application two megabits per second, it'll take two megabits per second.
If you give it four megabits per second, it's going to take four megabits per second.
So data is bursty and greedy.
It's sometimes smooth and benign.
In other words, it doesn't grab all the bandwidth that you give it.
But in a lot of cases, data applications are bursty and greedy.
They also tend to be drop insensitive.
In other words, if a packet is dropped, it doesn't matter because it's retransmitted.
A lot of data applications use TCP, which re transmits data.
If packets go missing, it's also delay insensitive.
So as an example, if packets are delayed by 500 milliseconds, it's not a major problem.
Now, this really depends on the type of data sent.
There are two major categories of data applications.
We have interactive data applications and non-interactive data applications.
Telnet, as an example, is interactive.
A large delay will degrade the user experience, but if you downloading a file using FTP delay isn't
that important.