When it comes to the quality of service on the wired side of things, I might
well have
a switch here and I've got a connection.
I need to send a bunch of packets I'm receiving from all these other sources.
And so this interface can become a point of congestion.
And I want to use the concept of queuing along with just identifying the
different packet
types by looking at my tags.
So it's the concept of classification and queuing.
Going to classify the packets based on their tags, I'm going to place them into WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Going to classify the packets based on their tags, I'm going to place them into
these different
queues and it allows me to make sure my high priority packets like voice over
IP get sent
out first.
And this works out great so long as all of this is part of the same device.
But now when we think about wireless communications, wireless communications
are based on a different
type of medium.
It's based on what we might call either the channel or maybe the airspace.
However we want to think about it. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
However we want to think about it.
But in general here, the key concept is that this is a half duplex medium.
Meaning that we can only have one talker at a time.
I know when I first learned about wireless communications, I was shocked to
learn that
we're not all just communicating at the same time.
We're actually taking turns.
We could have a thousand different wireless devices talking to the same or in
the same
vicinity as each other.
And every single one of them is going to sit there and listen.
If somebody else is talking, they're not going to start. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
If somebody else is talking, they're not going to start.
And so wireless is an amazing technology that it works at all given a lot of
the limitations
that we have, one of which is that it's half duplex.
So if we have an access point with even just a handful of wireless devices
around.
Now let's say that this device here, this device might actually have a priority
packet.
It might have to send a voice over IP packet onto the network. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
It might have to send a voice over IP packet onto the network.
And meanwhile we just have all kinds of low priority traffic everywhere else.
Everyone else is just watching some internet video streaming or something like
that.
So again, up here with wireless or with the wired switch rather, it was fully
in control
of all of the traffic we're considering.
We have full duplex links here.
And so I can completely control which traffic I send.
That just doesn't happen here unfortunately in the wireless space. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
That just doesn't happen here unfortunately in the wireless space.
We're all contending for access to the medium here.
It's actually a word that is contention based on that contending concept.
This contention is gaining access to the medium and we're all trying to gain
access to the
medium.
Now wireless actually has a set of rules and we want to make sure that we
establish fairness WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
establish fairness
in our access.
For example, if this device sends a packet or maybe technically a frame since
it's layer
2, but we send a wireless frame to this access point, well, I've got a bunch
more frames
lined up here to send.
Well, I don't want this device to necessarily get to send as many frames as it
wants.
Once it gains access, it needs to relinquish control and allow somebody else to
have a
turn after that.
And so our rules for fairness that have been built for wireless, they are built WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And so our rules for fairness that have been built for wireless, they are built
on a by
device type of fairness.
And these rules have a name.
We refer to this as the distributed coordination function or DCF.
That sounds really fancy, but when you break it down, it's a function, which is
a set of
rules.
And it's all about the distributed coordination of these different devices, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And it's all about the distributed coordination of these different devices,
meaning we have
a bunch of different devices that have to coordinate and they're distributed
out.
So it's not a centralized concept.
The access point isn't necessarily saying, okay, it's your turn, you send and
get a packet.
Okay, now it's your turn, you send and get a packet back.
We can build rules around that in certain, maybe high density deployment type
of situations,
but usually that minimum, it heavily reduces the efficiency of our wireless WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
but usually that minimum, it heavily reduces the efficiency of our wireless
space.
And so we'd rather all of these devices just take turns.
And we want to again, establish fairness and it's by device.
But if it's by device, then now once again, I have my priority packet that I
have to send,
but everybody else maybe is going to go first.
So I have to wait one, two, three, four, for four different devices to get to
send their
frames.
And then I get to send. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And then I get to send.
And without diving into the details of the distributed coordination function,
there is
still a chance that somebody else goes fifth, somebody else goes sixth, then
maybe I finally
get to go seventh because there's a, there's actually a possibility I pick a
much longer
time just randomly to send my packet and somebody else happens to get two
packets in before
I do.
So this is not good.
This is almost too fair.
It's too fair for things like voice over IP, but that air quotes there. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
It's too fair for things like voice over IP, but that air quotes there.
It's too fair for voice over IP packets and other prioritization just in
general because
now I can't actually send this packet with any priority.
I can send it faster than the other packets that I stack up, but I can't send
it faster
than anybody else gets to send on the network. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
than anybody else gets to send on the network.
And the fortunately, the wireless world recognized this pretty early on and
came up with an alternative
for the DCF.
And this is known as the hybrid coordination function.
The hybrid coordination function or HCF now, this is going to introduce the
concept of
quality of service mechanisms to wireless.
Before HCF came around, we didn't have a way of doing this now. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Before HCF came around, we didn't have a way of doing this now.
Fortunately, it's been around for a long time, but we do need to recognize that
this set
of functions is going to allow us to better contend for the medium when we have
priority
or specifically priority packet.
So if I have a priority packet, I should get more access to the medium.
So that does break the rules of the fairness, right? WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So that does break the rules of the fairness, right?
We said before it was almost too fair.
And that's a good thing.
We are making it so that now my device with the priority voice over IP frame
can actually
send it out faster than some of the other devices inside of the wireless cell
there.
Now we're going to throw a few terms out here just so we're all aware.
The hybrid coordination function came to us as part of the IEEE specification
802.11e,
which was designed to try to solve some of the problems that wireless networks WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
which was designed to try to solve some of the problems that wireless networks
were facing.
Furthermore, the Wi-Fi Alliance here, so the Wi-Fi Alliance is a separate
organization
from the IEEE.
The IEEE creates a specification, which is why we call things 802.11 in the
wireless
world, 802.11, you know, AX, for example, 802.11g, all of these specifications.
Well, the Wi-Fi Alliance takes that and creates a standardization. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Well, the Wi-Fi Alliance takes that and creates a standardization.
So where they've recently created Wi-Fi 6 out of 802.11ax, well, they took 802.
11e
and created the concept of Wi-Fi multimedia, which can be abbreviated as WMM.
And then out of all of this, we get the concept of a set of enhancements known
as the enhanced
distributed channel access or ECEDCA. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
distributed channel access or ECEDCA.
ECEA is going to define a bunch of these rules, including the hybrid
coordination function,
which allows us to, again, change the way that we gain access to the medium.
One of the first things that we do as part of this, and again, coming back to
our wired
QOS concepts here, is to create the concept of classification and queuing. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
QOS concepts here, is to create the concept of classification and queuing.
And the way that we do that is with a set of mechanisms known as access
categories and
user priority.
Access categories and user priority essentially give us a way to tag our
wireless packets
and allow us to classify them as well.
The big difference between our wired and wireless QOS is that this is a lot WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
The big difference between our wired and wireless QOS is that this is a lot
more hard-coded.
With Cisco-wired devices, I can create as many queues as I want.
I can put whatever tags into whatever queues that I want.
However, with wireless, we're going to find that specific tags here, these user
priority
tags, get mapped to specific access categories and the behavior is all pred
efined.
So it takes a lot of the thinking out of how to make a wireless network
properly tuned,
because it's already decided for us, although there are going to be some WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
because it's already decided for us, although there are going to be some
important considerations
we need to make as far as all of this is concerned.
Now it's worth noting that EDCA actually gives us a lot of other enhancements
as well.
We're going to find that, for example, we can use priority-based contention
windows, arbitrary
inter-frame spacing, transmit opportunity, T-spec, all kinds of different
wireless features that
allow for a much better way for us to share the medium.
But that goes well beyond the scope of Encore.
Within the scope of Encore, we're going to be a little bit more focused here on WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Within the scope of Encore, we're going to be a little bit more focused here on
the access
categories and user priority than on any of the other concepts.
But once again, if we do want to dive deeper into the world of Wi-Fi, we have
that content
here on CBT Nuggets website, and I encourage you to go look up the CWNP courses
that we
have here, which really just go as deep into wireless as is necessary.
But for now, we're going to stick within the scope of this course and this exam
.
So let's go ahead and take a closer look at access categories and user priority WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So let's go ahead and take a closer look at access categories and user priority
.
[BLANK_AUDIO]