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When it comes to the quality of service on the wired side of things, I might

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well have

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a switch here and I've got a connection.

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I need to send a bunch of packets I'm receiving from all these other sources.

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And so this interface can become a point of congestion.

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And I want to use the concept of queuing along with just identifying the

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different packet

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types by looking at my tags.

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So it's the concept of classification and queuing.

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Going to classify the packets based on their tags, I'm going to place them into
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Going to classify the packets based on their tags, I'm going to place them into

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these different

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queues and it allows me to make sure my high priority packets like voice over

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IP get sent

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out first.

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And this works out great so long as all of this is part of the same device.

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But now when we think about wireless communications, wireless communications

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are based on a different

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type of medium.

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It's based on what we might call either the channel or maybe the airspace.

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However we want to think about it.
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However we want to think about it.

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But in general here, the key concept is that this is a half duplex medium.

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Meaning that we can only have one talker at a time.

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I know when I first learned about wireless communications, I was shocked to

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learn that

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we're not all just communicating at the same time.

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We're actually taking turns.

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We could have a thousand different wireless devices talking to the same or in

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the same

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vicinity as each other.

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And every single one of them is going to sit there and listen.

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If somebody else is talking, they're not going to start.
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If somebody else is talking, they're not going to start.

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And so wireless is an amazing technology that it works at all given a lot of

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the limitations

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that we have, one of which is that it's half duplex.

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So if we have an access point with even just a handful of wireless devices

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around.

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Now let's say that this device here, this device might actually have a priority

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packet.

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It might have to send a voice over IP packet onto the network.
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It might have to send a voice over IP packet onto the network.

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And meanwhile we just have all kinds of low priority traffic everywhere else.

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Everyone else is just watching some internet video streaming or something like

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that.

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So again, up here with wireless or with the wired switch rather, it was fully

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in control

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of all of the traffic we're considering.

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We have full duplex links here.

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And so I can completely control which traffic I send.

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That just doesn't happen here unfortunately in the wireless space.
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That just doesn't happen here unfortunately in the wireless space.

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We're all contending for access to the medium here.

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It's actually a word that is contention based on that contending concept.

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This contention is gaining access to the medium and we're all trying to gain

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access to the

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medium.

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Now wireless actually has a set of rules and we want to make sure that we

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establish fairness
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establish fairness

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in our access.

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For example, if this device sends a packet or maybe technically a frame since

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it's layer

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2, but we send a wireless frame to this access point, well, I've got a bunch

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more frames

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lined up here to send.

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Well, I don't want this device to necessarily get to send as many frames as it

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wants.

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Once it gains access, it needs to relinquish control and allow somebody else to

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have a

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turn after that.

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And so our rules for fairness that have been built for wireless, they are built
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And so our rules for fairness that have been built for wireless, they are built

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on a by

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device type of fairness.

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And these rules have a name.

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We refer to this as the distributed coordination function or DCF.

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That sounds really fancy, but when you break it down, it's a function, which is

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a set of

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rules.

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And it's all about the distributed coordination of these different devices,
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And it's all about the distributed coordination of these different devices,

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meaning we have

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a bunch of different devices that have to coordinate and they're distributed

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out.

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So it's not a centralized concept.

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The access point isn't necessarily saying, okay, it's your turn, you send and

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get a packet.

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Okay, now it's your turn, you send and get a packet back.

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We can build rules around that in certain, maybe high density deployment type

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of situations,

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but usually that minimum, it heavily reduces the efficiency of our wireless
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but usually that minimum, it heavily reduces the efficiency of our wireless

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space.

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And so we'd rather all of these devices just take turns.

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And we want to again, establish fairness and it's by device.

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But if it's by device, then now once again, I have my priority packet that I

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have to send,

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but everybody else maybe is going to go first.

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So I have to wait one, two, three, four, for four different devices to get to

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send their

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frames.

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And then I get to send.
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And then I get to send.

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And without diving into the details of the distributed coordination function,

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there is

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still a chance that somebody else goes fifth, somebody else goes sixth, then

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maybe I finally

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get to go seventh because there's a, there's actually a possibility I pick a

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much longer

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time just randomly to send my packet and somebody else happens to get two

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packets in before

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I do.

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So this is not good.

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This is almost too fair.

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It's too fair for things like voice over IP, but that air quotes there.
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It's too fair for things like voice over IP, but that air quotes there.

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It's too fair for voice over IP packets and other prioritization just in

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general because

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now I can't actually send this packet with any priority.

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I can send it faster than the other packets that I stack up, but I can't send

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it faster

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than anybody else gets to send on the network.
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than anybody else gets to send on the network.

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And the fortunately, the wireless world recognized this pretty early on and

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came up with an alternative

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for the DCF.

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And this is known as the hybrid coordination function.

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The hybrid coordination function or HCF now, this is going to introduce the

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concept of

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quality of service mechanisms to wireless.

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Before HCF came around, we didn't have a way of doing this now.
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Before HCF came around, we didn't have a way of doing this now.

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Fortunately, it's been around for a long time, but we do need to recognize that

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this set

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of functions is going to allow us to better contend for the medium when we have

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priority

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or specifically priority packet.

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So if I have a priority packet, I should get more access to the medium.

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So that does break the rules of the fairness, right?
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So that does break the rules of the fairness, right?

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We said before it was almost too fair.

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And that's a good thing.

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We are making it so that now my device with the priority voice over IP frame

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can actually

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send it out faster than some of the other devices inside of the wireless cell

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there.

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Now we're going to throw a few terms out here just so we're all aware.

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The hybrid coordination function came to us as part of the IEEE specification

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802.11e,

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which was designed to try to solve some of the problems that wireless networks
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which was designed to try to solve some of the problems that wireless networks

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were facing.

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Furthermore, the Wi-Fi Alliance here, so the Wi-Fi Alliance is a separate

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organization

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from the IEEE.

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The IEEE creates a specification, which is why we call things 802.11 in the

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wireless

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world, 802.11, you know, AX, for example, 802.11g, all of these specifications.

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Well, the Wi-Fi Alliance takes that and creates a standardization.
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Well, the Wi-Fi Alliance takes that and creates a standardization.

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So where they've recently created Wi-Fi 6 out of 802.11ax, well, they took 802.

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11e

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and created the concept of Wi-Fi multimedia, which can be abbreviated as WMM.

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And then out of all of this, we get the concept of a set of enhancements known

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as the enhanced

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distributed channel access or ECEDCA.
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distributed channel access or ECEDCA.

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ECEA is going to define a bunch of these rules, including the hybrid

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coordination function,

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which allows us to, again, change the way that we gain access to the medium.

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One of the first things that we do as part of this, and again, coming back to

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our wired

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QOS concepts here, is to create the concept of classification and queuing.
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QOS concepts here, is to create the concept of classification and queuing.

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And the way that we do that is with a set of mechanisms known as access

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categories and

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user priority.

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Access categories and user priority essentially give us a way to tag our

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wireless packets

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and allow us to classify them as well.

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The big difference between our wired and wireless QOS is that this is a lot
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The big difference between our wired and wireless QOS is that this is a lot

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more hard-coded.

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With Cisco-wired devices, I can create as many queues as I want.

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I can put whatever tags into whatever queues that I want.

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However, with wireless, we're going to find that specific tags here, these user

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priority

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tags, get mapped to specific access categories and the behavior is all pred

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efined.

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So it takes a lot of the thinking out of how to make a wireless network

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properly tuned,

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because it's already decided for us, although there are going to be some
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because it's already decided for us, although there are going to be some

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important considerations

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we need to make as far as all of this is concerned.

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Now it's worth noting that EDCA actually gives us a lot of other enhancements

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as well.

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We're going to find that, for example, we can use priority-based contention

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windows, arbitrary

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inter-frame spacing, transmit opportunity, T-spec, all kinds of different

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wireless features that

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allow for a much better way for us to share the medium.

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But that goes well beyond the scope of Encore.

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Within the scope of Encore, we're going to be a little bit more focused here on
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Within the scope of Encore, we're going to be a little bit more focused here on

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the access

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categories and user priority than on any of the other concepts.

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But once again, if we do want to dive deeper into the world of Wi-Fi, we have

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that content

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here on CBT Nuggets website, and I encourage you to go look up the CWNP courses

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that we

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have here, which really just go as deep into wireless as is necessary.

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But for now, we're going to stick within the scope of this course and this exam

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.

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So let's go ahead and take a closer look at access categories and user priority
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So let's go ahead and take a closer look at access categories and user priority

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.

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