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Controlled vs. Random Access
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從本節課中
Random Access in Wifi Networks
In this lesson, we will investigate WiFi, another type of wireless network. Rather than having stringent power control algorithms as we saw for cellular, WiFi relies on "random access" methods to manage interference among users in the same location.
教學方
Christopher Brinton
Lecturer
Mung Chiang
Professor
So this brings us now to the term called Medium Access Control or MAC.
And we'll see that in the internet, MAC is a layer in what's called the Protocol
Stack. And we won't concern ourselves with that
yet. But just know that that is something that
the internet does. And it's a task managed by the internet.
And MAC defines how people share the network medium, right, so it's defining
how people can access the network. There's two different types of access out
there. There's Controlled Access, which is what
we looked at with [UNKNOWN], FDMA, TDMA, CDMA.
That's controlling, assigning, regulating, any of those synonyms, it's
doing that, it's controlling how people are getting on the medium.
Then there's what's called Random Access which is the way the Wi-Fi works.
So under control, we have our cellular and our regulated ways and under Random
Access, we, it's a different set of protocols.
They are all unified under MAC, so they are both types of MAC but it's just
better controlled or Random Access. So we can come back to our cocktail party
analogy to see the differences here again.
And with CDMA, as we said last time in the last lecture.
when you're at a cocktail party. If the host was having everyone speak
according to a CDMA standard. Which is kind of weird because when you
go to a cocktail party hopefully you're not talking about CDMA necessarily.
But they would ask people to adjust their volumes basically.
Right? Which is really the way that a cocktail
party would probably work is that everyone would be talking at the same
times. You'd just be, oh, adjusting your volume
so that no one was overpowering another person's conversation.
TDMA would be a really strange way to run a party.
It's when you assign people different time slots on which they would speak.
So, this person would be able to talk in his conversation then you'd get this next
group of people. It's hard to tell who's talking to who in
this picture but you get the idea. And then under WiFi.
WiFi itself isn't, you know, a multiple access like, we'll see more of those in a
minute. But with WiFi, you're allowed to speak in
this cocktail party. You'd be allowed to speak, as long as
nobody else wants to. There's no set rule.
Like there's no time slot that you're being assigned like in TDMA.
No one's telling you how you adjust your volume.
You just listen to say, okay, is there another conversation going on?
If not, then you'd say, well okay, then I can start talking.
That's the idea. So you just listened, you have some
sensing protocol, you listen. And then you can talk as long as nobody
as else is. And we allude back again to our stop
light versus our stop sign analogy. when you're sitting at the stop light
that's a way of controlling access to the intersection through the stop light.
Where as the stop sign is more so along the lines of random access where a
person's just looking and then going. And so they make their own decision in
the end. It's ultimately up to them whether
they're going to tran... Whether they're going to go or not.
So let's think a little deeper into Random Access, let's look at this example
over here. So, in these diagrams that we draw again
these circles are denoting the transmission ranges.
And here we have two access points D and E.
But note that the access points themselves also have to talk back to the
devices. So, they're really both transmitters and
both receivers. Just like we talked about in the last
chapter, the idea of a link. And then the other thing we should
mention is that there is a difference between the transmission range, the
sensing range and the interfering range. So, those are three different things that
you can there's a lot of research that goes into defining what they are and how
large they are. But for our purposes here, we're not
going to really make any distinction between them.
Just intuitively, if someone's in your transmission range; like when you're
speaking out loud, if somebody can hear you, it means they're near a transmission
range. So, you would know that B for instance is
in D's transmission range as we can see right now.
And C is in E's transmission range as well and A is also in there too.
And in terms of interference is if their transmission range collide at all.
So, at all. So, if when you transmit, the other
persons transmission can intersect with you whether that's happening at the
receiver itself or before you get to the receiver.
That will cause a collision and we're assuming that collisions in the worst
case scenario always going to cause both of the frames to be lost.
Again, these are assumptions and they're not always true in practice.
But it's going to simplify our analysis a lot in what we're going to be looking at.
And so, in this diagram right here, as we can see, there's always different
transmitters. Right?
And they're all, all their transmission ranges would intersect, if we drew the
circles out. We could draw the circles around C.
We could draw it around B, D, A and E. And, at most one of them can be
transmitting it at any given time. Because they're all going to interfere
with each other. Even though D and A are on a different
access point than C and E, they're still going to collide because they're all
within each others transmission ranges, alright.
So, at the beginning of every time slot, a device has to ask itself, should I
transmit or not? That's the question that they have to
answer. Do I transmit in this time slot?
Or do I not transmit in this time slot? And different Random Access protocols
will dictate how they answer that question.
So, if we look at this diagram right here.
At the bottom right here we have different time slots 1, 2, 3, and 4.
And for purposes of our discussion we will make another assumption which is
that each of these time slots. If someone decides to transmit, the frame
will be present for the entire time slot. So we can look at these different time
slots as being our standards of time here, which will make our analysis more
easy to lay out. So, in any one of these time slots, I
mean, let's suppose in the first half of it just A transmits.
So A decides that it wants to send a frame to D.
That's it. B and C don't decide, don't want to
transmit at all. So that's fine and A's frame will get
there. Everything will be okay.
Now, in B, in time slot 2, both B and C decide that they want to transmit at the
same time. But if B and C are transmitting, they're
going to collide with one another. And therefore, you're going to have a
lost frame. Now, in time slot 3, nobody decides to
transmit. So, we can call that, what we say, a
wasted opportunity. So someone could of transmitted if they
wanted but nobody decided to by virtue of the medium access protocol we're using.
And in the fourth time slot then, A, B and C all transmit, and again if any more
than one person's transmitting, we're going to have a lost frame.
So in that case the frame would be lost.
