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.