WEBVTT

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 In this next section we're going to
 dive into the details of CSMA CD.

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 So to begin with, before we go into that
 I have to explain what the difference

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 is between two terms of half
 duplex versus full duplex.

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 So this concept of duplex is not necessarily
 just something relevant to

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 ethernet. It's really a high level concept
 of how can devices communicate.

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 For example, if you and I are speaking
 and I say, well I'm going to use

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 simplex communication.

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 That's not on here.

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 Simplex. Simplex means I'm
 the only one talking.

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 Sort of like, I'm the dad, you're the
 child, you be quiet, you don't say

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 anything, I'm going
 to talk to you.

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 That's simplex, right?

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 Or to put it more technically, you're
 in your car, you're listening to

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 the radio, right?

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 That's simplex. Your radio is not transmitting
 back to the radio station,

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 it's just listening to stuff coming
 down from the radio station.

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 That's simplex. Now if we're going to
 have bidirectional communication,

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 both people talking back and forth,
 then we have to answer the question

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 of can they both talk
 at the same time?

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 Or does one person talk and
 then the other person talks?

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 Well, in human terminology, it's probably
 better if we take turns, right?

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 So we can not get each
 other irritated.

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 So that's called half duplex.

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 Half duplex means when I'm talking,
 you graciously listen, then when I'm

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 done, you talk, right?

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 We're in a therapist's office.

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 Here, I'll give you
 the talking stick.

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 Now you can talk now.

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 Mm-hmm. Okay, great, I heard you, I
 get the talking stick back, my turn.

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 So that's half duplex, right?

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 Full duplex is like a really bad therapy
 session where you and your wife

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 go, you're yelling at each other at
 the same time and nobody can hear

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 anybody else. That's full duplex.

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 Now you might think, oh,
 full duplex is bad.

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 Well, certainly if you're in a therapy
 session with someone that you're

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 not having a good time with, yes,
 full duplex would be bad.

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 In the concept of Ethernet or any kind
 of computing technology, full duplex

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 just simply means that we have some
 way that the two devices can talk

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 to each other at the same time and they're
 not stepping on each other's

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 toes. A can talk to
 B, B can hear that.

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 He can receive it at the exact same time,
 B is talking to A, that is full

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 duplex communication.

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 Now, when Ethernet first came out back
 in the late 1970s here, it was

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 half duplex. Now remember, Ethernet
 was based on this old Aloha network,

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 this wireless network communicating
 across the Hawaiian Islands and that

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 was based on the concept
 of half duplex.

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 That only one person could talk and if
 two people talked at the same time,

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 you didn't get an acknowledgement.


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 So you had to retransmit.

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 So the original CSMA CD, the original
 Ethernet was sort of based on that.

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 Only one person can use
 the cable at a time.

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 So Dr. Robert Metcalf said, okay, well,
 how are we going to do this in

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 a way that's fair?

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 And I want to make it a little bit
 better than the Aloha network.

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 I want to make it where the utilization
 of the channel is a little bit

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 more efficient than just anybody
 jumping on whenever they want to.

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 So he came up with CSMA CD and that's
 what we're going to jump into right

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 now. So the CS portion of it
 stands for carrier sense.

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 What is the carrier?

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 The carrier is that electrical signal
 that's going across the wire that's

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 transmitting the ones and zeros, that
 fluctuation, that change in electrical

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 voltage that's carrying
 ones and zeros.

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 So when an Ethernet adapter or an Ethernet
 NIC card is performing carrier

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 sense, basically it means it's in receive
 mode, it's listening to see

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 if somebody's talking, to see if there's
 any energy on that cable, see

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 if anyone's talking right now
 and sending ones and zeros.

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 So that's carrier sense.

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 Now in the way that Ethernet works,
 and we're going to go into this in

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 a little bit more detail, is that let's
 say that I'm an Ethernet adapter

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 and the IP process within the
 CPU has given me some data.

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 I put that into an Ethernet frame and
 we haven't looked at and now I want

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 to transmit all those bits as ones
 and zeros out of the wire.

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 So step number one, perform
 carrier sense.

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 In other words, listen to the wire, see
 if I can get detect any electrical

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 energy if someone's talking.

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 Now what if they're not, what if the
 wire is free, nobody's talking?

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 Well the way Ethernet works is it says
 okay, if the moment I perform carrier

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 sense it's free, I have to
 wait a little bit of time.

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 There's a little bit of time
 called the inter-frame gap.

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 You'll see that as IFG.

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 Sometimes people call
 the inter-packet gap.

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 Same thing. It's a little bit of time
 in microseconds that you wait and

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 then if nobody's talking by the time
 you're done with that time period,

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 then I can start putting
 my bits on the wire.

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 Now back with 10 megabit per second Ethernet
 back in the early 1980s when

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 that came out, that period of
 time was 9.6 microseconds.

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 Real short. So as humans we
 can't even detect that.

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 9.6 microseconds was the inter
-frame gap, then you could go.

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 Now when they came out with fast Ethernet,
 100 megabit per second Ethernet,

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 they said all right let's
 wait one tenth that time.

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 So instead of 9.6 microseconds, let's
 only wait 0.96 microseconds.

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 Not even one microsecond, just slightly
 less than one microsecond and

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 that's how they were able
 to get 10 times the speed.

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 Now they did some other
 things as well.

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 They sort of changed the way that ones
 and zeros were represented electrically

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 but that was also one of the
 fundamental things they did.

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 They shrink that inter-frame gap.

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 Now you might be thinking okay but what
 if I listen to the wire and it's

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 busy? Then what do I do?

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 We'll talk about that.

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 We'll get there in just a second.

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 All right so CSMA so the MA
 stands for multiple access.

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 So what this means is that Ethernet
 is the type of technology that says

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 okay if I have multiple units of data
 to send, multiple frames are so

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 stacked up in my memory.

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 Once I'm done transmitting frame number
 one, I can go ahead and transmit

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 frame number two, frame number
 three, frame number four.

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 In other words I can transmit multiple
 frames one after the other.

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 I might be thinking well don't all
 networking technologies do that?

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 Most of them do but not all.

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 For example if you would study networking
 oh I don't know 15 years ago

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 or something and you would learn
 about token ring as an example.

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 With token ring and older technology that
 I don't think anybody uses anymore,

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 the idea was once I put my frame on
 the wire then I had to wait for the

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 special little electrical, I had to
 wait for the frame to come back to

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 me and strip it off the wire and then
 release a little electrical pulse

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 called a token. So I couldn't immediately
 transmit my second frame, my

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 third frame, my fourth frame.

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 After every single frame the frame
 would go around the circle it was a

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 ring. You took your frame off the circle,
 you released this pulse and

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 then if you were lucky enough to get
 that pulse when it came back to you

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 you could transmit your next frame
 but you couldn't transmit one frame

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 after the other.

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 You couldn't do multiple access but
 with ethernet there's nothing like

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 that. There's nothing that you have to
 do between when you transmit frame

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 one and frame two and frame three.


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 That's why they call
 it multiple access.

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 And then ethernet performs
 collision detect.

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 So the idea is is that when a NIC card
 an adapter is transmitting on an

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 ethernet cable it owns that cable.


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 Nobody else should be talking, it has the
 full bandwidth, the full electrical

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 energy of that cable.

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 Now there are other technologies
 that don't do that.

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 For example think about, well think
 about your cable TV, right?

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 In cable TV you've got one cable but
 you've got different frequencies

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 on that cable which are like your
 radio in your car, right?

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 You've got all these different radio
 waves coming to you at the exact

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 same time right now and your cranium,
 your skull is being bombarded with

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 probably hundreds if not thousands
 of radio frequencies but if you're

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 a radio in a car you tune in to a particular
 frequency but just because

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 you're tuned into it doesn't
 mean those other ones stop.

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 They're still coming in you're just
 not paying attention to those you're

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 filtering them out and only listening
 to the one that you want to listen

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 to. But ethernet's not like that.

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 ethernet says okay only one person
 can go at a time you own the cable

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 it's all yours. And so when a device
 is transmitting remember the Aloha

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 network the Aloha network said okay
 when I'm transmitting I'm basically

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 deaf I can't hear anything I'm just talking
 and then once I'm done transmitting

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 I'll listen for an
 acknowledgement.

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 If I get an acknowledgement great
 my transmission made it.

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 Well Dr. Robert Metcalf said it's kind
 of inefficient I don't like that

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 so here's what he did.

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 He said okay when I'm speaking I'm
 listening at the same time and the

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 idea is as I'm talking I shouldn't
 hear anything maybe myself you know

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 if the electrical energy bounces back
 to me which hopefully it wouldn't

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 but I shouldn't hear any other bit
 sequences if I'm sending out 1011 I

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 shouldn't see anything coming back to
 me if I do that means a collision

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 has happened that means that my electrical
 energy has collided with somebody

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 else's electrical energy and what I
 was in the process of sending is now

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 meaningless garbage so that
 is called collision detect.

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 So let's look I'm going to go ahead and
 expand this so you can see a little

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 bit better. So in this particular environment
 we've got three PCs now

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 at the moment PCB is the PC that's transmitting
 and you can see here that

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 that device in the middle is a hub so
 he's transmitting into an Ethernet

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 hub and a hub's job is to simply take
 in though the electrical energy

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 the ones and zeros and they're just
 replicated out all other ports and

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 PCA and C would like to speak but they
 can't right now they have to hold

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 on to their frame because
 PCB is transmitting.

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 All right so both want to transmit a
 frame at the same time but they can't

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 because PCB is transmitting.

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 At some point PCB will be done and
 PCA and PCC will realize the PCB is

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 done they will receive the last bit
 and you might be wondering well how

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 do they necessarily know if they've
 received the last bit.

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 First of all when these bits are transformed
 on the wire there's very

0:11:02.560000 --> 0:11:07.340000
 specific rules about the the spacing
 of time between each bit.

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 I can't send a bit wait a microsecond
 send a bit wait 15 microseconds

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 send another bit again you can't do
 that so the rule one of the rules

0:11:16.220000 --> 0:11:19.520000
 of Ethernet is you know how long you
 can hold the voltage to a certain

0:11:19.520000 --> 0:11:23.680000
 level to represent a one or zero and
 how much time spacer is between the

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 next voltage change that represents
 a one or a zero.

0:11:26.860000 --> 0:11:30.740000
 So Ethernet knows that if a certain
 amount of time has elapsed the frame

0:11:30.740000 --> 0:11:36.960000
 must be done. Also if we're talking about
 802.3 the IEEE version of Ethernet

0:11:36.960000 --> 0:11:39.760000
 there's actually a field in there that
 we're going to see that says how

0:11:39.760000 --> 0:11:42.680000
 long is this frame so when you actually
 send your frame to somebody you

0:11:42.680000 --> 0:11:47.260000
 tell them how long it is so they explicitly
 know when it's finished but

0:11:47.260000 --> 0:11:50.180000
 either way whether you get that field
 that length field or whether you

0:11:50.180000 --> 0:11:53.920000
 see a certain amount of time elapsed
 you know when transmission from that

0:11:53.920000 --> 0:11:57.000000
 other guy is finished
 and the cable is free.

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 Okay so at this point PCB is finished
 PCA and C have been sensing the

0:12:05.040000 --> 0:12:10.480000
 cable and now they say oh the
 cable is free okay great.

0:12:10.480000 --> 0:12:15.660000
 Now at this point remember I said earlier
 I said okay during carrier sense

0:12:15.660000 --> 0:12:20.500000
 the first time you sense the carrier
 you see if it's free or not if it's

0:12:20.500000 --> 0:12:23.120000
 free you wait this period of time.


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 Do you remember what that
 period of time was called?

0:12:25.380000 --> 0:12:31.280000
 That was called the inter-frame gap
 or the IFG the inter-frame gap and

0:12:31.280000 --> 0:12:35.220000
 I said but what if you sense the cable
 and it's currently busy then what

0:12:35.220000 --> 0:12:38.360000
 do you do and I said we'll get to that
 that's what's happening right here

0:12:38.360000 --> 0:12:44.980000
 so PCA and C sense the cable and it
 was busy so they just listen listen

0:12:44.980000 --> 0:12:48.320000
 listen listen and the way the ethernet
 is designed is that once that cable

0:12:48.320000 --> 0:12:51.640000
 is free because they were already listening
 they were in that they were

0:12:51.640000 --> 0:12:54.980000
 holding a frame back and they were listening
 as soon as that cable becomes

0:12:54.980000 --> 0:12:57.600000
 free they instantly go.

0:12:57.600000 --> 0:13:01.540000
 Now you might say wait a second isn't
 that going to guarantee a collision

0:13:01.540000 --> 0:13:03.740000
 between A and C?

0:13:03.740000 --> 0:13:06.800000
 Yes it will but we'll see how that's
 dealt with in just a moment so that's

0:13:06.800000 --> 0:13:08.220000
 what's happening right here.

0:13:08.220000 --> 0:13:12.140000
 Not sure what happened with the vertical
 line connecting PC sort of floating

0:13:12.140000 --> 0:13:15.400000
 out there in the clouds but he is connected
 to that cable they both transmit

0:13:15.400000 --> 0:13:20.940000
 their frames at the same time and then
 what's going to happen within a

0:13:20.940000 --> 0:13:25.680000
 few microseconds is as those frames
 come into the hub or somewhere on

0:13:25.680000 --> 0:13:29.600000
 the wire between the hub the electrical
 energy is going to run into itself

0:13:29.600000 --> 0:13:33.580000
 and it's sort of like a wave of the
 ocean right if I have two waves that

0:13:33.580000 --> 0:13:38.040000
 merge what happens they get bigger
 they produce a bigger wave so if I

0:13:38.040000 --> 0:13:42.500000
 was saying well a wave of this height
 equals a one what's a wave of this

0:13:42.500000 --> 0:13:46.960000
 height I don't know it's not a one it's
 not a zero I don't know what that

0:13:46.960000 --> 0:13:50.520000
 is that's what's happening right here
 now this well-defined electrical

0:13:50.520000 --> 0:13:54.720000
 energy that met ones and zeros has now
 just become meaningless electromagnetic

0:13:54.720000 --> 0:14:01.320000
 interferences meaningless garbage and
 so PC A and C recognize that that

0:14:01.320000 --> 0:14:06.160000
 is a collision so what do they do as
 a result of that well they are in

0:14:06.160000 --> 0:14:10.060000
 the process of transmitting a frame
 when that collision occurred so when

0:14:10.060000 --> 0:14:15.060000
 that happens they transmit what's called
 a jam signal what is a jam signal

0:14:15.060000 --> 0:14:19.160000
 all a jam signal is it says however
 far you were into that frame maybe

0:14:19.160000 --> 0:14:23.040000
 I was five percent into transmitting
 it maybe it has 90 percent of the

0:14:23.040000 --> 0:14:28.320000
 way done however far I was whatever bits
 are remaining I throw those away

0:14:28.320000 --> 0:14:34.740000
 and I fill it in with 10101010 and
 that's called a jam sequence so jam

0:14:34.740000 --> 0:14:40.160000
 sequence is just populating the remaining
 portion of the frame with 10101010

0:14:40.160000 --> 0:14:45.480000
 and that everybody on the cable recognizes
 that as indicating there has

0:14:45.480000 --> 0:14:51.760000
 been a collision now as far as PCB is
 concerned he could care less I mean

0:14:51.760000 --> 0:14:56.320000
 he was listening that transmission
 but if it wasn't for him he doesn't

0:14:56.320000 --> 0:15:02.080000
 really care but A and C now they recognize
 okay there's a collision I'm

0:15:02.080000 --> 0:15:08.220000
 sending a jam signal so what do they
 do so at this point once a frame

0:15:08.220000 --> 0:15:13.580000
 has collided and you've completed your
 jam signal now what you have to

0:15:13.580000 --> 0:15:18.120000
 do is you have to stand back and you
 have to wait again but now because

0:15:18.120000 --> 0:15:21.180000
 you're waiting because you experienced
 a collision you're not going to

0:15:21.180000 --> 0:15:25.760000
 wait that nice tidy little inner frame
 gap that we talked about now what

0:15:25.760000 --> 0:15:30.740000
 happens is you're going to use what's
 called a random back-off timer and

0:15:30.740000 --> 0:15:35.720000
 the random back-off timer dictates how
 long will I wait after a collision

0:15:35.720000 --> 0:15:40.580000
 has occurred so you can see here it
 uses something now you're probably

0:15:40.580000 --> 0:15:43.800000
 never going to be tested on this but
 it's called the truncated binary

0:15:43.800000 --> 0:15:50.120000
 exponential back-off algorithm try saying
 that 10 times fast so basically

0:15:50.120000 --> 0:15:55.540000
 what does it says okay Ethernet at
 the physical layer has this concept

0:15:55.540000 --> 0:16:00.860000
 of something called a slot time and
 a slot time is basically okay if I

0:16:00.860000 --> 0:16:05.880000
 put one bit on the wire how long would
 it take to get all the way to the

0:16:05.880000 --> 0:16:11.500000
 end and all the way back to me again
 that is called a slot time now in

0:16:11.500000 --> 0:16:18.060000
 the case of 100 megabit per second Ethernet
 that's 5.12 microseconds don't

0:16:18.060000 --> 0:16:20.840000
 have to write that down by just giving
 you an idea of how short that is

0:16:20.840000 --> 0:16:28.820000
 5.12 microseconds is the slot time
 well what slot time have to do with

0:16:28.820000 --> 0:16:35.620000
 this slot time plays a factor into
 the random back-off because when I

0:16:35.620000 --> 0:16:40.860000
 detect a collision I say okay before
 I try to retransmit that frame I'm

0:16:40.860000 --> 0:16:46.260000
 going to have to pause and wait a certain
 amount of slot times how many

0:16:46.260000 --> 0:16:51.040000
 slot times should I wait that's where
 this algorithm comes into play so

0:16:51.040000 --> 0:16:57.860000
 the algorithm says you'll take 2 to the
 power of C in this case C is which

0:16:57.860000 --> 0:17:01.360000
 attempt is this is this your first attempt
 of the frame is this your eighth

0:17:01.360000 --> 0:17:05.060000
 attempt of the frame your fourth attempt
 so whatever a temp number it

0:17:05.060000 --> 0:17:11.640000
 is you put that where the C is minus
 one and then you randomly pick some

0:17:11.640000 --> 0:17:16.940000
 number between zero and whatever your
 calculation was so for example PCA

0:17:16.940000 --> 0:17:22.020000
 over here let me expand this a little
 bit this was PCA's very first attempt

0:17:22.020000 --> 0:17:27.400000
 at sending that frame so he plugs the
 number one in for C so 2 to the

0:17:27.400000 --> 0:17:33.280000
 power of 1 is 2 2 minus 1 is 1 and so
 he says okay I can pick from 0 to

0:17:33.280000 --> 0:17:37.680000
 1 slot times to wait as my random back
-off interval I'll just randomly

0:17:37.680000 --> 0:17:42.680000
 pick 0 bam I'm going to start going
 I'm going to start transmitting now

0:17:42.680000 --> 0:17:48.720000
 PCC that was his third attempt at sending
 his frame to the power of 3

0:17:48.720000 --> 0:17:54.280000
 is 8 8 minus 1 is 7 so he says okay I'm
 going to randomly pick some number

0:17:54.280000 --> 0:17:59.040000
 between 0 and 7 slot times to wait for
 my random back-off timer he says

0:17:59.040000 --> 0:18:04.440000
 okay I'm just going to randomly select
 5 so now even though they both

0:18:04.440000 --> 0:18:10.380000
 ran into each other on that last attempt
 now because A has randomly selected

0:18:10.380000 --> 0:18:15.000000
 a random back-off timer that is a shorter
 interval than C he gets to go

0:18:15.000000 --> 0:18:20.340000
 and you can see here that if a frame
 repeatedly gets collisions collisions

0:18:20.340000 --> 0:18:24.140000
 that number of C is going to increase
 your second time your third time

0:18:24.140000 --> 0:18:27.360000
 your fourth time so the amount of time
 you're waiting the slot times that

0:18:27.360000 --> 0:18:31.100000
 you're waiting gets bigger and bigger
 and bigger and the ethernet standard

0:18:31.100000 --> 0:18:36.580000
 says that that number of C can be no
 bigger than the number 10 so if I've

0:18:36.580000 --> 0:18:45.220000
 tried 10 times and to the power of
 10 is 1024 1024 minus 1 is 1023 so

0:18:45.220000 --> 0:18:52.120000
 if I've if I've now if this has been
 my 10th time and it collided I'm

0:18:52.120000 --> 0:18:56.580000
 done at this point the ethernet nigg
 card gives up it tosses the frame

0:18:56.580000 --> 0:19:00.940000
 it throws it away in memory and it has
 to uh I don't even think it's going

0:19:00.940000 --> 0:19:04.980000
 to report back upstream to the CPU hey it
 might it depends on the implementation

0:19:04.980000 --> 0:19:08.920000
 but basically that frame is done so
 something at a higher level like TCP

0:19:08.920000 --> 0:19:13.040000
 or something will have to realize hey
 never gone acknowledgement from

0:19:13.040000 --> 0:19:18.380000
 my TCP segment maybe I should send it
 back down the stack again so that