WEBVTT

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 So previously in the previous videos I've
 talked about how Ethernet originally

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 came out with just a single
 speed and a single duplex.

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 Basically 2.94 megabits per
 second and half duplex.

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 But over time, Ethernet speeds have greatly
 increased and Ethernet duplex

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 also changed from half duplex to full duplex
 where in today's modern networks

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 if you have your laptop per PC directly
 connected to a switch, which most

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 people do. Now you can do full duplex
 operation, meaning that you can

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 transmit bits to the switch.

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 At the exact same time, it's transmitting
 bits to you, which means you

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 don't have to do CSMA CD anymore.

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 So a couple of decades ago, Ethernet
 adapters started coming out that

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 were what were called
 dual speed adapters.

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 They could do 10 megabits per second
 or fast Ethernet 100 megabits per

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 second. And now we have triple speed
 adapters that can do 10, 100, 1000

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 Ethernet. And also these adapters were
 capable of either doing half duplex

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 or full duplex. So someone
 thought, you know what?

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 We should come up with some way that
 when you plug in your Ethernet cable

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 and that cables plugged into two different
 things, they can negotiate

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 between each other the fastest speed and
 the best duplex that they support.

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 Hence auto negotiation
 came into play.

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 So auto negotiation falls originally
 when it's first developed under the

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 802.3u specification under
 the clause 28 section.

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 And here's how auto negotiation
 basically worked.

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 So before auto negotiation existed,
 back in the days of just 10 megabit

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 per second Ethernet,
 old style Ethernet.

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 What would happen is as soon as you
 plugged in the cable to either end,

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 the NIT card connected that cable would
 start sending out a little pulse,

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 a little harpy. It was just a single electrical
 pulse every few milliseconds.

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 It was every 16 milliseconds
 as pulse would go out.

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 And that was called a normal
 link pulse or an NLP.

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 So if I had my 10 megabit per second
 PC connected to a 10 megabit per

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 second switch, when my green light
 came on, when my green LED came on

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 on my NIT card, that's because my NIT
 card was receiving these normal

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 link pulses every 10 milliseconds.


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 Then when fast Ethernet came out, they
 instead started sending out 16

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 of these pulses every 16 milliseconds,
 every 16 milliseconds, every 16

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 milliseconds, boom, boom, boom, boom,
 boom, boom, boom, boom, boom, right?

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 These 16, this was called a fast link
 pulse burst, a fast link pulse burst.

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 And then the 802.3U work group,
 they said, you know what?

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 Why don't we make use of this existing
 thing called a fast link pulse

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 burst and actually give it meaning above
 and beyond a simple heartbeat?

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 Let's turn this thing into what's called
 an auto negotiation message.

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 And so now, every single bit within
 that fast link pulse, whether it's

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 set to a zero or a one, will actually
 indicate what the supported speeds

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 are, what the supported capabilities
 are, if there's any faults, if we're

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 doing flow control using
 layer two pause frames.

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 So now this auto negotiation message
 was developed and it was based off

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 of this existing 16-bit sort of pulse
 called the fast link pulse burst.

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 Auto negotiation these days is
 on pretty much everything.

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 So any knit card you get on a laptop
 or a PC or a router or a switch is

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 most likely at minimum going to be
 a 10, 100 knit card, cable of 10 or

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 100. Many of them these
 days are 10, 100, 1000.

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 And how they are able to negotiate is
 by sending out this fast link pulse

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 message. And you can see here, here's
 an example of the 16 bits where

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 you can see the first five bits are reserved
 of 0001 and then the remaining

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 spaces can indicate what kind of speed
 and what kind of duplex you can

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 do. Now this is the original auto negotiation
 message that was 16 bits

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 long. If you're using a gigabit adapter,
 it's actually bigger than this.

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 It actually utilizes that 15th bit there,
 that one on the far right, setting

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 that to a one, which means there's another
 page, there's a next page present,

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 which means there's more auto
 negotiation bits to follow.

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 So when your knit card first comes up,
 it sends this and then it should

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 get within a maximum of sending three of
 these, it should get an acknowledgement

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 back. Meaning it should get an auto
 negotiation message back from the

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 peer with the acknowledgement bit set,
 acknowledging what supported speeds

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 and duplexes your peer
 is capable of doing.

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 And then based on that, you can negotiate
 to the highest speed and the

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 best duplex. Now an auto negotiation
 first was developed and this was

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 a while ago, this is like 20
 plus years ago probably.

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 And when it was in as infancy, it wasn't
 necessarily all that stable.

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 And the thing with auto negotiation
 was if you had two devices, like for

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 example, let's say that you had a switch
 that was capable of auto negotiation

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 connected to an older knit card on a server
 and that server did not support

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 auto negotiation.

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 Now let's say that knit card on the server
 was a fast ethernet knit card.

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 So all it could do was fast ethernet,
 100 megabits per second and it was

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 capable of full duplex or half duplex,
 but you had to set that manually.

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 So you think to yourself, oh, okay, well,
 on my server, I'm going to manually

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 set it for full duplex because after
 all it's connecting to a switch,

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 shouldn't be a problem.

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 Well, here's the problem
 that you would run into.

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 You plug it into the switch, the switch
 starts sending out its auto negotiation

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 message. What's it getting
 back from the server?

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 It's getting back just a regular fast
 link pulse burst, basically a pulse

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 of like all zeros or all ones.

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 I'm not sure which one it was, but
 it wasn't this combination of ones

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 and zeros. It was like 16 pulses
 all looking the same.

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 So the switch said, huh, okay, well,
 based on the fact that I got 16 of

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 those pulses and based on sort of the
 electrical energy used and the signal

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 and the encoding, I know that what
 I'm connected to is a fast ethernet

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 device. So the switch said, even though
 I can do 10 and 100, I know I

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 should be doing 100 because if that
 device I'm connected to was only a

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 10 megabit knit card, I would have only
 gotten a single pulse, a single

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 normal link pulse and that's
 not what I received.

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 So I can all, I can, the switch
 says I can figure out the speed.

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 I got the speed nailed down.

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 He's doing 100. But that device I'm
 connected to, I don't know what it

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 is. I know it's a knit card capable
 of fast ethernet, but is it a knit

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 card in a laptop, in a PC, in a
 hub, another switch, no idea.

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 And that device I'm connected to isn't
 telling me what duplex it can do.

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 So the auto negotiation device would
 say, hmm, since he's not telling

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 me, I'm just going to play it safe and
 I'm going to go down to half duplex.

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 And so in the early days of auto negotiation,
 when it first came out,

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 you would see a lot of white papers
 and documents that would say, oh,

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 just turn it off, statically hard code
 your speed and your duplex on both

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 sides. Because if you don't do that,
 you might end up with a situation

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 where you get half duplex
 instead of full duplex.

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 And that's not what you want.

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 But these days, here we are, 20 plus
 years later, auto negotiations very

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 stable. It's on everything and you
 don't have to worry about it.

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 It just works. So this is one of those things
 that just works in the background.

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 But if you're like me and you're wondering,
 okay, well, how does my knit

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 card know what speed it should select
 or what duplex it should select?

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 It's because of this.

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 It's because of auto negotiation.

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 And that concludes this particular
 video on auto negotiation.

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 In the next video, we're going to talk
 about the Ethernet frame structure.