WEBVTT

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 This video, which is part of the CCT
 routing and switching curriculum,

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 is titled Differentiating
 Layer 2 Technologies.

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 My name is Keith Bogart, and I will
 be your instructor for this video.

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 As far as the video overview is concerned,
 in section 3.0 of the CCT routing

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 and switching, it has
 this on the blueprint.

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 It says differentiate between these
 Layer 2 technologies, the list sees

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 off from Ethernet, down
 to optical, and etc.

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 Now, right here, right off the bat,
 I have a problem with this bullet

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 point, and I will talk to you a little
 bit more about this in about 30

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 seconds. Namely, that some of the things
 listed on here are not Layer

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 2 technologies, specifically
 DSL and optical.

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 As you learn about networking technology,
 I don't know as you're watching

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 these videos if you've done any
 studying of the OSI model yet.

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 But in the OSI model, you have the whole,
 everything that could be done

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 in networking broken up into seven
 different functional layers.

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 Layer 1 is called the
 physical layer.

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 This means everything having to do
 with electrical impulses, how do we

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 represent a 1 in 0 via electricity,
 or how do we represent it via light?

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 What's the cable look like?

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 How thick is it?

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 How many wires should it be?

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 How long should it be?

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 All that stuff is called Layer
 1 or the physical layer.

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 And that is where DSL
 and optical reside.

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 Layer 2 is not that.

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 So what is Layer 2?

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 So in the OSI model, Layer 2 is
 called the data link layer.

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 So physical layer is Layer 1, Layer
 2 is the data link layer.

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 And the data link layer is actually
 subdividable into two sublayers.

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 The LLC sublayer, which stands for the
 logical link control, and the MAC

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 sublayer. So let's look at the MAC
 sublayer first real quickly.

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 So we know that Layer 1 is defining
 the cable, what's in the cable, how

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 1's and 0's will be actually physically
 represented by either laser light

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 or electricity or radio frequencies
 or something like that.

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 Well, the MAC sublayer says, OK, now
 that we have all that taken care

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 of, the MAC sublayer says things like,
 how do I know when I can access

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 the cable? How do I know when it's
 my turn as opposed to when I should

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 wait if somebody else is talking?

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 Or should I even be concerned
 about that?

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 When I access the cable, when I put the
 data on there, do I need to address

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 it in such a way that the destination
 knows that it's for them?

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 Or do I not care about that?

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 If I do need to address it,
 was that address look like?

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 How big should it be?

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 What do the bits represent
 in that address?

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 When something is coming in on the
 cable, how do I know it's for me?

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 All of that is stuff that the MAC
 sublayer is responsible for.

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 So if there is a protocol that does that,
 if someone develops a protocol,

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 the answer is questions like, how do
 you know when you can talk on the

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 cable? How do you know when
 you should stop talking?

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 When you get something on the cable,
 how do you identify that it's for

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 you? When you transmit something on the
 cable, how do you identify specifically

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 who it's for? If the protocol answers
 those types of questions, that is

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 a layer two protocol.

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 Where DSL and optical, they don't
 answer those questions.

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 They're physical layer stuff.

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 They're not concerned
 with that stuff.

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 Similarly, at the data link layer, we have
 the logical link control sublayer,

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 which is stuff, for example, like,
 okay, we know that one layer up at

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 the networking layer, there's a lot of
 protocols that reside at the networking

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 layer. There's IP version four, IP version
 six, Apple talk, IPX, all kinds

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 of stuff. So here we are at the physical
 layer and we got, we got electricity

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 coming in at different voltages representing
 ones and zeros, ones and

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 zeros. Now, the Mac sublayer looks at
 those ones and zeros and ones and

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 zeros. This is okay.

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 According to the protocol that I run,
 according to the rules that I follow,

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 this first set of ones and zeros represents
 the destination address.

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 Is that my address?

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 Oh, yes it is. That's my address.

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 Okay, great. I'm going to
 start picking this up.

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 Okay, now all the ones
 and zeros stop.

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 Okay, I have my frame.

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 Here's my frame.

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 I know it's for me.

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 Now, the next question, this thing says,
 is okay, I'm going to pass this

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 up to the LLC sublayer.

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 Mac sublayer says I'm done.

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 I know this is for me.

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 I got it all. I verified
 it's okay.

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 It's not messed up in some way.

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 And by the way, that's also something
 the Mac sublayer does.

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 Error checking. Are any of these ones
 and zeros screwed up from the time

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 that they were transmitted
 by the sender?

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 Not going to talk about how it does
 that, but that's a responsibility,

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 the Mac sublayer.

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 So the Mac sublayer says,
 my job is done.

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 Hey, LLC sublayer.

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 Here's the frame.

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 Here's all the ones and
 bits I picked up.

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 It's for us. It's okay.

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 Here you go. So the LLC sublayer gets
 it and says, okay, I know my job

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 is I need to pass it upstream.

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 But I got a lot of stuff up there.


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 I got IPv4 that might be
 waiting for something.

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 IPv6, Apple Talk,
 IPX, other things.

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 The LLC sublayer is responsible for figuring
 out how do I send it to IPv4?

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 How do I send it to IPv6?

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 Whatever is up there, the next layer
 at the networking layer, how do I

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 get it to the appropriate
 protocol up there?

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 And similarly, all these protocols,
 you know, in your laptop, if you're

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 running IPv4 and IPv6 at the same time,
 which these days, you probably

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 are, even if you're
 not aware of it.

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 That means your laptop's processor is
 creating IP, is creating data with

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 IPv4 addresses, creating data with
 IPv6 addresses, and all that stuff

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 is going down to your
 Ethernet NIC card.

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 Well, it's the LLC sublayer that's responsible
 for grabbing all that stuff.

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 So the NIC card can
 talk to layer three.

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 That's the responsibility of LLC.

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 So once again, if a protocol knows
 how to do that, that is a layer two

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 protocol. Once again, DSL and optical,
 they don't know how to do that.

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 They're physical layer stuff.

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 And typically, we say that devices that
 are used at the data link layer

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 are switches and bridges.

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 But really, anything with a NIC card
 operates at the data link layer.

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 Really, you can't connect to a network
 without operating the data link

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 layer. Your laptop operates
 at the data link layer.

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 A router operates at
 the data link layer.

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 A Wi-Fi access point operates
 at the data link layer.

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 If you're not touching the data link
 layer, you have no connectivity to

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 the physical layer.

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 And if you have no connectivity to the
 physical layer, you're not networking.

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 You're not talking to anybody.

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 So let's differentiate between these various
 different layer two protocols.

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 So I have include on here the actual
 real true layer two protocols.

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 Notice that in the bullet point, even
 though it mentions serial and optical

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 and DSL, those are not
 layer two protocols.

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 So I'm not concluding them
 in my chart right here.

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 So let me zoom in on this here.

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 And let's just go through each one of
 these things as a bullet point so

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 we can sort of differentiate
 between them.

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 Okay. So the true layer two protocols,
 all the ones I could come up with

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 off the top of my head, were the various
 flavors of ethernet, asynchronous

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 transfer mode, that's ATM, integrated
 services, digital network, that's

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 ISDN, frame relay, the point to point
 protocol, PPP, and the high level

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 data link control
 protocol or HDLC.

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 Now these are all things that answer
 those various questions.

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 All these protocols I wrote here answer
 the questions of how do I know

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 when it's my turn to go?

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 When something comes in off the wire,
 how do I know if it's for me?

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 When I'm putting something on the wire,
 do I need to address it to the

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 destination? And if so,
 how do I do that?

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 All these protocols answer
 those questions.

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 So how are they different
 from each other?

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 Well, in a variety of ways.

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 Number one, some of these protocols
 are used in local area networks.

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 They're specifically designed to be
 used between devices that are fairly

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 close in proximity to each other.

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 So that is ethernet, fast ethernet,
 and gigabit ethernet.

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 Now there are gigabit ethernet can blur
 the lines because if you're doing

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 gigabit ethernet over fiber optic cabling,
 you can actually have that

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 go for quite a bit of distance.

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 So gigabit ethernet could potentially
 be used in WANs as well.

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 But usually when we're thinking of
 ethernet, we're thinking of LANs.

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 We're not thinking of WANs.

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 Now what about WANs?

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 In other words, what layer two protocols
 are designed to operate across

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 vast distances, hundreds
 or thousands of miles?

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 That would be everything else.

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 So all these layer two protocols here
 were specifically designed to operate

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 across wide area networks.

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 Broadcast based, what
 does that mean?

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 Some of these protocols were designed
 around this assumption.

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 They were designed around the assumption
 that when I put data on a wire,

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 there are going to be many potential
 devices connected to the same wire

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 as me. Which means that when I put
 data on the wire, I have to address

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 that data. I have to put some sort
 of destination address so that even

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 though everybody's going to see it,
 only the guy actually want to pick

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 it up will pick it up.

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 There's nothing I can do to prevent
 everybody from seeing it.

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 We're all connected
 to the same wire.

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 But only the guy who needs it is actually
 going to pay attention to it.

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 That is called broadcast based.

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 For example, when you and a bunch of
 your friends get into a room and

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 you all start talking together,
 that's broadcast based.

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 Even if you're the only one talking,
 everybody's going to hear you.

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 You can't help that.

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 So all the flavors of ethernet were designed
 around broadcast based networks.

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 Which means ethernet needs to specifically
 address its data to the destination

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 device that it's going to.

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 Because we want to make sure that people
 who don't need it, don't pick

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 it up and listen to it.

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 These other protocols from ATM all the
 way down to HDLC are what's called

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 point to point based protocols.

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 Which means that this protocol has
 the assumption that when I put data

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 on the wire, it can
 only go one place.

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 There's not multiple destinations
 on this wire.

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 When I put data on the wire, there's
 only one in place it can go to at

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 the other end. ATM, ISDN, Frame Relay,
 PPP, and HDLC are all like that.

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 Now as you learn about Frame Relay,
 that's a little bit debatable.

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 But Frame Relay is definitely
 not broadcast based.

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 There is no way to put a Frame Relay
 frame on a wire and have multiple

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 devices see that one single
 Frame Relay frame.

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 Hence, it is not broadcast based.

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 Require setup signaling.

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 What's that referring to?

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 Okay, so some of these protocols are
 sort of like when you pick up the

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 phone. Right? Now, if you're in the
 White House and you've got the Red

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 phone in front of you and you pick it
 up, you don't have to dial a number.

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 You are automatically connected
 to the Pentagon.

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 Right? But at your house,
 that's not the case.

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 When you pick up that phone, it's not
 going anywhere until you type in

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 some digits, which means you are
 sending signaling information.

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 You're telling the telephone company
 via those digits you're punching

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 in, this is the remote destination,
 in this case, telephone number that

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 I want to talk to.

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 And then the telephone company sets
 up a path or what we call a circuit

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 between you and the
 remote telephone.

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 And now you can talk
 across that line.

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 But when you first pick up the phone,
 there's no circuit established.

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 You have to send some sort of signaling
 information to say, this is where

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 I want the circuit to end.

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 So, ATM, if you're using what's called
 switched virtual circuits, that's

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 what SVC stands for, is like that.


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 ISDN is definitely like that.

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 ISDN uses like a phone number.

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 Frame relay can also be set up with
 something called switched virtual

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 circuits. And PPP was designed around
 the assumption that it could go

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 multiple places, but you have to signal
 first and say where you want to

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 go. As opposed to all of the flavors
 of Ethernet are always on.

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 Right? When you put an Ethernet frame
 on the wire, you don't have to first

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 say, hey, not, not, not wire.

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 Are you awake? Come on,
 I need to use you.

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 You don't have to do that.

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 The wire is always there.

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 It's always available.

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 You can immediately put
 your data on that wire.

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 So all flavors of Ethernet
 are like that.

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 If you're using what's called a permanent
 virtual circuit, which means

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 a circuit that's always up, like that
 red phone connecting the white house

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 to the Pentagon, that would be
 a permanent virtual circuit.

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 It's always available.

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 It's always up and always ties two devices
 together that's always available.

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 So ATM and frame relay when using permanent
 virtual circuits is always

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 on. Doesn't need any signaling.

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 HDLC was designed around the assumption
 that the circuit's riding across

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 is always up. HDLC and doesn't
 have any kind of addressing.

0:13:53.240000 --> 0:13:57.320000
 When you put data inside of an HDLC
 header, there's no layer to address

0:13:57.320000 --> 0:14:00.580000
 there. HDLC says, hey, why
 do I need to address it?

0:14:00.580000 --> 0:14:01.980000
 It can only go to one person.

0:14:01.980000 --> 0:14:06.080000
 At the other end of the line,
 there's just one person there.

0:14:06.080000 --> 0:14:10.880000
 And then lastly, some of these layer
 two, some of these specifications,

0:14:10.880000 --> 0:14:20.320000
 for example, all the flavors of Ethernet
 as well as ATM and ISDN not only

0:14:20.320000 --> 0:14:24.580000
 reside at layer two of the OSI model,
 they also reside at layer one of

0:14:24.580000 --> 0:14:29.000000
 the OSI model. For example, if you open
 up an Ethernet document, you'll

0:14:29.000000 --> 0:14:33.340000
 see in there not only answers to the
 questions that layer two has, which

0:14:33.340000 --> 0:14:35.580000
 is how do I know when I can go?

0:14:35.580000 --> 0:14:37.660000
 How do I know when I
 should stop talking?

0:14:37.660000 --> 0:14:39.960000
 When something comes in, how
 do I know it's for me?

0:14:39.960000 --> 0:14:41.640000
 Those are all layer two questions.


0:14:41.640000 --> 0:14:45.000000
 But the Ethernet document also answers
 questions like, was the cable look

0:14:45.000000 --> 0:14:46.500000
 like? Does it use copper?

0:14:46.500000 --> 0:14:47.720000
 How many strands of copper?

0:14:47.720000 --> 0:14:49.300000
 How long can the cable be?

0:14:49.300000 --> 0:14:51.180000
 What kind of voltage
 is used for one?

0:14:51.180000 --> 0:14:52.520000
 What's used for a zero?

0:14:52.520000 --> 0:14:54.120000
 That's all layer one stuff.

0:14:54.120000 --> 0:14:59.400000
 So all the flavors of Ethernet in addition
 to ATM and ISDN also specify

0:14:59.400000 --> 0:15:03.260000
 layer one as well as layer two.

0:15:03.260000 --> 0:15:06.860000
 And that's a pretty good breakdown there
 of differentiating between our

0:15:06.860000 --> 0:15:08.420000
 layer two protocols.