Minggu, 05 Februari 2017

Early Ethernet

We strongly recommend you to read the article first to increase knowledge about the Fiber Optic, Gigabit Ethernet, The Physical Network before proceeding to the next page.

- The overwhelming majority of all wired networks in existence in the world today are Ethernet, and pretty much every one of those Ethernet networks out there has some kind of box, a switch, with cables running out to the individual host, but, interestingly enough, with early Ethernet, it didn't work that way. Right here in front of me is an example of the first generation of Ethernet. This was called 10Base5. You had this huge yellow cable that was running up in the ceilings, and then you had this big box.

This thing is actually screwed in through a vampire connector. You tear into it, and you hook it in. And this transceiver made your connection, and then from the transceiver, you had this little connector here, which would go down to your Ethernet cable. So this guy would be way up in the ceilings, and then your Ethernet cable would be plugged in your computer, and wherever you wanted to put a computer, you just made a drop, and that's where the term drop comes from when we're in the networking world. Now, no one's used 10Base5 in a long time.

Yes, I'm sure somewhere in Ypsilanti, Michagan somebody's running a 10Base5. I don't know, but these don't exist anymore. However, for the Network+, we need to talk about Segmented Ethernet, where there is no switch. There is just one big cable up in the ceiling. Let me show you how this works. Here's an old Ethernet cable up in the ceiling. So let's start plugging computers in, so we can screw in transceivers, and then we connect computers to the transceivers, and, in fact, we can plug in a number of these up to a certain limitation.

But if you notice, everybody is on one cable, so in order for them to talk at the same time, we have to learn about something called CSMA/CD. The best way to see how CSMA/CD works is to. Well, we're gonna recreate the animation you just saw. Now, first of all, CSMA/CD stands for Carrier Sense Multiple Access/Collision Detection. So let's watch this in action. So this little black cable here is my big Ethernet segment up in the ceiling some place, and I'm gonna put three drops on this particular segment.

Now, yeah, there's a cable that's connected to a computer as well, but I don't wanna put that in here, so appreciate that it does exist, but it's not here. If this computer right here wants to talk, we can't have anybody else talking, so the CS stands for Carrier Sense. Basically, before any of these computers start to talk, they're gonna listen on the wire to be able to hear if anybody else is talking. If they don't hear anybody, then they can go ahead and talk. Now, let's say this guy wants to talk,. So he sends up, comes up from the computer, and he sends a frame up.

This frame actually propagates both directions on the bus, and that's fine. And as it goes, it will hit whatever computer who's supposed to be picking up that Ethernet frame, and life is good. Now, the challenge you have here is that when you're looking at this, you gotta keep in mind that the frame is going both directions. So take a look here, and you'll see as the frame's going both ways, it's gonna hit the ends, and the problem we have with this is that when it hits the end of a cable, we're gonna have what's known as reflection, and the signal will actually bounce back.

And that's a bad thing, so on all of these old Ethernet segment type networks, we put these things. These are blocks, but we put terminating resistors on the ends, so it would eat the signal by the time it got to either end of the segment. That's great. So we've got multiple computers on here. That's MA, Multiple Access. We have Carrier Sense, so now we have CSMA. The last part is Collision Detection. The one thing we can't have is two computers talking at the same time. And with Ethernet speeds being as high as they are, it's common for two computers, even though they're listening and they don't hear anybody, to start talking at the same time.

Boom. You get a collision. Now, electrically, it's fairly easy to determine this, and the network cards can go, "Oh! We got a collision." So they can deal with it simply by grabbing dice. So I grabbed a couple of my old D&D dice here. And let's say these two guys have a collision, so one would roll his dice, and the other guy would roll his. And whatever number came up, that would be how long they would wait in milliseconds before they tried to retransmit. Now keep in mind I have 20-sided dice here, but their dice were, like, 65,000 sides, so the chances of them rolling the same were pretty small.

So that's CSMA/CD, Carrier Sense Multiple Access/Collision Detection. It does exist in more modern Ethernets, the ones that use switches, but in a very rare and weird way. So pretty much we'll just keep it to this type of Ethernet. The once exception is a type of Ethernet called 10Base2. 10Base5 was a real pain to work with, big, thick cables, screwing vampire connections into cables, literally cutting holes in ceilings and making drops wherever the transceiver happened to land.

We needed a, kind of like a poor man's 10Base5, and that was called 10Base2. 10Base2 stands for 10 megabits per second baseband with 200 meter segments. Although it was really 185, two stood for 200 meters. Anyway, I've got some example 10Base2 for you to look at right here. So here are two network cards, and they're 10Base2, and let's take a look at them in detail. First of all, this little connector is called a BNC Connector.

Don't ask me what BNC stands for. There's about five different versions, and I don't think any of them are right. So you would have a BNC connector, and this was actually a T connector, so if you take a look at this, you'll see that it's kinda shaped like a T. And so you'd never plugged a cable straight into it. You always had a T connector. Now what you'd do is you would just go ahead, and you'd plug these two together like this. And this is a terminating resistor, and at the ends of the cables, you'd put a terminating resistor, so you are now seeing a complete 10Base2 connection right here.

But what if we wanted to add another computer? No big deal. We'll just take off the 10Base2 terminator. Don't we all keep network cards in our drawers? And here's another 10Base2, and what I'm gonna do here is I've got another T connector and a chunk of cable. So I'm just gonna plug this in, take the terminator from him 'cause he's not on the end anymore. He's gonna be on the end. And make another connection.

So you take a look at this. We have an extended 10Base2. You could but up to 30 devices on one 10Base2 segment. So, yes, this is a very old style of Ethernet. It is on the current Network+, so make sure you understand what a BNC connector looks like, make sure you understand that you have terminators on the end. It's 10 megabits. It runs to a 200 meter segment, although it's really 185. And, hopefully, you'll never see it again after your Network+.

By the way, my email address is all over this series. If you happen to be using 10Base2, send me an email and tell me about it. I'd love to hear.
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