Physical Cabling

In this section we’re going to go over the physical cables that links all of your infrastructure together. Whether it’s over the WAN, or internally at your organization. See, a lot of network engineers spend a lot of time at the command line, but neglect some of the physical infrastructure that our devices use to communicate. To be a good network engineer, you really need to be aware of both the physical requirements and limitations of the different types of cables you’ll be using, and also just to recognize the types of cables that you’ll see in your day-to-day work.

Shared media vs. Point-to-point

First up, let’s talk a little bit about the difference between shared media and point to point. For point-to-point a good example is your serial connections. These are your connections that you’d end up getting from a service provider typically, and this might be a T1, the 1.44 megabit link. It might come to your equipment on an RJ45, but the wires are connected a little differently, or it might come in on a larger serial connector.

In shared media you end up having a broadcast domain, you also have a collision domain whereas in point to point in a serial connection the two devices know that it is a point-to-point connection and they know that there is only one other device on that link so they can communicate accordingly.

LAN media standards

I want to list a couple standards here for LAN media. 1000BASE-T is unshielded twisted pair (UTP) copper, the common ethernet links with RJ45 connectors. The “T” indicates this is UTP, whereas your 1000BASE-CX is coaxial. If you’re not familiar with coaxial, this would be like the cable TV wire that came into your house. Where you have one individual copper wire and then shielding all around it and it’ll come into the connector with a screw top that’s crimped onto the end to be able to screw into the device.

The 1000 in 1000BASE is gigabit, for 1000 megabits. Coaxial has a maximum distance of 25 meters before it needs to have a booster of some kind or a repeater. 1000BASE-T, or the unshielded twisted pair, UTP, has a max distance of 100 meters. This is at gigabit speeds mind you, that changes a little bit if you’re using a different category of cable and you end up with 10 gig connection instead of one gigabit connection.

Just to give you a small sense here as far as the types of fiber media that’s out there, our 1000BASE-LX, that is a longer distance single mode gigabit fiber. The LX standard has a rated distance of 10km. The 1000BASE-SX is short range, this is only your multi-mode fiber. Multi-mode is a more general type of fiber, multi-mode allows more types of light and this has a much shorter distance though because it is more general and it’s not so highly tuned to just one wavelength of light. You can only get a maximum of 550 meters out of your multi-mode fiber though, and that is of course dependent on the wavelength of light that you’re using.

Fiber Connectors

I want to list what the different fiber connectors look like and also here a table as far as the color coding for your different types of fiber cables. For the exam, you don’t need to know these, however it will likely be very helpful in your job. The jacket being the color of the actual jacketing on the cable indicates what type of cable it is. Orange and Aqua indicates multi-mode, and your single mode will be indicated by yellow. I think that is a good thing to know that the color of the cable and the type of connector, the color of the connector, that you’re looking at as far as fiber goes does have a meaning here, that it is trying to tell you what type of cable it is and what it’s used for.

Single-mode vs. multi-mode

Let’s do a little rundown of the difference between single mode and multi-mode cable. We already said single-mode can have longer cable runs, we saw up to 10km of cable it can be run before it needs any kind of repeater or signal booster that is a that’s a long distance, and there’s much longer depending on the transceiver used. Multi-mode is much much cheaper though the cable itself is much cheaper, the transceivers are much cheaper, unless you actually need the distance for single mode then multi mode is going to be the way you’re going to go probably just due to cost.

Power over Ethernet (PoE)

lastly I wanted to cover Power over Ethernet (PoE) basics. So power over ethernet, if you’ve never heard of this it’s a technology that allows you to take the regular 1000BASE-T ethernet cable that we saw previously and we can transmit electrical power over it. So for security cameras, access points, even some routers, you can have just one cable going to it for both power and data. You can have more flexibility as far as where you’re placing these devices because you don’t need to have power nearby. If you’re installing wireless access points you won’t need an electrician to run power to each, only a network cable.

The PoE standards have progressed over time, where 802.3af is the oldest standard having been released in 2003, 802.3at is newer, and 802.3bt is new. These have progressed so that as our devices have gotten more complex, and we’re doing things like high power wireless access points and IP cameras, IP phones, etc.

The bottom row of the table this shows the cable category that is needed in order to be compatible with these standards. For 802.3af, we only need CAT3 cable in order to be able to supply that PoE standard. The maximum wattage received at the end device does decrease with CAT3, as it is not as well shielded and loses a lot of that power through the distance of the line. With CAT5 we’re going to get much closer to that 15 watts.

802.3at supplies a maximum of 30 watts, and 802.3bt supplies a maximum of 60 watts. I recommend you know these standards here for the exam. This would include the standard designations and the maximum power each provides.

Power over ethernet can be supplied from a switch, the switch injects power into the lines. A lot of times you’ll see on a PoE switch that not all of the ports are PoE capable, unless it’s a rather expensive switch. Even in such a case, it’s rare to find a switch that has the power budget to supply maximum PoE to all ports simultaneously. Be aware of the total power budget of the switch, and that switches supplying PoE will get much hotter than those that don’t.

The way devices negotiate PoE to be delivered is the supplying device will supply a very small amount of power enough so that if this is not a PoE device it will not damage it, and then it’ll expect a small response back. There’s a little chip in the end device that’ll send a response back saying “Yes I am PoE capable, please send me power” and when it does that this will go ahead and start sending power enough so that they can negotiate which standard or amount of power that they actually need to send. It won’t just be negotiating to a particular standard, our end device can draw as much power as it needs or as little power as it needs up to the maximum amount allowed by the standard that is supported.

PoE can also be supplied by an injector that’s separate from the switch and connected in-line. with a PoE injector, let’s say your switch is not a PoE capable switch, you can still supply PoE to devices. So this injector is a separate little box that is usually the size of something like a laptop power adapter. It takes in ethernet on one side and then it also plugs into a power outlet and spits out your ethernet with PoE on the other side and this is what you would use if you have a non-PoE switch but you need to connect a PoE device.

I’ve seen these used a lot of times with IP phones that in a lot of offices, maybe an older office that didn’t put in a Power over Ethernet switch or maybe there’s just not enough people there where they thought it was justified to put a PoE switch.