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Ethernet

Ethernet is one of the most popular networking technologies. I don't claim to know a lot about it, but here are some of my notes. I am sharing them in case anyone else finds them useful for doing their own home wiring. Use at your own risk, I am assuming no libablity for your following my notes. You may find this useful if you are putting ethernet into your home for use with a turn key NAT server so that multiple computers can use a single broadband connection.

Etherent can use a variety of connectors. Co-Axial cable (like cable TV uses) with BNC connectors was the original standard. It required a "T" type connector, and if that connector ever came off of the back of your computer, you brought the whole network down. Some people still like BNC style ethernet because it requires no "hub". The most popular wiring standard for ethernet is using "twisted pair". This is what I am familiar with, and what I will cover here. Most ethernet cards have a "transceiver" built into them, but some do not. For these, you need an external transceiver. For 10MB/sec ethernet, the interface to an external transceiver is called an AUI port. For 100MB/sec ethernet, the external interface is called an MII port (this is rarely seen; twisted pair as the defacto standard for ethernet preceeded 100MB/sec; where you do see an MII port is on an ethernet card with a twisted pair port right next to it).

Twisted Pair is commonly mistaken for telephone wiring. Telephones also use a form of twisted pair wiring. Telephones use either RJ-11 or RJ-12 plugs. Both are the same size, RJ-11 has 4 wires in the plug, and RJ12 has 6 wires (usually used for 2-line phones). Ethernet uses RJ-45 plugs. These look amazingly like RJ-11/RJ-12 plugs, only they are bigger. RJ-45 has 8 wires. You may have noticed that telephone handsets have a similar, even smaller (than RJ-11) 4 wire plug.

Wire is rated, by quality, into "catagories". Catagory 3 wire (or CAT3 for short) is suitable for 10MB/sec ethernet. Catagory 5 wire is required for 100MB/sec ethernet. Having said that, most people just install CAT5 even if they are only using 10MB/sec ethernet. Cable comes in a flat "satin" format, like phone cable, or as a round cable with a covering made of PVC (Poly Vinyl Chloride - SP?) or a material known as Plenium. PVC is the most common (round cable is easier to work with, IMHO, and PVC is cheaper than Plenium). Cable is commonly sold in rolls of 500 foot or 1000 foot (actually, now-a-days, it comes in a box). Pick a color that you will not mistake for other cables, but one that will not stand out too much.

Inside the PVC casing, you will find that the 8 wires are twisted, in pairs, and a "string". There are 4 pairs of wires. The pairing is important to avoid crosstalk (the flow of electricity in one wire causing a magentic field which in turn causes a flow of electricity in another wire). The wires are colored. Each pair has a solid and striped wire. For example, there is a wire that is solid green and it is paired with a wire that is green and white striped. Of course, the wire itself if copper, the colors are on plastic coatings over the wires. In an RJ-45 plug, the wires are always numbered in a consistant way. If you hold the plug in front of you, pointing away from you (as if you were about to plug it into a wall jack), with the "tab" pointing down, towards the floor, the wire on the left most side is wire #1. The wire on the right most side is wire #8. I'll try to come up with a picture. Wires #1 and #2 are one twisted pair, wires #3 and #6 are another, wires #4 and #5 are another pair, and lastly wires #7 and #8 are a pair. A common beginner's mistake is to try to use #3 and #4 as one pair and #5 and #6 as a pair (if you ask me, it's a common mistake because it seems more logical). Ethernet, really, only uses 4 of the wires. It uses wires #1 and #2 (one pair) and #3 and #6 (another pair).

Does it matter what color wires are used in which pair in the cable? Strictly speaking, no. As long as #1 and #2 are a pair and #3 and #6 are a pair, you are fine. Having said that, there are two common standards for what colors of wires to use for which number, and it's probaly best to follow one of the standards:

You'll notice that they only differ in which color (orange or green) is used for #1 and #2 and which color is used for #3 and #6. What is important is that both ends of the same cable follow the same standard. It is not important that two cables plugged into each other follow the same standard as each other, so long as both ends of both cables match the other end of that same cable. It might be a good idea, for consistancy, to stick with whatever standard your wall plates use. You may even want to put in a matching wall plate on either end of your longer cables that are strung through the walls, just to make sure that they agree with each other in terms of standards, and then have another shorter cable between your computer (or other network device) and the wall plate.

Lastly, to make a twisted pair ethernet cable, you'll need a crimping tool. Do your self a favor, and get one that does both RJ-45 and RJ-11/RJ-12. You never know when you will need to put in a phone wire too.

For RJ-45 jacks, you have many options. They make wall plates that have screws on the back, but most of them have a "punch down" block. I have to admit, in the past I have used the ones with screws because I don't have a punch down tool. You can use a punchdown type of wall plate with a small slotted (i.e. straight, not Phillips head) screwdriver to punch the cable into place. You can then trim the excess cable off (if you want it to look nice) with some wire cutters. If you are going to be doing a lot of punchdown work, buy a punchdown tool. The important part about using a punchdown tool is to pay attention to which side is going to trim the wire off. To use the ones with screws, you can strip the PVC off of the cable exposing the twisted pairs, and then strip some plastic off of the ends of the twisted pair wires, unscrew a screw slightly, wrap the twisted pair wire around the body of the screw, and screw it back down. Probably not the recommended method, but it works, and you save yourself $45 on a punch down tool. If you are using a screw type wall plate, I recommend sitting down and figuring out which screw is for wire #1, etc. and write, with a perminant marker, on the back of the plate, next to the screw, the number of the wire. When it comes time to wire it up, this will help you to remember which wire goes where. Remember, the jack is going to be numbered backwards from the plug. You may want to get an extra wall plate and plug and take a small bit of wire and wire yourself up a small example, for your reference.

As for making the cables, string the cable where you want it to go, and cut it off at the desired length. Leave a little bit extra to play with (maybe 6 inches). I like to use "lineman's pliers" to cut the cable with. Strip some PVC off of the cable. With the twisted pairs will be a string like material. With some small scissors, cut it off about even with the PVC. Untwist the pairs. Then, with your fingers, straighten out the pairs (they tend to retain some of the "twist", making them difficult to line up) by pinching the wire slightly, starting at the PVC, and pulling towards the end of the wire. Straighten each wire individually. Between your index finger and your thumb of your left hand (you can use either hand, I'm indicating left and right for example's sake), hold the wire that you are going to use for wire #1. It works best to hold the wires with your fingers pointing in the same direction as the wire (this will be useful when you put the wire into the plug). With the rest of you fingers in your left hand, hold the cable against your palm. With your index finger and thumb of your other hand, move wire #2 in place next to wire #1. Repeat this process until you have all the wires in a line, relatively close together (as close as you can get). Take your lineman's pliers, and cut the twisted pair wires to the same length. Carefully take the RJ-45 plug in your right hand and bring it up to the wires in your left hand (tab side down, of course). Gently push the wires into the plug using your left hand fingers (don't let go of the wires to do this, as you move your fingers forward, rock your index finger so that it is wider apart at the end of the finger tips). When you are sure that the wires are far enough into the plug that their order will not change (i.e. they have started into their individual holes), let up with your left hand fingers and push the wire into the plug until it won't go in anymore. Holding the plug onto the end of the cable, check the colors of the wires to make sure that they are correct. If they are, crimp. If not, then take the cable out and restart with straightening the wires. The age old question is, how much PVC to strip off, i.e. how much of the colored wires should be showing. If you have too much wire showing, then when you crip, then PVC is not being griped by the end of the plug, which is not good. If the wires are too short, then when you shove the cable into the plug, the wires are not long enough to reach into the plug to make a connection. It's best to make the wires too long and trim them down if needed (when you even them up with your lineman's pliers). As a rule of thumb, the wires should stick out of the PVC about the width of your thumb. When you are striping the PVC off, however, leave more wire and cut it off when you even the wires up - longer wires will be easier to work with when it comes to putting the wires into the proper order.

For home use, 10MB/sec ethernet is plenty fast enough. I would, however, use CAT5 cable because most devices these days will switch between 10MB/sec and 100MB/sec, and so if you have 2 devices that both support 100MB/sec, they will talk to each other at those speeds. Besides, it's getting very hard to buy anything less than 100MB/sec (though 10MB/sec stuff is much cheaper, if you can find it).

Cross-over cables are the ethernet equivalent of a "null modem" cable for serial ports. First, check to make sure that your hub does not have a special port designated as an "uplink" port. There may be a button so that if the button is pressed, it is an uplink port and if it is not pressed, then it is a normal port. An uplink port is an ethernet port that is already crossed over - you can use a normal cable with it. To make a cross-over type cable, on one end of the cable, wire everything as normal. On the other end, the wire that should go into wire #1, put in wire #2. The wire that should go into wire #2 goes into wire #1. Likewise, the wire that should go into #3 goes into #6 and the wire for #6 goes into #3. It may be easier to work with if you cut the other 2 twisted pairs off, even with the PVC. If you do so, be very careful that you hit the hole for wire #6. Mark your cross over cable as such. I like to take a perminent marker and color the sides of the RJ-45 plug as a visual clue to me that it is a cross over cable.

If you have long cable and a short cable, always make the cross over cable be the short cable. If you have to redo it a few times to get it right, you are redoing the short cable, not the long one. You may even consider getting an RJ-45 coupling just to have a long straight through cable up to the coupling, and then have a short cross over cable from the coupling to the destination. Having the non-standard cable be the short cable is a good rule of thumb for just about any kind of wiring.

If you have only 2 computers, you do not need a hub, you can use a cross over cable to connect them to each other.