Straight Through and Crossover type Cables
First off, before you decide to rewire your house, be fully aware of the dangers involved, be warned that if you wire the Ethernet cards wrong you could permanently damage them or your computer. Another thing worth knowing is that if your wiring is too long (more then 100m I believe), or too close to noise sources (power lines or anything generating a strong magnetic field) the data can be corrupted and your network will not function properly. Finally, you should probably know what parts are needed and have the required tools before you get started. Having said that, let's talk about the network a little, and maybe shed some light on what your options are, and how you may save some money.
There is a number of ways to network computers together, but the two most common ways are via the 10-base or 100-base Ethernet Cable which itself is split in two modes of operation:
The first involves 2 Ethernet Cards and one cross-over cable, the second requires one extra piece of equipment, the HUB. Needless to say the cheapest is the first option, peer to peer. Unfortunately, it is not the most elegant or the most techy way to do things, nor is it open to much expansion. The second option, using a Hub, is much more elegant, and expansion becomes a simple matter of plug and play, as long as the hub has enough ports available.
Peer to Peer in this mode the two computers communicate with each other directly. What this means is that what one computer transmits is received by the next directly, and vice versa. For this reason this type of network is for two computers only. (unless one computer has 2 or more NICs installed, in which case it can be connected to as many computers as it has NICs) If both computers have two lines one for Transmitting (let's cal it Tx) and one for receiving (let's call it Rx) then communication can be done by having wires two crossing wires connecting the Tx of one to the Rx of the other, and the other way around. Hence you would require a crossover cable for this kind of operation. This of course implies that only two of the 4 pairs are used, which is true, in fact you are interested in the green and orange pair only, the others are for another connection or for a phone line. A rule of thumb is that if a cable has an Ethernet card connected to both ends, then the cable must be crossed over.
Using a hub is the more expensive way to go, but you get what you pay for. It is open to expansion, and much easier to wire. The nice advantage is that there can be multiple computers on this network, an 8-port hub is common these days, and allows for up to 8 computers to be networked together. In this mode of operation, the computers do NOT talk to each other directly, there is an intermediate step, the hub. Each computer communicates with this device, which redirects the flow of data as required. Since this device is designed for this purpose it's ports are wired with the Tx and Rx lines reversed, thus a straight through cable would connect the Computer's Tx line with the hub's Rx line and the other way around. So each computer must have a straight through cable connected between it and the hub. Again, only 2 of the 4 pairs is used, the other can be used for another connection, or a phone line. As a rule of thumb, if you have a cable with a NIC on one end and a Hub on the other, the cable must be straight through.
The cable itself is called CAT 5 cable (you probably seen the most common type the blue cable), contains a set of 8 conductors. The 8 conductors inside are paired up to form 4 pairs each containing a solid colored wire and a white striped wire. The conductors in each pair are twisted around each other giving it the name twisted-pair. This is done to minimize it's vulnerability to EMF noise. (The physics behind it is simple, by twisting the wires around each other you minimize the cross-sectional area (in any direction) of any loop formed by the pair, thus minimizing the induced current formed by a changing magnetic field.) The colors used are Orange, Green, Blue, and Brown, thus the pairs are Orange & White/Orange, Green & White/Green, Blue & White/Blue and Brown & White/Brown. The ends of this cable is fitted with an RJ-45 head. Which looks just like a phone jack (RJ11) but instead of 6 connections it has 8(bigger). Now depending on what kind of cable you need, straight through or crossed over determines what order the conductors are connected to the RJ45 heads. Below are pictures showing what the cable and heads look like:
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This is the CAT5 cable, you can use sections up to 100m long, anything beyond that may cause data loss due to attenuation, and noise interference. Although the most common color is blue, you may find outher colors such as yellow, red, and green. Getting different cable colors can help you organize large networks, but generally, this is not a problem. |
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This is the RJ45 head or plug, the contacts are numbered 1 through 8 starting from the left. The trick is how to old the plug when you decide where left is. Holding the plug as if you are about to plug it in, with the clip facing down, you count 1 - 8 from left to right. Although on this page you will have pictures, other documentation, will tell you what color cable go with what number so it's worth knowing how it is numbered. |
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This is the tool used to fix the heads onto the cable, it has two purposes, one is to push the copper blades (connectors) at the end of the plug, through the isolation on the cable, and the other is to push a plastic wedge into the cable so that the plug will not slide off the cable throughout the cables use. (This does not mean that you can hang from this plug, so don't try it.) Some of these tools also have a built in wire stripper, you can use to strip the blue outside isolation to expose the conductors. However the conductors themselves need not be individually stripped. |
Now let us look at what order the wires go into the RJ45 head. When building a cable, one end should have the standard T-568B wiring; this is also known as the straight through configuration and is depicted in the chart below. Now the other end of the cable determines what type of cable it is. If the T-568B standard wiring configuration is used again the cable becomes a straight through cable, and should be used for computer to hub patches. If on the other hand, the T-568A standard configuration is used then the cable becomes a crossed over cable, which is used for computer to computer patches.
In short T-568B to T-568B makes a straight through cable while T-568B to T-568A makes a crossover cable.
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Standard T-568B or
Straight. This configuration should be on one end of the cable, no matter if you have a straight through, or a
crossed over cable. If used on both ends, then the cable is Straight through. The technical info here is:
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Standard T-568A or Crossed. This configuration should never be used on both ends, only on one end and only when a crossover cable is desired. else the cabe becomes a straight through cable, with the green and orange wires interchanged, take a look for yourself. |
That is all you need to know to successfully wire your small home office. Somewhere I have mentioned that peer to peer can be used even with more then 2 computers, this is true and it is the way my network is wired at home. One computer (the server) has 3 Ethernet cards installed in it, one for the High-Speed Internet, the other two are connected to the other members of my network (an upstairs computer and another downstairs computer). Using Windows XP the two network cards are bridged together, which allows all of the computers to see each other and share one internet connection. This configuration works fine for me and I have only chosen this path because I had an abundance of networking cards available to me, and the cheapest hub I had found at the time was about $60 CAD, so the free alternative won.