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Degradation plagues digital signals on ordinary wires |
Ethernet is yet another one of the wonderful things that came from Xerox research, but that Xerox never particularly profited from. You should also thank them for the graphical user interface, the mouse, and the laser printer.
The original Ethernet ran in special coaxial cables (with a center conductor and a shield, like cable TV cables.) Today, that's called "thicknet" and it is virtually non-existent. Next came a cheaper coax called "thinnet" or "10base2". There's still some of this around, but it is rarely used for new installations because it is a total pain to maintain. Instead, Ethernet now nearly always runs over twisted pair cables, just like phone lines, generally called "10baseT", for 10 MHz on twisted pairs, or 100baseT for 100 MHz. Data cables and telephone hookup cables are all data communications cables, and they follow the same or related standards. It also runs on wireless connections using radio signals (Apple's AirPort) or Infrared, and in your house over your existing telephone lines using Intel's Home Network system.
Then came voice. Voice is a fairly low frequency (up to about 3K hertz) signal that is easily carried for fairly long distances by a pair of wires. Phone companies strung billions of pairs of wires, marring the view from almost every window in America, and then hired really smart geeks who realized they could carry more than this one low frequency signal over a pair of wires, so eventually they found ways to multiplex several voice lines onto a single pair of wires. While this saved money and made it economical to keep the rat's nest of wires hanging in the sky, it ultimately limited the bandwidth available on ordinary phone lines to voice frequencies, as higher frequencies couldn't be wedged into the multiplexing system. See "Why My Phone Company Won't Give Me ADSL", later in this web site.
Then came modems. They put digital data onto ordinary voice phone lines by using sounds to represent the 1's and 0's in the data transmission. The earliest modems only did 110 Baud - about 10 characters per second - because that's as fast a teletype machine could print. Later came 300 baud, 1200 baud, 2400 baud, 9600 baud, 28.8 Kbaud, and finally 56 Kbaud. All these modems use sounds within the audible voice spectrum to transmit digital data. The higher data rates are obtained by encoding multiple bits into a single sound, plus using sophisticated digital signal processing to recover the information at the other end. But it's still going over lines designed to only carry voice traffic.
Then came digital transmissions. Digital data can be sent over a pair of wires, but there are some problems to deal with. A digital signal is a step function: an instantaneous rise from one voltage to another. But on a long wire, various electrical effects will cause the step to get stretched out into a more gradual transition. There are also potential problems with reflections: when a pulse goes down a wire and hits the other end, there is a new pulse that bounces back down the wire, just like when a wave hits the wall of a swimming pool. Also, when you transmit digital information down a wire, you lose signal strength as the wire gets longer. You are also susceptible to noise getting into the wire, like from fluorescent lights or motors. All of these effects can make it hard to correctly interpret the 0's and 1's at the receiving end of the line.
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Degradation plagues digital signals on ordinary wires |
Telephone wiring also uses a kind of cable called "Silver Satin". This is the relatively flat cable usually used from the wall to the phone itself. This kind of cable should not be used for data, as it does not have the twists in it that are essential to reducing electrical noise that will cause data errors.
Most communications cable today consists of twisted pairs. The twists are put in by thousands of prisoners in China who were laid off when Americans stopped buying tennis shoes with the laces already in them and started putting in their own shoelaces instead. The purpose of the twists is to prevent electrical noise from disrupting the data signals. The two wires are used in a differential mode; that is, the signal is the electrical difference between the two wires. If they are tightly twisted together, then any electrical interference that gets into one wire will also get into the other one, so there is no net difference. This is the secret to reliably high-speed data communications over twisted pairs of wires.
The standard connector for telephone equipment in the US and for data connections everywhere is the modular connector. For voice service, the 4-pin or 6-pin connector, called "RJ-11" is used. For data, an 8-pin version, called "RJ-45" is used.
This picture shows the wiring for the plug side of an RJ-11 connector. Can you say "BRGY"? (Think about cheap red wine; I know I do.) The diagram is shown with the "hook" - the little thing you press on to get the plug out - underneath. RJ-11 sockets (the thing the plug sticks into) always have the colors indicated on the connector terminals.
Note that the connector on the other end of an RJ-11 connector is wired in reverse order. That is, if you stretch the cable out flat, the Black wire stays on the left all the way to the other end, including through the connector with the hook oriented down also. Also note that the RJ-11 connector has six potential terminals on it. Only the middle 4 are normally used, and sometimes only the middle two. Line 1 is the center pair: red and green, as in Christmas.
Eight-conductor data cable contains 4 pairs of wires. Each pair consists of a solid (or predominantly) colored wire and a white wire with a stripe of the same color. The pairs are twisted together. To maintain reliability on Ethernet, you should not untwist them any more than necessary (like about 1 cm). The pairs designated for 10 and 100 Mbit Ethernet are Orange and Green. The other two pairs, Brown and Blue, can be used for a second Ethernet line or for phone connections.
There are two wiring standards for these cables, called "T568A" (also called "EIA") and "T568B" (also called "AT&T" and "258A"). They differ only in connection sequence - that is, which color is on which pin, not in the definition of what electrical signal is on a particular color.
T-568A is supposed to be the standard for new installations, while T-568B is an acceptable alternative. However, a lot of off-the-shelf data equipment and cables seem to be wired to T568B. T568A is designed to be consistent with phone system wiring. It's important not to mix systems, as both you and your equipment will become hopelessly confused.
Pin Number Designations for T568B
| Color Codes for T568B | |||
| Pin | color | pair | name |
|---|---|---|---|
| 1 | wh/or | 2 | TxData + |
| 2 | or | 2 | TxData - |
| 3 | wh/grn | 3 | RecvData+ |
| 4 | blu | 1 | |
| 5 | wh/blu | 1 | |
| 6 | grn | 3 | RecvData- |
| 7 | wh/brn | 4 | |
| 8 | brn | 4 | |
Note that the odd pin numbers are always the white with stripe color.
The wires connect to RJ-45 8-pin connectors as shown below:
RJ-45 wiring for Ethernet (T568B standard)
Note that the blue pair is on the center pins; this pair translates to the red/green pair for ordinary telephone lines, which is also in the center pair of an RJ-11. (green=wh/blu; red=blu)
Pin Number Designations for T568A
The T568A specification reverses the orange and green connections, so that pairs 1 and 2 are on the center 4 pins, which makes it more compatible with the telco voice connections. (Note that in the RJ-11 plug at the top, pairs 1 and 2 are on the center 4 pins.) T568A goes:
| Color Codes for T568A | |||
| Pin | color | pair | name |
|---|---|---|---|
| 1 | wh/grn | 3 | RecvData+ |
| 2 | grn | 3 | RecvData- |
| 3 | wh/or | 2 | TxData + |
| 4 | blu | 1 | |
| 5 | wh/blu | 1 | |
| 6 | or | 2 | TxData - |
| 7 | wh/brn | 4 | |
| 8 | brn | 4 | |
And to end the discussion of wire colors, here's a translation table from the BRGY used for two-pair lines to the more colorful combinations used in larger lines:
Translation Between Alternative Color Code Systems
| Pair
Number |
Color | Alternate Color |
Pin Number |
|---|---|---|---|
| 3 | White/Green | White | 1 |
| 2 | White/Orange | Black | 2 |
| 1 | Blue | Red | 3 |
| 1 | White/Blue | Green | 4 |
| 2 | Orange | Yellow | 5 |
| 3 | Green | Blue | 6 |
Straight-Through vs Cross-Over
In general, the patch cords that you use with your Ethernet connections are "straight-through", which means that pin 1 of the plug on one end is connected to pin 1 of the plug on the other end. This is different
from voice telephone cables. Voice cables are "crossed" end to end; data cables aren't.
The only time you cross connections in 10BaseT is when you connect two Ethernet devices directly together without a hub. Then you need a "cross-over" patch cable, which crosses the transmit and receive pairs, the orange and green pairs in normal wiring. See the local networking for more details on doing this, along with some setup info for the sorry excuse for an OS called "windoze".
Really Big Wires
Tim Courtemanche (timc@acti.net) reports that the real telco pros have a scheme for dealing with gobs of wires (more than any nethead needs). It's outlined below, just in case you feel like climbing a pole and re-doing your block some Saturday. Isaac Wingfield was the first supplier of this data, but seems to have gotton his primary and secondary colors reversed, according to Tim. If Isaac begs to disagree, we'll have a face-off and get someone from Lucent to referee. Tim also reports that the signal on the primary color wire is called "TIP" and the one on the secondary is "RING". Some of you may recall when 1/4-inch phone plugs were actually used for telephones and their parts were ring, tip, and sleeve. Those of you still in knee-pants when I had my first legal hangover can think about stereo headphone jacks. "RING" is as in the little circular part of the plug that's not the tip or the sleeve, as opposed to the one-ringy-dingy, two-ringy-dingy kinda ring. Tim also reports not wanting you to experience any "tipovers" whilst climbing the neighborhood phone poles, but has not yet shared exactly what a tipover might be. (afternote: Well, Tim told me it's just when you reverse the two wires in the pair. I thought it would be something way more exciting than that, and am really disappointed that such a great word has such a boring meaning.)
The primary colors are white, red, black, yellow, violet, and the secondary colors are blue, orange, green, brown, slate, in that order. Pairing runs through all primaries with the first secondary, then all primaries with the next secondary, and so on. (For more than 25 pairs in a cable, 25 pair groups have a loose wrap of nylon thread, where the thread colors follow the same sequence.) Anyway, by this standard, the pairs become:
1) white,blue
2) white,orange
3) white,green
4) white,brown
5) white,slate
6) red,blue
7) red,orange
...and so on
Note: An artist friend told me that "slate" is what us Crayola types call "grey". Or is it "gray"? Or is it just Netscape without a colored background?
RS-232 is a definition of signal names, functions, and voltages. Some systems use an updated standard, called RS-422 instead. For many years it used a standard "D"-shaped connector with many more pins than the protocol needed. Today you see it in little round DIN-8 connectors a lot.
A side-note on those DIN-8 connectors:
This is clearly a connector designed by a committee, as it is the least user-friendly connector I've ever seen. How could anyone - even a committee of DINS, whoever they are - come up with a round connector with no significant cues about what way to plug it in? I know, there's a little square do-hickey in it, and if you put on your glasses and get the light right, you can see how to line up the plug and socket. But how much brains would it take to have put a flat side on the connector or something?
Anyway, here's the description of a cable I made to hook up an older router to my Mac. The router needed an RS-232 connection for control, so I had to identify the pins and figure out how to make a cable. For your RS-232ing pleasure, here's the results.
It is possible to put an RJ-45 connector onto a 4 to 8 conductor cable if you're a masochist. Or if you build ships in bottles for fun. For everyone else I strongly recommend going to your nearest CompUSA, or Fry's if your up to it, and just buy cables with RJ-45s on them off the shelf. I think they make these with machines, as it is almost impossible for a human to get all the wires stuck into the right spot on the connector. I'm a human and I know. If you insist on doing this yourself, get a good crimping tool.
RJ-45 sockets on the other hand are pretty easy. Just make sure you get one that's rated for Cat 3 or Cat 5. If you do, it will have the color codes for the wires (orange, white/or, green, white/gr) marked on it.
I highly recommend the Leviton products for wall mounts. They have faceplates that accept up to 4 different sockets, and they have sockets that snap into the faceplates for RJ-45, RJ-11 (telephone), speakers, cable TV. They aren't expensive relative to the alternatives, and may be available at your local Home Depot.
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