Telephone and LAN Wiring

It seems that every year or so I have to crawl around under the floor of my house and add/remove/replace some of the wiring. Our house is 20 years old (at least) and contains a lot of wire strung wherever anyone could put it. The person who built the house had a fanatical interest in telephones and installed phone jacks on every wall together with a very impressive system of speaker wires, alarm wires, intercom wires, TV cables and power cables. All in all quite a nest of snakes. I have added to this by installing a LAN.

Recently I decided to upgrade my LAN wiring from its original form to rather better quality Category-5 cable and since this involves being in the crawl space (or creepy-crawly space as it should really be known) I thought I'd string some new phone and TV wires while I was at it. 

I decided to bring all the cables (LAN, phone and TV) back to a central point in a closet under the stairs. This is pretty much a requirement for the LAN but is also possibly helpful for the phone system if I ever want to install a local phone switch. I am also considering doing a better job of distributing the various video feeds we have (VHS, DVD etc) around the house. So a central design for all cabling seems like a good idea.

Along the way we get to put a lot of plugs on cables. There are several established color coding systems for LAN and phone wiring and I have created diagrams for each possible case.

Phone Wiring

Telephone wiring hasn't evolved all that much since Alexander G. Bell first hooked a couple of Coke cans together with some hairy string. As soon as Mr. Bell had more than two working phone he found that there was a need to employ an old lady call Gladys to help one phone user get connected to another. Alex built a big machine for Gladys that contained a lot of plugs and sockets. Gladys connected calls by putting plugs (called jacks) for call circuits into the sockets for each subscriber.

Gladys in her younger days

A more complete history can be found at http://www.privateline.com/

Why am I telling you all of this? Only because the plugs used in prehistoric telephone exchanges had parts whose names have held over into the modern era. Here's a picture of a typical telephone plug used in a switching center:

 

The tip of the plug was connected to one phone wire and the ring to the other. In most modern systems the connections are still often called tip and ring.

The tip connection was at 0V and the ring connection at about -48V. 

Most home phone systems (until very recently) were installed using one or two-pair cable. The cable wire colors were green, red, black and yellow. Originally the green and red pair was for the voice circuit and the black and yellow pair was used to provide a low voltage from a transformer to the telephone to light the dial. More recently the black and yellow pair is used for a second line. 

Wire Tip/Ring Voltage Function
Green Tip 0 Line 1
Red Ring -48 Line 1
Black Tip 0 Line 2
Yellow Ring -48 Line 2

Modern phone cables are terminated using modular jacks. These jacks can accommodate up to six wires (for three phone circuits). They are typically used with cable having one or two pairs and wired as shown below. The phone companies refer to particular cabling arrangements by RJ numbers. This stands for Registered Jack. The two common forms used at home are RJ11 and RJ14.

This is the simplest case where only one line is used. The line connections use the center pair of conductors in the jack.

This is the other common case where all four wires are terminated in the jack. This case uses the center pins for line 1 and the pins on either side for line 2.

Very recent wiring in homes is done with Category-3 or better cable which uses a different color scheme. The color scheme used allows for up to 25 pairs of cables (50 wires) with each individual wire having a different color or at least each pair of wires being identified with a unique set of colors.

There are five base colors: white, red, black yellow and purple. These are used to identify groups of wires. There are also five tracer colors: blue, orange, green, brown and slate (gray).

The idea is that a pair of wires is based on one of the base colors with the 'ring' wire also sporting the tracer color. So for example the first pair of wires is nominally white with the ring wire having a blue tracer. In practice this varies and all we can really say is that the tip wire is mostly the base color and the ring wire is (sometimes) mostly the tracer color. So for the first pair of wires there are four common possibilities:

Tip wire color Ring wire color
White Blue
White with Blue tracer Blue
White with Blue tracer Blue with White tracer
Blue with White tracer Blue

This sort of thing annoys me. If you are going to have a standard, have only one way of doing things not four.

The phone companies are installing buried cables to homes around here that contain six pairs. The colors used in the cable to my house are:

Pair Tip Ring
1 White with Blue tracer Blue
2 White with Orange tracer Orange
3 White with Green tracer Green
4 White with Brown tracer Brown
5 White with Slate tracer Slate
6 Red with Blue tracer Blue

You may also find that the ring wires contain a tracer of the main group color:

If your home uses the newer Category-3, 4, or 5 wiring your internal phone wires will consist of pairs of blue and white and orange and white cables. If you have to wire them into RJ11 or RJ14 jacks, they should look like this:

These colors are common in commercial installations but may be coming to a house near you soon.

LAN Wiring

When I say LAN I mean 10-Base-T or 100-Base-T not coax or some other strange silly IBM system.

A lot of LAN wiring was done with Category-3 wire which was good for 10 Mbit systems. Most new installations are done with Category-5 cable which is good for up to 100 Mbit systems. If you are wiring a home, I'd put in Category-5 cable for everything - phones, LAN, intercom, the lot. The cable doesn't cost all that much and in any case the cable cost is very low compared to the cost of actually putting it in the walls etc.

Category-5 cable comes with different numbers of pairs per cable. Most common for LANS is 2-pair or 4-pair cable. If you are being economical (tight) you can use 2-pair cable. You wire an RJ45-style jack like this:

Note that although an RJ45-style jack has 8 connectors only 4 are used for LAN cabling.

It is more common to use 4-pair cable to wire LANs (even though only 2 are used for the LAN). If you do this you can conveniently also use the outlets for phones since the common RJ11 and RJ14 phone jacks will fit in the outlets and the center pair will serve as the line 1 pair as usual.

There are two standards for wiring 4-pair cables into the jacks: EIA/TIA 568 A and EIA/TIA 568 B. Once again this is very silly since we only need one way. 568B is the AT&T standard. Most of the rest of the commercial world ueses 568A. I have adopted 568A for my own system at home. 

NOTE: If you put a 568A plug on one end of a cable and a 568B plug on the other end you get a cross-over cable which you can use to link two hubs. This can also be very confusing if someone else wires part of your network using one scheme and you do the rest differently. Pick a scheme and use it everywhere.

Notice that in both cases it's the green/white and orange/white pairs that carry the LAN signals. The blue/white pair is reserved in case you use the cable for a telephone circuit and the brown/white pair is reserved for the FBI which uses it to spy on your network traffic. Actually although it seems wasteful, the spare brown/white pair can come in handle if a rodent selectively eats one of the other pairs. In my experience though, rodents eat more than one pair - all colors being equally tasty.

Network Doubler

If you run 4-pair Cat 5 cable to all your outlets and to your patch panel you can make some simple patch cords to use at either end and get 2 LAN circuits over each cable.

The green and orange pairs are the normal circuit. The blue and brown pairs are used for the second circuit.