Jack is one of those English words that can mean virtually anything you want it to mean…just like some recent technical standards. All of our cars have jacks, although we rarely if ever use one to change a tire. The Jack of Diamonds is a knave and known as a hard card to play in Pinochle and in social commerce. Jacks, as a game, is favored by young girls as it has been for centuries. There are lumberjacks, jacks-of-all trades, a well known recent president, Jack Kennedy, car hi-jackers and jack as currency. And on the street someone may be told, “You don’t know Jack.”

Control4 Home Automation

Our interest in jacks is more specific: the phone and data jacks sprouting up on walls all over modern residences. Jacks are used to connect phones, computers, intercoms, fax machines, doorphones, even whole house music. Why jacks? Why not just “hardwire” everything? Because today’s homeowners live in a dynamic world. New things replace old, new lifestyles mean rooms change their function and jacks let you plug things in and out, as you wish.

In the case of LANs and other data circuits, jacks also serve a technical function. They are the simplest way to ensure a broadband, reliable connection. Moreover, jacks provide great access points for testing and troubleshooting.

What else is there to know? Aren’t all those jacks RJ-11’s, RJ-45s, or something? And aren’t they all the same? Can’t I just plug my router into one and my telephone into another? And interchange them if I want to? They seem to fit together mechanically.

Would that life were that simple.

To begin with, don’t be so sure you know what an RJ-45 is or an RJ-11 for that matter. At last count, there were eight ways an “RJ-45” could be wired.

So for perspective, some basics and a little history. At one time all telephones were in fact hardwired and owned by the phone company. It was not unusual for the same phone to remain in place 20 or more years. However, by the mid-1970’s deregulation was on the horizon and Bell Labs developed the modular phone concept. That meant owners could buy a phone in a store and plug it into a standard jack at home. All future devices like modems and answering machines would connect with the same jacks and plugs.

The modular jack design was very clever. Instead of leaf type contacts, small gold plated wire springs comprise the electrical contacts. That ensured both low cost manufacturing and low loss transmission of voice and data. Moreover, the approach allowed enough design diversity to cover a wide range of telecom applications. Here is what evolved.

Miniature Modular Telephone Jacks (that’s the official name) come in different sized molded plastic housings: 4 positions, 6 positions and 8 positions. Position refers to the number of possible wiring contacts that a housing accommodates.

Modular plugs, the connectors that mount on the end of a cable, are available in the same three mechanical sizes. The plugs have gold plated conductors molded into a plastic housing. A locking tab or tang secures the plug with a click when inserted into a matching jack. In the field, installers press connectors onto the ends of telephone cables using a hand tool. Notice that cable pair one is always assigned to the center pins, but the center pin numbers are different on each size of jack or plug.

Back to the language for a moment. The arrangement of jacks and plugs is often referred to as a male and female configuration. The slightest acquaintance with anatomy makes that clear. What is not clear is why we don’t call the female gadget a Jill instead of a Jack. Oh well, what do you expect from an industry that says something is On when it is Off–Hook and Off when it is On-Hook?

The 4P (four position) housing handles up to four contacts (4C). The 4P size commonly connects a telephone handset to its base via a curly cord. It is a 4P4C device. That is, a four position housing serving four conductors.

Residential and commercial site wiring uses the 6P (six position) and 8P (eight position) sizes.

To describe a modular jack, specify both the number of positions and the number of contacts. Specifying the number of contacts is important because the 6 position housing is used in telephone work as an RJ-11 (6P2C) for one line, an RJ-14 (6P4C) for two lines or RJ-25 (6P6C) for three lines. It’s simple math, each phone line has two conductors.

OK, it is time to talk seriously and in a finer granularity about RJ- jacks and plugs. Historically, homes in North America were wired by the phone company according to Universal Service Ordering Codes known as USOC codes and pronounced You Sock. Bell System discipline ensured that technicians installed wiring conforming to the USOC specifications on the service order.

When the Federal Communications Commission (FCC) deregulated things, they used USOC codes as their model and specified USOC numbers in Part 68 of their rules and regulations. Part 68 covers what is necessary to interconnect privately owned equipment to the PSTN…the Public Switched Telephone Network. Their rules required testing, certifying and registering everything, and that means everything, connected to the PSTN. By the way, Part 68 rules have the force of law in the USA.

So, when the FCC started registering telecom jacks…that’s what RJ stands for: Registered Jack…they used USOC and Bell System specs. Now here is where almost everyone gets confused.

RJ stands for registered jack and the terms RJ-11, RJ-14, RJ-21, RJ-22, RJ-25, RJ31X and RJ-45 are in general use. But in fact, those designations are a misnomer and often cause confusion. Ever wonder why there are RJ-11 jacks but no RP-11 plugs?

Because: Originally RJ-11 referred to a circuit configuration with a jack. Not the jack itself. Since officially there was no RJ-11 jack, no little piece of hardware recognized as an RJ-11 jack, there is no RP-11 plug. Instead, there is a “miniature 6 position jack” and a “miniature 6 position plug”. As Ripley might say, “Strange but True.”

A real RJ-11 as described by the Universal Service Order Code is shown below.

This is how the FCC views an RJ-11. This is how the telephone company files its tariffs. They describe it as a bridged single line connection.

USOC also addresses RJ-11C and RJ-11W jacks. Those designations show up occasionally but, by and large, they are OBE. (Overcome by Events…obsolete.) The C suffix means the circuit appears in a jack mounted on the surface. RJ-11C implies the installer will mount a little connector box on the baseboard. These boxes, commonly called “biscuit boxes” have a 6P2C or 6P4C jack on the side.

A W suffix meant the circuit was for a wall-mounted telephone. When an installer saw RJ-11W on his service order, he knew that he should install a jack mounted in a housing with two shiny protruding steel studs. A wall telephone hangs on the studs.

While biscuit boxes are still handy, especially for additions to an existing system, today new residential phone jacks are flush mounted. The builder or his subcontractor puts a single gang electrical junction box on the wall where a phone is desired. The phone installer pulls a cable to the box before the drywall is nailed up and taped. Later he installs an…ahem…RJ-11 jack with a faceplate that matches an electrical outlet. The whole thing is flush with the wall, much neater and more esthetically pleasing.

RJ-11s in the real world. Away from regulators, bureaucrats and pedants, everyone knows an RJ-11 isn’t some FCC sanctified circuit. It is an electronic component, a part, a physical interface connecting one phone pair. Similarly, an RJ-14 handles two pairs and an RJ-25 three pairs. All telecom people know an RJ-45 has 8 positions, handles all 4 pairs of a CAT-5 cable and was originally used for data wiring.

OK, OK, what do we call the jack used in an RJ-11 wiring configuration? Easy. The jack is a 6P2C miniature modular jack. It mates with a 6P2C modular plug. But we live in an imperfect world and few if any 6P2Cs or RJ-11s are ever installed. Why? Because all phone cables have at least four conductors. There is no cost difference between 6P2C and 6P4C jacks. If you have four conductors in the cable, why not install an RJ-14 (6P4C) and use all the copper in the cable you just bought? And, in fact, that is what everyone does.

RJ-14s in the real world. The USOC code for an RJ-14 says it provides bridged connections to the tip and ring conductors of two separate telephone lines. Bridged just means that the wires may also connect to other extension phones in the house. Daisy Chained is the operative word. RJ-11s and RJ-14s are quite similar. Look into an RJ-11 and you see two gold wire springs. Look in an RJ-14 and you see four gold contact springs. That’s the only difference.

Bonnie and Clyde and other twisted pairs: A caveat about that second phone line. Around 1950, Ma Bell started using four conductor, inside wiring cable for all residential phones. Called JKT, Jake, Quad or D-Wire, it uses the familiar red-green-yellow-black color code. The red/green conductors handle the wire pair for line one so yellow/black can handle the wire pair for line two. Right? Well, maybe.

The Bell System never intended JKT to handle two phone lines. The yellow/black wires were supposed to power the night light in a Princess phone or provide an auxiliary signaling path but not to serve a second line. Putting two phone lines in the same cable requires that the wires serving each line must be twisted around each other. This prevents crosstalk and electromagnetic interference. In effect, the intruding voltages in the paired conductors cancel each other out.

JKT is not paired. The four conductors are laid randomly inside the cable sheath. If you use the yellow/black wires for a second phone line, it may work. But it also may pick up electrical noise from furnaces, air conditioners, blenders, hairdryers and refrigerators. Moreover, if you have a dial up internet call on one line and the other phone rings, it may cause the Internet connection to drop.

Whether an RJ-14 arrangement lets you use the vacant wires for a second line depends on how the wire was routed and how lucky you are. Nevertheless, the best practice is to use an RJ-14 instead of an RJ-11.

RJ-25. Eventually the phone company switched from JKT to 3 pair inside wiring in homes. This is a true UTP or Unshielded Twisted Pair cable. Each pair can serve a phone line without interference. If your home’s phone cable color code is Red/Green, Yellow/Black and Blue/White, you have the newer 3 pair wiring and can use an RJ-25 jack for your telephone connections. An RJ-25 is a 6P6C modular jack. Very much like the RJ-11 and RJ-14 but with all positions populated with conductors.

RJ-31X The FCC considers the RJ-31X a mandatory jack if you have dialup alarm service. Burglars casing a home look for a telephone near a window, break in and knock the phone off-hook. A few seconds later, the alarm system kicks in, detects the intrusion and tries to send an alert to the police or alarm company via the telephone line. Alas, it cannot. The phone line is busy.

The RJ-31X solves this problem. It wires in series with the incoming phone line, ahead of all telephones, PBXs, Interphones or other devices. The alarm sending unit plugs into the RJ-31X and when it does, the phone line routes through the alarm unit. When the alarm system detects a break-in or when a smoke detector trips, the alarm system opens the phone line downstream of the RJ-31X. Then it goes off hook towards the telephone central office, gets dial tone and dials the alarm. Thus, it doesn’t matter if the phone is on hook or off hook.

Finally, if the alarm system is unplugged for service, the RJ-31X contacts close internally, routing the phone line directly to the house phones. RJ-31X jacks are available as part of structured wiring systems or as standalone units mounted in a biscuit terminal box.

RJ-45 or Welcome to a Can of Worms. RJ-45s are the most common jacks installed in new homes…especially in new homes built with structured wiring. Moreover, Ethernet LANS all interface through RJ-45s. That may be why everyone thinks they know what an RJ-45 is. The result is that too often the customer orders one thing and the installer puts in another.

Progressing from RJ-11 to RJ-14 to RJ-25 was nice and orderly. There was physical compatibility in that plugging an RJ-11 into an RJ-14 or an RJ-14 into an RJ-25 had ensured that you would connect to complete pairs. As shown earlier, pair one (line one) always appears on the centermost set of pins). The next two out are the second pair, and so on.

USOC codes formed a nice basis for the original FCC standards but when the 8P8C jack appeared, the Bell System no longer controlled standards and the FCC tried to please everyone who had an interest in the PSTN and a lobbyist in Washington.

RJ45, 8P8C Plug Pin Assignments
Shown with locking tab up

Meanwhile, back at the Labs, Ma was showing hardening of the arteries. She first established an 8P8C jack standard, (RJ-61X). Its wiring plan followed the scheme of the other modular jacks and nested lines 1,2,3,4 on successive pins starting from the center of the jack. However, 8P8C jacks are mostly used for data. Ma’s plan put one conductor of pair four on pin 1 and the other on pin 8. OK for voice but a bad scene for high speed data. It remains the USOC 8P8C “analog” standard.

Then Bell Labs came to with a special data modem jack called the RJ-45S. It was an 8P2C layout with two other conductor pins used for a terminating resistor. The physical size was modular 8P but it also had a keying feature to inhibit interconnection. The RJ-45S is history but it contributed its name to the RJ-45 fray.

To fix the high-speed data problem, ATT (founder of the Bell System) came up with ATT 258A. This is an 8P8C jack with pairs one and two on the same pins as the RJ-14 and RJ-25. However, pairs three and four do not nest symmetrically but have their conductors side by side on either end of the RJ-45. Yes, this is called an RJ-45 too. ANSI our national standards institute and the EIA/TIA specify the ATT 258A as the T-568B.

T-568A/T-568B. ANSI and EIA/TIA were not finished with RJ-45s. They also have a T-568A which is another 8P8C standard. It is similar to the T-568B except that the orange pair two and green pair three are interchanged. Before we go, remember there also are 10BaseT Ethernet and ISDN configurations.

Enough! Isn’t there an easier way? Well, yes. The manufacturers enforce a standard for actual RJ-45 jacks and plugs by making them to FCC dimensions and specifications. As for home and light commercial wiring, you only need to think of T-568A and B.

* Commercial systems use T-568B, the old ATT 258 layout.
* Residential systems use T-568A as a de facto standard by industry consent.
* Federal Government contracts use T-568A as a specification requirement.

Caveat: Not everyone follows these rules. Some commercial guys who occasionally wire homes use the commercial standard T-568B in residential work. As long as a home uses one or the other…not both, and if the installer leaves a job record on site, little harm is done.

Category Cables

All offices and most new homes use “category” cables for low voltage telephone, LAN and data wiring. Most often these are called Cat 5 or CAT 5 but like the term RJ-45 jack, Cat 5 is a very generic label and there are several categories of note. Another label that applies is UTP or Unshielded Twisted Pair.

All Cat 5 cable used in homes consists of eight conductors of 24 gauge (AWG) solid copper wires. Each individual conductor is insulated with PVC plastic according to a standard color code. The eight conductors are twisted into four pairs and then covered with a common, flexible, plastic sheath. The result is a compact cable of very high quality. As it is manufactured in huge quantities, the price is surprisingly low.

Cable categories range from Cat 1 to Cat 7 but only a few are of interest in residential wiring. The difference between them is largely their bandwidth and resistance to various forms of crosstalk and other mutual interference. The main physical difference between say, Cat 3 and Cat 5 is the number of twists per foot of the conductor pairs. The cable type and other pertinent information is printed periodically on the outside sheath

Cat 3 handles data at speeds up to 16mbits but is mostly used only for voice. Cat 5 handles data up to 100mbits. It was superseded by Cat5e but the name lingers. Cat 5e handles data up to 1000mbits. It is the standard for most home wiring today. Cat 6 handles data up to 400mhz. It is used in most new office wiring and in high-end home installations.

In days of yore…yore? Anyway, until recent years, an electronics installer spent a lot of time figuring out which wire screwed down to which terminal pin. Or what color wire got soldered to pin 23. One of the great things about Category or Structured Wiring is that an installer need never worry about to which pin number which color wire connects. Manufacturers take care of that by routing PCB traces to the proper color terminals on a 110 punch down block or to the correct pins of an RJ jack.

The figure above shows how Tropical Telecom Corp maps the differences in pin numbers in their residential phone products. An installer terminates the Cat 5 cable by matching the color of the cable pair to the color swatch printed on top of the 110 block. Similarly, the installation tech could plug a Cat 5 into the RJ-45 jack and be sure the connections match. The curvy lines are the printed circuit board traces on the Tropical Telecom module that makes it all match.

Fixed standard color coding is one of our biggest time savers and a good example of why structured wiring, jacks and Cat 5 are such an efficient way to wire homes and small offices.

About that 110 Block

This is another good thing that Bell Labs and Mother Bell bequeathed to us. 110s are small IDC terminal blocks. That’s IDC as in Insulation Displacement Connector. Terminations on an IDC block are made by punching down the wire with a special tool. The wire is cut to the approximate length and placed in a 110 slot of the proper color. It isn’t stripped of its insulation nor otherwise prepared. The punch down tool forces the wire in between a pair of calibrated sharp edges that force the insulation aside and make a secure electrical and mechanical connection. 110 block connections are more reliable than screw terminals or soldering and infinitely easier to make. The tool even automatically trims off excess wire beyond the terminal.

Some 110 blocks show dual color makings to allow use with either T-568A or T-568B schemes. This can be confusing to installers. Therefore, best practice is to avoid such blocks in residential jobs.

Often modular RJ-45 jacks (called keystone jacks) have 110 type punch down blocks on the back to make connections with the Cat 5 before mounting into a wall outlet box.

Tropical Telecom Corp. doorphone showing 110 block and RJ45 jack. Orange device is plug-in screw terminal header for a remote door strike.

A note or two in passing. The predecessor of the 110 block was a big ungainly thing called a 66 block. It too is an IDC device, reliable and secure. But it is lousy at transmitting data. The 110 block is cheaper, oriented to Cat 5 and color coded. That is why professional installers don’t use 66 blocks in new work.

In Europe, Asia and some parts of the Northeast US, Krone IDC blocks are used in lieu of American style 110 blocks. They are very similar but not identical, have some slight technical advantages and require different installation tools. Krone was a German company and the Fortress Europa concept made them very popular as a protest against US product dominance. We have the last laugh however. Krone is now owned by an American company from the Mid-West.


Modular connectors and 110 blocks require a few specialized hand tools. While gas pump pliers and screwdrivers won’t get the job done, modern wiring techniques don’t require anything expensive or exotic. Your local Radio Shack, Home Depot, or neighborhood hardware store has what you need. or, order online.

First priority is a good modular phone plug crimper. Make sure the one you buy has both 6P and 8P dies for RJ-14 and RJ-45 plugs. Expect to pay $25 to $45 for a reasonably good one. Buy a small bag of modular plugs while you are at it. They are sold on the same shelf. Ideal and Palladin are good brands.

Next you need a Punch down tool. These range from light duty plastic tools to heavy duty fall-off-the-truck-on-the-highway-and-it-won’t-break tools. Plastic ones range from those so cheap that Leviton packages one free with their keystone modular jacks. Another yellow plastic device is for sale under $5, has a punch down tool, and a handy Cat 5 sheath stripper. For professional installs, I use an ancient Dracon spring loaded punch down tool but these little plastic tools are amazingly durable and fit in a shirt pocket for troubleshooting chores.

Wiring Homes To The US Standard

Walking down the street one day, near our factory in Shenzhen China, I came across a sign recently translated into English. It read, “We Specialize in Everything”. That’s almost like the standards situation in the US. It seems possible to wire a home terminating the wires at random and still claim you meet the US standard. In telephony, like the Chinese shopkeeper, we have a standard for everything.

Nevertheless, competent installers and engineers use best practices. Moreover, competent homebuilders and owners demand best practices. That means structured wiring. Use Cat5e or Cat 6 and homerun everything back to a central cabinet. Run cable to every place the human mind can conceive of ever needing a communications device. That includes doorphone runs to front and rear entrances. And extra phone jacks near the TVs for satellite control modems. Best practices call for terminating those runs on modular jacks for phone and data circuits. They mean flush mounted jacks in walls using j-boxes or mud rings.

Any modern home should go beyond telecom wiring. RG-6U coax cable, even fiber optics need consideration but this article is about telecom jacks. And, it is time to look at one of the common dilemmas faced by homeowners who insisted on best practices and standards and still can’t plug in their phones.

Here is what happens. Today’s phones are made with the simplest (and cheapest) connection device possible and that is a 6 foot two-conductor line cord with a plastic 6P2C modular jack. If your home has Cat 5 cables and RJ-14 jacks in your walls, you can just plug in the phone. But I wonder…and so should you…about what the installer did with the extra pairs of wires. I said wonder…not worry. Ask the installer to demonstrate how he treated the extra pairs and make sure he terminated pair one, the blue/white pair on the center pins of the wall outlet jacks. If your home has RJ-25 jacks there is only one pair left over to wonder about.

If you have RJ-45 jacks there are no surplus pairs. All four terminate on the RJ-45. but now you do have a problem to worry about. RJ-11s, RJ-14s, RJ-25s all use the same 6P physical housing. So not only do the center pins always carry pair one, but the plug from your phone always lines up mechanically when it snaps in.

However, the RJ-45 is made with an 8P housing. Certainly pair one will be on the center pins but the plug from your phone will make only a sloppy mechanical connection. “Gee is that a problem? I see people do it everyday.” Yeah, and I see people run red lights everyday, but I don’t do it. The consequences of putting a 6P plug into an 8P jack won’t get you T-Boned by a semi but it can cause problems. Some phone plugs get mechanically latched in the RJ-45 housing and won’t release. That ruins the jack.

Often when you plug a 6P into an 8P jack, the plug gets inserted crosswise. The Hawaiian word for that is kapakahi, in case you were wondering. A kapakahi plug can cause a short circuit between the pairs inside the RJ-45. In modern homes, you generally have more than one phone line. Your family phone may run on the blue/white pair one but the home office may be on another pair. The short circuit can knock it out of action.

Some RJ-45s serving LANS use POE or Power Over Ethernet. Shorting out one of those pairs is not hazardous but can result in expensive grey smoke coming out of the data boxes.

There are several solutions. Neatest is to buy a small Allen Tel jack insert. That snaps into the RJ-45 and provides a straight channel for the smaller 6P phone plug. No more kapakahi installations. Graybar stocks the adaptors.

Another way is use a breakout box. This has a short cable with an RJ-45 plug and a box with four RJ-11 jacks. This “breaks out” each pair and delivers it to its own jack. These are sometimes configured as dongles, hanging from the wall outlet. Similar devices are available that mount inside the wall box. In that case, the Cat 5 terminates on a 110 block inside the wall and the front plate has four RJ-11s. This is handy if you have a home office and need a way to plug your FAX machine into your dedicated FAX line.

Now that you know more about telecom jacks then you ever wanted to know, take a break. Buy a set of jacks. They are a great stress reliever and I’ll bet you never get more than 10 in one swoop when the ball is in the air.

Tom Moore is CEO of Tropical Telecom Corp. a US company that designs residential phone and intercom systems. Among Tropical’s design customers are Broan-NuTone, OnQ Legrand, and Linear Open House. Mr. Moore was CEO/Managing Director of Nortel Asia based in Singapore and earlier held technical and management positions with New Jersey Bell, Illinois Bell and Hawaiian Telephone companies. While serving in the US Army Signal Corps, Tom was with a unit supporting the UN Military Armistice Commission at Panmunjom, Korea. Tom’s BS degree is from the University of Illinois.

As a founder and CEO of Intelect, Inc, a NASDAQ company, Mr. Moore received an award in the White House from President Ronald Reagan for his work on air traffic control and air defense systems in 33 countries.

Mr. Moore is a Senior Member of the IEEE, a member of the Airplane Owners and Pilot’s Association (AOPA) and the Man Will Never Fly Society. He is a board member of the Hong Kong Business Assoc. of Hawaii and in 2006, conducted a four-part seminar for the State of Hawaii titled Dispelling the China Business Myths. In early 2007, U.S. Secretary of Commerce Carlos Gutierrez designated Mr. Moore a member of the Hawaii/Pacific District Export Council.