Background Information on Jack Standards

Common usage is to refer to 6 position jacks and plugs as RJ11 and 8 position jacks and plugs as RJ45.
This is really incorrect!

6PxC and 8P8C are the "technically correct" terms used to refer to six pin and eight pin modular jack and plug hardware used for both voice and data connections.  Six pin position hardware is commonly available that only has connection points (and pins) for two or four wires.  Six pin hardware is correctly referenced as 6P2C (6 pin positions/2 connections), 6P4C, or 6P6C depending on how many connection points are available, or using the generic 6PxC designation for any six pin hardware.  I am not familiar with any eight pin hardware that doesn’t support eight wire connections and every reference I know of uses the "8P8C" nomenclature. 

Comparison of
"USOC RJ" and EIA/TIA 568A/568B
Jack Standards

The "RJ" (registered jack) designations are Bell System USOC (uniform service order codes) for the combination of the hardware, how wires are connected, and what signals the jack is used for.  Because the RJ configurations were the defacto standard when modular connectors became widely used for telephone service, some RJ designations have been incorporated into Part 68 of the FCC rules (47 CFR Part 68).  For the same reason, RJ11 and RJ45 have become synonymous with 6PxC and 8P8C hardware, even though the designations are often used incorrectly to refer to other jack configurations. 6P2C, 6P4C, and 6P6C configurations are all commonly referred to as RJ11, but they really equate to RJ11, RJ14, and RJ25 depending on whether they are used for one, two, or three analog voice lines.  Four analog voice lines on an 8P8C is an RJ61, but that is almost universally referred to as an RJ45.  RJ45 is really the USOC designation for a single pair "programmed data equipment" line connected on pins 4 and 5 of 8P8C hardware (simplex, receive only data (teletype) line that is nearly non-existent in today's service offerings).  RJ45 is also commonly used to refer to Ethernet on 8P8C hardware, which "almost always" uses 8P8C hardware configured according to EIA/TIA 568A or 568B standards.  EIA/TIA 568A or 568B standards have absolutely no direct relationship with USOC RJ configurations except that they happen to use the same connector hardware.  (Yes, there are some special "non-8P8C" connectors used for Ethernet in very specialized industrial equipment, but you won't find them on "consumer goods".) 

Oddly, EIA/TIA standards recommend using 8P8C jacks for both voice and data, but seem to ignore the fact that the pinning for 568A/568B neither one match RJ25 or RJ61 pinning, which means a non-standard adjustment has be made someplace in order to connect to standard three-line or four-line analog telephone equipment.  That's because EIA/TIA 568A/568B is really a compromise between the voice and data worlds.  The original EIA/TIA desire was for a configuration to keep the pairs isolated to improve data signal quality.  USOC RJ configurations were around first and RJ11/14/25/61 pairs are arranged "split" from the middle of the connector to the edges, which meant that any configuration that kept pairs isolated wouldn't work with standards already in place for telephone connections.  So 568A matched the center two pairs to RJ14 and isolated pairs 3 and 4 on the outside.  As I understand it, somebody (I think it was ATT) thought the second pair should be moved so that it would be isolated rather than the third pair and had enough clout to get EIA/TIA to adopt 568B as an alternative standard. 

Regarding use of 6PxC plugs in 8P8C jacks: Yes, that will work fine.  Some techs will claim that the plug can go in crooked and cause opens or shorts.  If the hardware is built correctly, that is not likely to happen unless the plug is forced in crooked.  If the hardware is built with really sloppy tolerances, I suppose it could happen, but I've never been able to identify a case where it was a problem.  One thing I have noted is that if an 8P8C jack is used with 6PxC plugs, pins 1 and 8 of the jack tend to bend upward permanently, which means the jack may not work with 8P8C plugs in the future if desired.  That's not generally a problem, since the jacks will probably always be used for the original purpose. 

6PxC and 8P8C hardware are also used for many things besides telephone and Ethernet. Some video security equipment uses these type connectors.  Some industrial control and telemetry equipment uses them.  I am even aware of some specialized automobile control electronics that use "RJ11" connectors according to the installation manual.  I doubt connecting an analog telephone line to auto mobile control electronics would be recommended! 
You can click on any of these graphics to see a larger version.





So, what jacks should you use and how should you connect the wires?

If adding to an existing installation, the simple answer is "match the existing."

For new commercial installations, my preference is to use all 8P8C jacks.  If data and voice cables will be routed to separate wiring hubs (patch panels/frames), I recommend using 568B configuration for data jacks and digital (ISDN or proprietary) voice jacks and RJ61 for analog voice jacks, with different color keystone jack modules for each jack configuration.  Some techs prefer using 568A configuration for all jacks, which is a good alternative especially if all cables will be routed on a combined voice/data wiring hub.  Plenty of small/medium commercial installs are still going in with 6P4C jacks in RJ11/14 configuration for voice, because it's what everybody understands and expects.  Voice service in larger commercial installs tend to be digital (ISDN or proprietary) PBX based service, which normally uses EIA/TIA 568A or 568B configured jacks.  When using the 568A or 568B standard, ensure that the same standard is used at both ends of the cable.  If the hardware is only marked with one 568A/568B standard, the other configuration is easily achieved by simply swapping the positions of pairs 2 and 3. 

For new residential installations, I recommend using 8P8C jacks with 568B configuration for data and dual 6P4C or 6P6C jacks for voice and DSL, with four-pair cable terminated as two RJ14 jacks, with pairs 1 and 2 on the first jack and pairs 3 and 4 on the second jack.  Using dual RJ14 jacks lets two or more lines be connected so that they can be accessed most easily.  For example: If two lines are available, at the wiring hub line 1 is cross-connected to pairs 1 and 4 and line 2 is cross-connected to pairs 2 and 3; a single line device connected to the first jack connects to line 1; a single line device connected to the second jack connects to line 2; two line devices can use either jack.  For three or more lines, the cross-connects would be done as best suited your needs.  Of course, if using a wiring block as your wiring hub, you can use different cross-connect arrangements for each jack according to your needs, as illustrated on my page about adding a third line.  If you are using a fancy residential "structured media center" as your wiring hub, then your flexibility will be constrained by the options available on the bridging modules used in the center.  Most structured media centers don't allow a great deal of flexibility when used with more than one line.  There are more advantages to using blocks instead of a media center than just saving money. 

I generally recommend terminating as many pairs as possible on all jacks.  That is a significant advantage if you decide to add more lines on previously installed jacks, because the wires will already be connected to the jacks.  Believe me, it's frustrating to have to re-work all of the jacks in a home to add a second or third line.  Connecting the unused pairs also provides additional strain relief for the cable and avoids having to decide whether to coil or clip the spare wires.  That is why I only stock 6P6C voice jacks. 

Does all of that really matter?  My answer is yes.  Using the wrong terminology can lead to confusion and unnecessary expense.  Want a longer cord for a two-line phone?  If the package says RJ11, better make sure it really has 6P4C plugs and two-pair cable, or you might be making another trip back to the store.  No big deal, except gas is getting pretty expensive. 

However the problem can get worse.  Suppose you're setting up a small business in your basement and need three phone lines on three desks.  You buy a three-line phone for each desk and you order three phone lines.  Then you decide to have one of your employees oversee the jack and cable install because you're busy and can't be there when the install is done.  You give the employee a note for the tech ordering the lines be connected to RJ11 jacks at three locations.  If the tech doing the install has an order to install an RJ11 jack at three different desks, he'll naturally assume single line service (RJ11) at each desk.  He might use two-pair cable from the demark to each location, since only one pair would be needed to support single line service and the tech probably has several boxes of old two-pair cable he's trying to use when possible.  When he's done, everything will be done as per the order.  However, when the three-line phones are connected to the jacks, they will each only have access to one line, each different from the other, and the intercom functions won't work, since they use frequency modulation of one of the voice line pairs to transmit the intercom signals.  However, if the tech doing the install has an order to install three RJ25 jacks at three different locations, he should understand that all three jacks need all three lines and everything should work fine when he's done.  And if not, you have a legitimate complaint with the tech.  But, if the problem resulted from you requesting the wrong type jacks, then the tech is not a fault and you'll probably have to pay for the correction or do it yourself.  If two-pair cable was used for the initial job, having the tech come back to change three RJ11 jacks to three RJ25 jacks would probably cost as much as the initial install, to say nothing of the time and frustration.  What's the difference between RJ11 and RJ25?  In this scenario, at least about $300 for tech time, 16 hours of lost productivity from the two people you just hired, a full day of lost sales, and a day of frustration. 

So the standard for jacks is that there are a lot of standards and you should pick one based on your preference and what type lines and equipment you expect to use. 

One last note:  JACKS and PLUGS:  Jacks are "female" devices, normally found on walls or and on equipment.  Plugs are "male" devices, normally found on line cords.  Hopefully, the ramifications of confusing jacks and plugs when making purchases are obvious!

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