So, we've discussed RJ45 plugs used for Ethernet applications, and the difference between solid and stranded. But now we're going to hop in the DeLorean and go back in time a little bit to take a look at its predecessor, the RJ11. Superficially, the two modular plugs appear similar: both are clear plastic terminals at the end of cables that plug into similar looking jacks in your wall. But they are most definitely not interchangeable. Anyone who has accidentally grabbed an Ethernet cable and tried to cram it into a telephone jack (and vice versa) could tell you that it's not likely to get the desired result.
How are They Different?
As we mentioned, the two plugs looks somewhat similar. But take a closer look-see, and you'll notice right off the bat when you compare the two that RJ45s are longer. Additionally, as you may have read in the aforementioned article, the true name of that plug is 8P8C, because it has eight pins and eight connections, meaning 8 wires can be run through it. RJ11 jacks, on the other hand, are six position, meaning both the modular plug and the jack can utilize up to six wires, or three pairs: pair being a term used in telecommunications to represent the twisting of two copper wires together, called, appropriately enough, a “copper pair.”
Certification is another way to broaden the gap between the two. While RJ11 has no certifications, RJ45 is classified in many levels: CAT3, CAT5e, CAT6, etc…Though we tend think of RJ45s as the go-to terminal for computer networks today, Apple actually used RJ11s for their “Apple Talks Network” to connect several computers and printers together, using one pair for voice, and a second for data. However, this fell out of favor when ethernet wires, such as 10baseT, came into play around the early 90's.
Nowadays, there's a pretty clear separation between the usage of the two plug types. So if RJ45 is for data networks, you may be asking, when should you break out the RJ11s?
What are RJ11s Used For?
The short answer is: phones. RJ11s are mainly used in phone lines.
The longer answer: The format is designed to carry voice or analog signals. They've been popular in home and business environments since before the 1980's, when many business telephone systems would use up all three copper pairs. The first pair would be used on two way talk, the second for incoming data (time display, speaker phone, line appearance, etc…), and the third pair would be used for bringing power to run the phone. The format is still used today in home and business phone lines, as well as fax machines and dial-up modems.
So that's that, right? RJ11=phone, RJ45=computer network. Well, yeah, mostly, but like we said, the former can and has been used to tie data networks together before the widespread adoption of its successor, and likewise the latter actually can be used for phone lines, as we'll get to in just a minute.
A Brief History Lesson
So where did all these different plugs get their start? They haven't been around forever; I've seen those caveman displays in museums and they were definitely not using RJ11s to plug in their phones. In fact, my memory is a little fuzzy but I don't think they had phones at all. And I also am pretty sure Alexander Graham Bell didn't plug in his old timey phone with one. (though he DID invent the first optical cable). But no, ultimately, AT&T was the company responsible for the formats, and many others, and has signified the modular jack application.
Though RJ11 has typically been the standard for telephones, AT&T actually used RJ45s for their phone systems, all the way back in the 1970s. AT&T's method still holds true today for some installers who prefer to use an RJ45 jack. If the patch panel is properly terminated and pinned out by the installer, it can be used for either an Ethernet cable for a computer network or a phone system. However, unless you are a trained installer, it's always best to stay on the safe side and use RJ11 modular and jacks for voice or analog applications, except when dealing with VOIP (Voice over IP)…in which case you'll want to stick with RJ45.
Hopefully that gives you at least some insight into why you can't shove a Cat5e cable into your phone jack. If not, then we really can't help you.