The RJ11 is a six position jack or modular plug. The jack as you know is usually installed on a wall and the modular plug is the little clear plastic thing that can plug into the jack. The reason it is called a six position jack is because the jack can connect to up to 6 wires. In the telecommunications world when we talk about copper wire, we use the word pair, copper pairs. A pair of wires is obviously 2 wires and they are twisted together to form one pair. In the case of an RJ11, it can handle up to 3 pairs.

An RJ11 format is basically designed for voice or analog circuits. It’s normally what you would see in your house phone or fax machine. IT can also be used for modems and other analog devices. Back before 1990, a lot of business phones would use all 3 pairs of an RJ11 to run their business telephone sets. The reasoning was that they needed the first pair for a two way talk path, the second pair for data going to the phone such as time display, line appearance, whether or not you are using the speaker phone, etc. and the third pair was often used for power. Some business phone systems back then actually would use pair one for line one and pair two for line two and pair three for line three. In that case, that particular phone would not use a phone system brain often called “key service unit” or “pbx”.

RJ11’s are different than RJ45’s in that they do not hold certifications. An RJ45 can be a cat3, cat5e, cat6, etc. An RJ11 is not rated with any category requirement from IEEE. Even though back in the late 80’s and early 90’s RJ11’s were used on the “Apple Talk Network”, a design that Apple used to network several Apple computers and printers together, Apple would always use the second pair in an RJ11 for their network, allowing the first pair to be used for voice. This fell out of vogue quickly when Ethernet over twisted pair standards such as 10baseT and others came into existence in the 90’s. It is interesting that old AT&T systems in the 70’s and 80’s were the first ones to use RJ45’s for their telephones. Some installers today still use RJ45’s for telephones and therefore just plug in an RJ11 into the RJ45 jack. This is perfectly acceptable if you now the proper pin out on the jack. It gives you a margin of flexibility when it comes to using the same jack for other applications such as a computer network. If properly pinned out and terminated on the patch panel in the data room, it can be easily changed from an analog or digital phone system to an Ethernet computer. RJ11’s are never used for Voice over IP. If you look closely, they are not used in handset cords either even though some people seem to confuse the two.

RJ standards were originally formed by AT&T “the phone company” years ago to signify a modular jack applicaton. Not only are there RJ11 and RJ45 but there are other jacks called RJ31x and others, and the list goes on.

To be on the safe side, only use RJ11’s for voice/analog applications other than Voice over IP.