Monday, February 12, 2007

Do You Know Registered Jack?

You're no doubt quite familiar with the terms RJ11, RJ45 and perhaps RJ48. You'll immediate recognize them as the connectors used for telephone and networking equipment. But do you realize that these aren't mere catalog numbers or even de facto industry standards. They are part of a official telecommunications standard in the Code of Federal Regulations established by the FCC. They're not just connector jacks, they're Registered Jacks.

Registered Jacks is where the initials RJ come from. There are numerous RJ types, but you are only likely to run into a few of them. Each RJxx was established by a USOC or Universal Service Order Code under the Bell Telephone System. The code specifies the jack, the plug and the wiring scheme. There is also a suffix letter, such as C for flush mount or surface mount. Other letters include W for wall mount, S for single line, M for multi-line, and X for complex jack.

Let's take a look at some of the more popular Registered Jacks. RJ11 is the one familiar to both consumers and businesses. It is the interface used for single line analog telephones. A phone, computer or modem will likely have a RJ11C jack unless there is a hardwired lead and plug attached directly to the device. In the kitchen, a RJ11W is a wall mount jack that will both connect and hold a telephone set.

T1 lines are terminated in "smart jacks" at the demarcation point, usually in the telephone closet of the business. The smart jack acts as a network interface to protect the line. The user interface is most often a RJ-48C connector. This registered jack describes an 8 position jack with four active wires, one pair for transmit and one pair for receive. Two pins are assigned for cable shield integrity and two are reserved for future use.

Another T1 line connector that is used less today than in years past is the RJ48X. The "X" means complex jack. In this case it refers to shorting bars that connect the transmit to the receive pairs when no plug is inserted into the jack. The reason this is done is to loop-back the signal for testing purposes.

What's called an RJ45 connector today looks much like an RJ48, but the RJ48 is more specialized in that it has a keyed connector and a notched jack. A RJ45 cord will plug into a RJ48 jack, but not the other way around.

The ubiquitous RJ45 connector system that is so popular in networking connections today is actually somewhat different than the original RJ45 Registered Jack specified by the telephone company. The original RJ45 was a keyed connector that used two pins for signal and two for a programming resistor. This connector system is rarely seen today but the RJ45 naming convention has been adopted for a similar size connector used almost universally in Ethernet twisted pair cabling.

The correct nomenclature for the RJ45 connector we use today is an 8P8C modular connector. The 8P8C stands for 8 positions, eight conductors. In other words, all pins are connected. The wiring scheme follows one of two Electronic Industry Association standards, either T568A or T568B. T568B is equivalent to the old AT&T 258A standard, but T568A is preferred for new installations. Both standards use all 8 pins but are wired differently. Within a particular physical plant, one standard or the other is implemented. Mixing the two can cause problems with pairs being swapped at different ends of a wiring run.

Now you know Registered Jack. Well, at least a brief introduction to the subject. If your business has a need for telephone lines or computer networking bandwidth, The Get instant line quotes now.

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