What is a RJ-45 Port?

Kurt Inman

An Registered Jack 45 (RJ-45) port is a data port often found on computers, network routers, switches and hubs. It is commonly used for an Ethernet or serial connection with an 8 position 8 conductor (8P8C) jack. In the past, some modems and telephones included a port that used a true RJ-45 jack. Although the 8P8C jack is not identical to the RJ-45, the data port commonly retains the name.

RJ45 ports on the back side of a router.
RJ45 ports on the back side of a router.

The most common RJ-45 port uses four of the 8P8C wires for Ethernet communications. Pins 1 and 2 are the transmit pair, while pins 3 and 6 are the receive pair. The port generally connects devices using a Category 5 (CAT5) or Category 6 (CAT6) twisted-pair cable. This port and cable combination typically supports communication speeds of up to 1 gigabit per second (Gbps). Unlike a true RJ-45 jack, the 8P8C jack includes all eight conductors and does not have an extra "key" tab.

Cat 5 cable with RJ45 plug.
Cat 5 cable with RJ45 plug.

An RJ-45 port for serial communications may use all eight wires of the 8P8C modular connector. Some Integrated Services Digital Network (ISDN) connections often use up to six of those wires. Two conductors may carry power while the other two are Tip/Ring signals. Another type of ISDN device may use four wires for transmit/receive pairs instead of two for Tip/Ring.

The original port design for use with a modem — now obsolete — followed the RJ-45 specification. The true jack has eight positions, but only two conductors that carry Tip/Ring signals. Pins 7 and 8 short out a resistor on the device, indicating the presence of a cable. RJ-45 is one of several registered jacks used for telephone connections. Registered Jack 11 (RJ-11) is almost always used for household phone wiring.

A modern gigabit Ethernet device is better served with a modified RJ-45 port. Augmented RJ-45 (ARJ45) and GigaGate 45 (GG45) are newer standards designed for high-speed communications. Both support 10 Gbps and faster connections using CAT6 and Category 7 (CAT7) cables. ARJ45 connectors come in two varieties: one for 8-pin CAT6 cables, the other for 12-pin CAT7 cables. In contrast, one GG45 connector can accommodate either type of cable.

Siemon Labs' TERA is a different type of high-speed connector. It does not use the RJ-45 form factor, but can be plugged into the same port with a special patch cord. Often used to carry broadcast video, it is also designed for speeds of 10 Gbps and faster.

A Quick History of the Registered Jack Standard

The Registered Jack standard was first developed during the 1970s when the Bell System created its Universal Service Ordering Code. The USOC resulted from the need to meet Federal Communications Commission requirements. It identified telecommunications equipment and complied registration program for customer-supplied telephony devices and cabling.

The first Registered Jacks were first invented in 1973 by Bell Labs. RJ specifications encompassed wiring, physical construction, and signaling functionality. Almost 30 kinds of Registered Jacks are officially recognized today in the United States. Their use has expanded worldwide, including the RJ11.

Despite the subtle differences between the two, you’ll still hear an 8P8C connector referred to as an RJ45 port. This usage became common when professionals who worked with telephone equipment and networks started specializing in computer networks. Since the two connectors look similar, they just started calling the 8P8C connectors “RJ45s” and the moniker stuck.

The Advent of Ethernet and 8P8C Connectors

Registered Jacks date to the 1970s. So does Ethernet, which was also invented in 1973. Created at Xerox’s Palo Alto Research Center, Ethernet was a solution to connect multiple computers even over lengthy distances. When the Palo Alto team partnered with Digital Equipment Corporation and Intel, they created the 10 Mbps data transfer standard that was ratified by the Institute of Electrical and Electronic Engineers.

How Ethernet Helped Usher in the Digital Age

Ethernet began rolling out commercially in the early 1980s when 3Com began creating Ethernet circuit boards and network cards. After the World Wide Web became a thing in the early 1990s, Ethernet technology experienced explosive growth. Speeds eventually increased, making many things possible: long-distance communication, scientific and technological innovations, e-commerce, social networking, expanded educational opportunities, and more.

Over 30 years later, gigabit internet is becoming more commonplace. Speeds of 1 Gbps are available in many U.S. markets. We now also have 400-Gbps internet, with the latest 802.3cu 400G Ethernet standard approved by the IEEE in 2021. The fastest data transfer speed on record is 319 terabits per second, achieved in 2021 by Japanese researchers. You’d need optical fiber to support speeds like these, but such developments are on the horizon.

Ethernet 8P8C Vs. True RJ45

Today’s Ethernet cables vary in length and use 8P8C connectors. So how did the 8P8C standard come about for data cabling? You can thank the Electronic Industries Alliance, which created the eight-contact design based on existing RJ45 connectors. Perhaps that’s part of the reason for the confusion. With the design and appearance similarities, the RJ45 nickname for Ethernet ports and cabling became a thing. But the original RJ45 connector has a small key on its right side. Ethernet cables with 8P8C connectors will never have that key. So while unkeyed 8P8C connectors can fit into an RJ45 port, the reverse is impossible.

Why We Still Need Ethernet Cables

Home and commercial users have grown increasingly reliant on wireless data networks. Yet wired ethernet connections remain an essential part of communications infrastructure. Wireless simply isn’t feasible in some situations, but access and security are key issues.

Speed and Stability

When maximum speed is desired, an Ethernet cable and port are the best equipment to use. That’s because Ethernet connections are faster than Wi-Fi thanks to the cabling that carries your data signals electronically. Using these direct connections avoids potential pitfalls that can occur with wireless connections:

  • Data loss or degradation
  • Slowed or blocked signals
  • Signal interference

These problems can result from any number of causes: distance, physical obstructions, and other wireless networks. Even equipment such as cordless phones, microwaves, and alarm systems may generate interference that can impair your wireless network’s performance. But with hardwired Ethernet cables and 8P8C connectors, you avoid these issues. The only way an Ethernet connection can go faulty is if the cable or the connector breaks.

Improved Security

Besides signal speed and reliability, an RJ45 port and cable afford you other advantages with your data network. As you probably know, a Wi-Fi network requires a login and password for access. Assuming that you aren’t hiding your SSID, any person can use a smartphone, laptop, desktop, or another Wi-Fi-capable device to see your network’s existence.

Your network’s visibility to others doesn’t automatically mean that someone can gain unauthorized access. They need to hack your passwords to accomplish that. Yet anyone nearby with enough knowledge to break into your private Wi-Fi network can do so. But with a wired network, Ethernet cables are required to even link up and use it.

A network cable tester can be used to determine if an issue lies with the cable or the RJ-45 port.
A network cable tester can be used to determine if an issue lies with the cable or the RJ-45 port.

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Discussion Comments

anon979889

I've been using RJ45 for years and will continue to in our installations, with people now using 12v for lighting and various appliances Rj45 has been revitalized.

I'm using POE to send 48 volts through the extra wires, and then using switch supplies at the end to adjust to 12volt for lighting etc.

So the RJ45 is far from dead, with signal and power in a single wire, that is very hard to beat.

It' s useful in home automation and surveillance, and is very often used in modern energy low powered houses, instead of the standard 220-250v we use here.

CoffeeJim

@summertime, I agree with your thoughts on a new connection type for computer networking, I just hope that it is as easy as the older style coaxial connections to create ends for. Those were the days.

If it is going to be successful in the marketplace, it will have to be easy to use for modern day information technology professionals.

summertime

While I agree with the previous comment concerning the very easy of use that RJ-45 connectors inherently now have because of it's time on the market, I do believe that we need to move on in our efforts to keep up with increasing network traffic.

Eventually we will need to push our technology sectors and companies toward using a faster connection type. While fiber optic networking has the amazing ability to transfer insane amounts of data in a very short amount of time, there is problems using this technology on a consumer level. Not many at home users will have any clue how to properly polish the glass connections that require high levels of training to complete. It's just not practical. What we need is a solid way of creating home networking cables that exceed the current gigabit standards. Better yet, they should be wireless.

spreadsheet

I remember the first time I had ever attempted to change and crimp an RJ-45 connection. As the author states, there are eight individual wires in four twisted-pairs inside of the category 5, 5e or 6 wiring that is used in conjunction with RJ-45 connectors. This great amount of small wiring being placed into exact slots in the tip of an RJ-45 connector makes the task extremely difficult.

Luckily, the job I was doing to replace a gigabit switch 8 port box came with a testing device that ensured my connections were solid. It still took too long and over three failed attempts to get both ends of the new uplink cable connected at full strength and proper pin output.

jeancastle00

It is amazing to me that that the RJ-45 network port has been able to remain in existence this entire time. Not many computer systems or types of cable connectors and protocols last more then a decade and the RJ-45 tip is one that certainly has proved it's worth. Used in essentially every type of computer network setup, the rj45 cable is ubiquitous in offices, houses and military complexes across the globe.

I think it is important for us to continue to keep standards such as this to ensure that compatibility across systems can remain easy to use. It drastically reduces costs when we are able to use the same test and connection equipment for many years instead of changing standards too often for service companies to stay profitable.

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