Introduction of the RJ45 interface

Introduction of the RJ45 interface

Posted on by FS.COM

Generally, cables can transmit information along their length. To actually get the information where it needs to go, you need to make the right connections to an RJ45 connector. Your cable run needs to terminate into a connector, which needs a jack to plug into. RJ45 is a standard type of physical connector for network cables, which is especially used for Ethernet networking. And “45” is the number of the interface standard. Recently, RJ45 connectors are commonly seen with Ethernet cables and networks. Ethernet cables with RJ45 connectors are also called RJ45 cables. Modern Ethernet cables feature a small plastic plug on each end of the cable. The plug is inserted into RJ45 jacks of Ethernet devices. Have you ever had some confusion about RJ45 interface? That’s what we’ll explore today.


RJ45 Connector

RJ45 connector is the most common twisted-pair connector for Ethernet cables and networks. “RJ” means “registered jack”, which is a standardized telecommunication network interface for connecting voice and data equipment to a service provided by a local exchange carrier or long distance carrier. The physical connectors that registered jacks use are mainly the modular connector and 50-pin miniature ribbon connector types. RJ45 connector is an 8-position, 8-contact (8P8C) modular plug and jack, which is commonly used to connect computers onto Ethernet-based local area networks (LAN). RJ45 cable plug is usually made of a plastic piece with eight pins on the port. Four of the pins are used for sending and receiving data, and the other four are used for other technologies or power networking devices.

As we all know, there are two wiring schemes: T568A and T568B, which are used to terminate the twisted-pair cable onto the connector interface. Two standards define how the RJ45 pinouts to arrange the individual eight wires when linking RJ45 connector to a cable. These wiring layouts have their own color convention, and following the convention is important to ensure electrical compatibility. The differences of T568A and T568B in color conventions are shown in the figure below.

T568A & T568B

With regard to the two standards, there are two different connectivity forms. The T-568B wiring scheme is by far the most common, though many devices support the T-568A wiring scheme as well. If both ends of the patch cords are wired on the basis of one standard, it is a straight through connection. Both the standards can be used for straight through cable. If not, it is a crossover connection. Some networking applications require a crossover Ethernet cable, which has a T-568A connector on one end and a T-568B connector on the other. This type of cable is typically used for direct computer-to-computer connections when there is no router, hub, or switch available.

RJ45 VS. RJ11

Several other types of connectors closely resemble RJ45, the RJ11 connector used with telephone cables is one of such connectors. The close physical similarity of RJ45 and RJ11 makes it difficult for an untrained eye to tell the two apart. RJ11 connector is a 6P2C (6 position 2 contact) modular connector – only uses six positions rather than eight positions, which make them less popular than RJ45 connectors.


RJ45 and RJ11 are two commonly used jacks, each with their own specific purpose. The biggest difference between them is that they are used for different applications. RJ45 is used in networking, where you connect computers or other network elements to each other. RJ11 is the cable connector that is being used in telephone sets. Aside from the application, another difference is the number of wires in their connectors. If you look closely at both connectors, you would see that there are only four wires inside an RJ11 while there are eight wires inside an RJ45. As a consequence, RJ45 connector is a little bit bigger than RJ11. It is then quite easy to deduce that you cannot plug in an RJ45 connector to a RJ11 slot but the opposite is possible. Although the smaller size of RJ11 makes it easier to be plugged into the RJ45 slot, it is not recommended to do so since this may damage the device that adopts the RJ45 slot. With proper knowledge and training, some people have been able to use RJ45s all over their house instead of RJ11s. At present, RJ45 jacks are usually placed on the wall outlets inside people’s houses to reduce the number of visible wiring when using VoIP handsets that are rapidly gaining popularity.

Main differences:
  • RJ45 is used with Ethernet cables in computer networking while RJ11 is used in connecting telephone units.
  • RJ45 contains more wires than RJ11.
  • RJ45 is physically bigger than RJ11 to accommodate the extra wires.
RJ45 Network Interface Cards

Network interface card (NIC) is a slot-based card that connects directly to the motherboard of the computer. It provides an electrical interface for the computer to the network and completely complies with the Ethernet standards. A proper termination is needed for connecting the LAN media and the proper motherboard. The most current type of media in LAN networks is Cat 6 Ethernet cable connected to devices using RJ45 connectors. So its network interface card must have an RJ45 modular jack that matches with the patch cable. In terms of the RJ45-equipped devices, most only come with one available port. But there are those that can host up to four ports on one NIC.



RJ45 connectors are the key part of Ethernet connectivity to transmit voice and data media. They were developed as much smaller and cheaper replacements to the older telephone installation methods of hardwired cords. The easy plug-n-play style reduces the difficulty of installation. Compared with RJ11, RJ45 is suitable for more applications, such as Ethernet networking, telecommunications, factory automation and so on. It is frequently used for networking devices including Ethernet cables, modems, computers, laptops, printers, etc.

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