Copper Ethernet networks depend on properly installed connectors and correctly arranged conductors. Even when the correct cable category is used, a poor termination can prevent a connection from working, reduce a gigabit connection to 100 Mbps, or cause intermittent network problems.
CompTIA A+ technicians should be able to identify RJ45 and RJ11 connectors, recognize T568A and T568B wiring, distinguish straight-through cables from crossover cables, terminate an Ethernet cable, and use a punchdown tool on keystone jacks and patch panels.
Understanding these physical components helps technicians install network connections and troubleshoot failures caused by incorrect wiring, loose conductors, damaged locking tabs, and poor terminations.
RJ45 Ethernet Connectors
The connector commonly called an RJ45 connector is used on twisted-pair Ethernet cables.
Technically, the connector used for Ethernet is an eight-position, eight-contact modular connector, often described as an 8P8C connector. However, the term RJ45 is widely used in networking courses, equipment documentation, and certification exams.
An RJ45 connector is commonly attached to:
Cat 5e cable
Cat 6 cable
Cat 6a cable
Ethernet patch cables
Router connections
Switch connections
Network interface cards
Wireless access points
Network printers
Security cameras
Voice over IP phones
The connector allows the eight conductors inside an Ethernet cable to make electrical contact with an Ethernet port.
Eight Positions and Eight Contacts
An RJ45 Ethernet connector has eight conductor positions and eight metal contacts.
Each conductor inside the cable is inserted into a separate channel inside the connector. When the connector is crimped, the metal contacts are pushed into the conductors.
The eight conductors are arranged as four twisted pairs:
Blue pair
Orange pair
Green pair
Brown pair
All eight positions are normally used for Gigabit Ethernet and faster twisted-pair Ethernet connections.
Older 10 Mbps and 100 Mbps Ethernet standards commonly use only two of the four pairs. This is why a damaged cable may continue operating at 100 Mbps but fail to negotiate at 1 Gbps.
The Locking Tab
The plastic tab on an RJ45 connector helps hold the connector securely inside the Ethernet port.
When the connector is inserted, the tab locks into the port. Pressing the tab releases the connector so it can be removed.
A damaged locking tab is one of the most common physical Ethernet problems.
When the tab breaks:
The connector may not remain securely seated.
Small movements may interrupt the connection.
The link light may repeatedly turn on and off.
The switch port may experience port flapping.
The device may disconnect when the cable is bumped.
The cable may fall out of the port.
A patch cable with a broken locking tab should normally be replaced.
Some patch cables include a molded boot around the locking tab. The boot helps prevent the tab from catching on nearby cables and breaking.
Metal Contacts
The top of the RJ45 connector contains eight metal contacts.
During crimping, the crimping tool pushes these contacts downward. Each contact cuts through the insulation of one conductor and makes an electrical connection with the copper inside.
If the connector is not crimped correctly, one or more contacts may fail to reach the conductors.
This may cause:
No network connection
Missing wire pairs
Reduced Ethernet speed
Intermittent connectivity
Packet loss
Cable-test failures
A technician should inspect the connector and confirm that all eight contacts appear evenly pressed into place.
Cable Entry
The Ethernet cable enters through the rear of the RJ45 connector.
The individual conductors pass through separate channels toward the front contacts. The outer cable jacket should also extend into the connector.
When the connector is crimped, a strain-relief section presses against the cable jacket. This helps secure the entire cable rather than placing stress directly on the individual conductors.
If the outer jacket does not enter the connector far enough, pulling or bending the cable can place stress on the conductors. This may eventually cause one or more wires to loosen or break.
Standard and Pass-Through Connectors
A standard RJ45 connector contains internal channels that stop the conductors near the front of the connector.
A pass-through connector allows the conductors to extend completely through the front. The excess wire is trimmed during crimping.
Pass-through connectors can make it easier to:
Verify the wire order
Confirm that every conductor reached the front
Keep the conductors straight
Reduce incorrect terminations
However, the technician must use a compatible pass-through crimping tool that trims the conductors correctly.
Standard and pass-through connectors can both create reliable cables when properly installed.
Common Causes of RJ45 Connector Failure
RJ45 connectors may fail because of:
Broken locking tab
Conductors inserted in the wrong order
Conductors not reaching the front
Poor crimp
Outer jacket not secured
Excessive untwisting
Bent or damaged contacts
Corrosion
Moisture exposure
Incorrect connector for the cable category
Incorrect connector for solid or stranded conductors
Repeated cable movement
Excessive pulling force
Many connector failures can be located using a cable tester or by replacing the suspected cable with a known-good cable.
RJ11 vs. RJ45
RJ11 and RJ45 connectors have a similar modular appearance, but they are designed for different purposes.
RJ11 is primarily associated with telephone connections. RJ45 is associated with Ethernet networking.
RJ11 Connectors
An RJ11 connector is smaller than an RJ45 connector.
It is commonly used for:
Analog telephone lines
Fax machines
DSL modem connections
Telephone wall jacks
An RJ11-style plug often contains two or four active conductors, depending on the telephone configuration.
The connector may physically contain up to six positions, but not every position must contain a contact or conductor.
Common telephone cable configurations include:
One telephone line using two conductors
Two telephone lines using four conductors
The center contacts are commonly used first.
RJ45 Connectors
An RJ45 Ethernet connector is wider than an RJ11 connector.
It contains eight positions and eight contacts. Modern Ethernet connections normally use all eight conductors.
RJ45 connectors are commonly used for:
Computers
Switches
Routers
Network printers
Wireless access points
Security cameras
Servers
Voice over IP phones
Network wall jacks
Size Difference
An RJ11 plug is physically narrower than an RJ45 plug.
An RJ11 plug may fit loosely inside some RJ45 Ethernet jacks. However, it should not be inserted into an Ethernet jack because it can bend or damage the outer contacts.
An RJ45 plug is too wide to fit into an RJ11 telephone jack.
Technicians should identify the port before connecting a cable.
Conductor Comparison
| Feature | RJ11 | RJ45 |
|---|---|---|
| Common purpose | Telephone or DSL | Ethernet networking |
| Physical size | Smaller and narrower | Larger and wider |
| Common conductor count | Two or four | Eight |
| Common cable | Telephone cable | Twisted-pair Ethernet cable |
| Typical wall connection | Telephone jack | Network wall jack |
| Common devices | Phone, fax, DSL modem | Computer, switch, router, printer |
Wiring Differences
Telephone wiring normally places the active conductors near the center of the connector.
Ethernet wiring follows T568A or T568B and uses carefully arranged twisted pairs.
Telephone cable should not be substituted for Ethernet cable. It may contain too few conductors, lack proper pair twisting, and fail to meet Ethernet performance requirements.
Practical Identification
A technician can distinguish the connectors by examining their width and number of contacts.
An RJ45 Ethernet plug:
Is wider
Has eight visible contact positions
Is normally connected to Cat 5e, Cat 6, or similar cable
Is used with Ethernet ports
An RJ11 telephone plug:
Is narrower
Usually has fewer active contacts
Is connected to thinner telephone cable
Is used with telephone or DSL ports
T568A and T568B Wiring Standards
T568A and T568B define the order in which the eight Ethernet conductors are placed into an RJ45 connector or terminated on a keystone jack or patch panel.
Both standards use the same four colored pairs. The main difference is the position of the orange and green pairs.
The blue and brown pairs remain in the same positions in both standards.
Either T568A or T568B can support Ethernet when properly installed. The most important rule is to use the selected standard consistently.
Connector Viewing Orientation
When reading the wire order, hold the RJ45 plug with:
The gold contacts facing upward or toward you
The locking tab facing downward or away from you
The cable entering from the rear
Pin 1 on the left
Pin 8 on the right
The conductors are then read from left to right.
T568A Wire Order
The T568A wiring order is:
| Pin | Wire color |
|---|---|
| 1 | White-green |
| 2 | Green |
| 3 | White-orange |
| 4 | Blue |
| 5 | White-blue |
| 6 | Orange |
| 7 | White-brown |
| 8 | Brown |
T568A Visual Order
Contacts facing up; pins read left to right
Pin: 1 2 3 4 5 6 7 8
Wire: White/Green | Green | White/Orange | Blue | White/Blue | Orange | White/Brown | Brown
Pair placement:
Green pair: Pins 1 and 2
Orange pair: Pins 3 and 6
Blue pair: Pins 4 and 5
Brown pair: Pins 7 and 8
T568B Wire Order
The T568B wiring order is:
| Pin | Wire color |
|---|---|
| 1 | White-orange |
| 2 | Orange |
| 3 | White-green |
| 4 | Blue |
| 5 | White-blue |
| 6 | Green |
| 7 | White-brown |
| 8 | Brown |
T568B Visual Order
Contacts facing up; pins read left to right
Pin: 1 2 3 4 5 6 7 8
Wire: White/Orange | Orange | White/Green | Blue | White/Blue | Green | White/Brown | Brown
Pair placement:
Orange pair: Pins 1 and 2
Green pair: Pins 3 and 6
Blue pair: Pins 4 and 5
Brown pair: Pins 7 and 8
Comparing T568A and T568B
| Pin | T568A | T568B |
|---|---|---|
| 1 | White-green | White-orange |
| 2 | Green | Orange |
| 3 | White-orange | White-green |
| 4 | Blue | Blue |
| 5 | White-blue | White-blue |
| 6 | Orange | Green |
| 7 | White-brown | White-brown |
| 8 | Brown | Brown |
The blue and brown pairs do not change. Only the orange and green pairs exchange positions.
Which Standard Should Be Used?
Both standards can support modern Ethernet.
T568B is commonly found in many commercial Ethernet installations in the United States. T568A may be required in certain government, residential, or organization-specific installations.
A technician should follow:
Existing building standards
Project documentation
Organizational policy
Local installation requirements
The wiring labels on the patch panel or keystone jack
When repairing an existing cable, the technician should determine which standard is already being used and match it.
Consistency Matters
Using T568A on both ends creates a properly wired straight-through cable.
Using T568B on both ends also creates a properly wired straight-through cable.
Using T568A on one end and T568B on the other creates a crossover cable.
A common mistake is assuming that T568B is inherently faster than T568A. It is not. Both provide the same Ethernet performance when correctly terminated.
Straight-Through vs. Crossover Cables
Straight-through and crossover Ethernet cables use the same type of twisted-pair cable and RJ45 connectors. The difference is the wiring standard used at each end.
Straight-Through Cable
A straight-through cable uses the same wiring standard on both ends.
Possible configurations include:
T568A on both ends
T568B on both ends
The conductor connected to pin 1 on one end is connected to pin 1 on the other end. The same is true for every other pin.
Straight-through cables are the most common Ethernet cables used today.
They are commonly used to connect:
Computer to switch
Printer to switch
Router LAN port to switch
Wireless access point to switch
Security camera to PoE switch
VoIP phone to switch
Patch panel to switch
Wall jack to computer
Crossover Cable
A crossover cable uses T568A on one end and T568B on the other.
This causes the orange and green transmit-and-receive pairs to cross.
Crossover cables were historically used to connect similar types of Ethernet devices directly.
Examples included:
Computer to computer
Switch to switch
Router to router
Hub to hub
Older Ethernet equipment expected transmitting and receiving to occur on specific pins.