When making network cables or troubleshooting network issues, many people look at the eight colorful thin wires inside the RJ45 connector and wonder: "It's just for internet access-do we really need so many wires?" In reality, each wire has a specific role: some handle network data, others carry telephone signals or power for surveillance cameras, and some are reserved as spares. Let's break down the function of each of these eight wires.
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First, let's cover a key fact: why are there eight wires in a network cable?
Inside the cable, there are eight copper conductors twisted together in pairs, forming four twisted pairs. This twisted structure allows them to cancel out electromagnetic interference during signal transmission. That is precisely why the technical name for a network cable is "twisted pair."
Stripping the cable jacket: what the eight internal wires represent and their functions - CSDN Blog
Here's something many people don't know: 100 Mbps internet actually only requires four wires.
For 10 Mbps and 100 Mbps Ethernet, only wires 1, 2, 3, and 6 are used to transmit data. The other four wires remain largely idle in a 100 Mbps environment. This explains why some people can get online by cutting corners and crimping only four wires-though, as you'll see later, doing so leads to endless problems.
The eight wires in a network cable: function breakdown, color coding, and buying guide | Akihabara Official Website - Shenzhen Akihabara
Let's go through them one by one.
Wires 1 and 2: The main lanes for outgoing data
These two wires act as the network's "dispatchers." Whether your computer is sending messages, transferring files, or uploading data, these two wires handle the transmission of all outgoing information.
Under the T568B standard (the most common wiring method in China), wire 1 is white-orange, and wire 2 is orange.
Wires 3 and 6: Dedicated to receiving network signals
These two wires act as the "receivers." Whether you are streaming videos, browsing the web, or downloading files, these two wires handle all incoming data from the external network.
Wire 3 is white-green, and wire 6 is green. Here is a specific detail: the arrangement of the wires inside the cable deliberately separates wire #3 and wire #6, placing wires #4 and #5 between them. This design prevents signal interference caused by the lines being too close together, thereby avoiding lag or stuttering during internet use.
Consequently, for 100 Mbps networks, only four wires-#1, #2, #3, and #6-are actually needed.
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Wires #4 and #5: Idle at 100 Mbps, but actually multi-functional
These two wires essentially sit idle at 100 Mbps speeds, yet they possess several hidden functions.
Function 1: Connecting legacy landline phones
In the structured cabling of older office buildings and residential complexes, a single network cable is often used to handle both internet access and telephone calls. Voice signals travel over wires #4 and #5, eliminating the need to run a separate telephone line.
A quick note, however: using wires #4 and #5 for telephone signals can cause some interference with network data transmission. This method is largely an expedient solution for older cabling setups; it is not recommended for newly renovated homes.
Function 2: Powering surveillance equipment (PoE)
When installing surveillance cameras or wireless AP (Access Point) panels, there is no need for a separate power connection; the network cable itself can supply power. In 100 Mbps PoE setups, wires #4 and #5 are specifically designated for power transmission-wire #4 connects to the positive terminal, and wire #5 connects to the negative terminal.
Transmitting data and power over a single cable is the key advantage of PoE (Power over Ethernet). Standard PoE can deliver up to 15.4W of power, which is sufficient for typical surveillance cameras.
Function 3: Key players in Gigabit networks
Upon upgrading to Gigabit broadband, wires #4 and #5 are officially "activated" to participate in data transmission. Gigabit networks require all eight wires to function; if even one is missing, the network cannot achieve full speed.
Wires #7 and #8: Emergency backups
These two wires can be thought of as "emergency backups."
They remain largely inactive in 100 Mbps networks. However, in Gigabit and 10-Gigabit networks, all eight wires must work in unison; without them, the full bandwidth cannot be utilized. Additionally, if any of the primary lines (1, 2, 3, or 6) suffer a break or poor contact, lines 7 and 8 can serve as temporary backups to prevent a total network outage.
What is the actual difference between 100 Mbps and Gigabit (1000 Mbps) speeds?
In short:
100 Mbps internet: Four wires do the heavy lifting (1, 2, 3, and 6), while the remaining four stand by.
Gigabit internet: All eight wires are utilized; none can be left out.
Landline support: Wires 4 and 5 transmit telephone signals, allowing a single cable to serve dual purposes.
Surveillance (PoE power): Power is delivered via wires 4, 5, 7, and 8, eliminating the need to run a separate power cable.
Practical advice: Never cut corners by crimping only four wires.
While crimping only four wires might work for basic internet access in the short term, you will likely encounter various minor issues later if you upgrade to Gigabit broadband, install surveillance cameras, or connect a landline. Fixing the connectors later is time-consuming and tedious.
Therefore, when making a network cable, make sure to crimp all eight wires properly, following the T568B standard (White-Orange, Orange, White-Green, Blue, White-Blue, Green, White-Brown, Brown). Doing it right the first time will save you a lot of trouble down the road.
