Why use twisted pair

Mon Dec 26 11:34:07 CST 2022

0_Why-use-twisted-pair-Hornmic

Why use twisted pair in ethernet?

Use twisted-pair cables for the physical connection of the ethernet. Why is this? What makes twisted-pair cable the dominant network cabling choice?

There are two main reasons, both related to Electromagnetic Interference (EMI).

1. Twisted-pair wires can greatly reduce the electromagnetic interference radiated outward from the conductor.

2. Twisted pair wires can reduce the impact of external electromagnetic interference on the wire itself.

Both of these characteristics are very important if the network cable needs to be arranged in bundles with various other cables over long distances (such as data centers or distribution boxes).

Reduces EMI radiation to the outside

As long as there is a current signal in the wire, it will definitely radiate EMI, which in turn affects the surrounding cables, which is commonly referred to as "crosstalk".

EMI radiation can be compensated for by additional shielding, but the famous Mr. Bell has invented a brilliant way to counteract EMI.

His idea is to use two wires, one of which sends the original signal, the other sends the exact opposite of the original signal. In this way, the two wires would radiate exactly the opposite EMI, and they would cancel each other out.

1_single-wire-to-transfer-data

2_differential-pairs-to-transfer-data

In electrical engineering, these two wires are often referred to as "differential pairs" and can be represented as TX+ and TX-.

This invention allows for wiring solutions that do not require extensive shielding, and is one of the reasons why unshielded wires are currently used so much.

Reduces external EMI absorption

Even with the above "differential lines", we cannot avoid all external electromagnetic interference. Wireless networks, Bluetooth, satellite communications and cell phones can become a source of spurious radio waves in space.

But fortunately, Bell has emerged again and designed a very simple but effective program to shield electromagnetic interference.

This design is based on a basic concept of EMI: the closer you are to the source of EMI radiation, the stronger the interference you will receive. If the two wires are alternately close to the EMI source, they will absorb the same amount of radiation. This is shown in the figure below.

3_Reduces-external-EMI-affect

Although the voltage on the right side of the wire is different from the left side, the voltage difference between the two wires is always the same, always 100 V. The effect of the EMI on the two wires is equal. After a simple calculation and transformation, the final 100V voltage difference can be based.

Note: In fact, the voltage disturbance caused by EMI is at the level of microvolts (µV).








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By HornmicLink_Henry @221226 11:42