What Is A Twisted Pair Cable?
Jul 29, 2023
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As mentioned before in the previous blog post, The twisted pair cable is absolutely ubiquitous in our production and daily life because they are used for signal transmission between various devices, such as Ethernet cables, communication lines, encoder wires, motor feedback wires, and more. Especially with the increasing digitization of various devices, twisted pair cables are needed in more and more application scenarios.
But have you ever wondered why we must use twisted pair cables for signal transmission? In this article, let's discuss what is a twisted pair cable and how these twisted pair cables work.
1. The source of interference
Signals (whether analog or digital) and the basic form of power transmission is electric current. This current generates an electromagnetic field around the cable, causing noise interference to surrounding circuits.
This crosstalk between cables increases with the length of the transmission line. The birth of twisted pair cables was precisely to eliminate this interference.
2. What is twisted pair cable?
Twisted pair is a type of wiring where two insulated wires are twisted together in a specific pattern (usually in a clockwise direction).
The most significant feature of this wiring is its ability to eliminate external electromagnetic interference (EMI) during signal transmission. This includes interference from other unshielded cables or crosstalk from nearby wires. Twisted pair cables can be used for transmitting both analog and digital signals.
Alexander Graham Bell, born in 1847, is the inventor of twisted pair cables. Yes, he is the same great scientist known for inventing the telephone.
3. How does twisted pair cable eliminate interference?
In a pair of twisted wires, the two cores transmit signals with equal amplitude but opposite phases (polarities). At the receiving end, these signals are detected using a differential amplifier, which is what we commonly refer to as differential signal transmission.
Noise sources couple noise into the wires through electric or magnetic fields and tend to couple equally onto the two twisted wires. As a result, noise generates a set of common-mode signals on the twisted pair. The interference/noise generated by one wire will cancel out the interference/noise generated by the other wire because they are essentially equal in magnitude but opposite in direction. When the differential signal is received, it cancels out the noise at the receiver, resulting in a clean, interference-free signal.
4. Why not use coaxial cables?
Some people may say that coaxial cables are known for their excellent interference resistance. Indeed, about a decade ago (and even now), coaxial cables were widely used for transmitting cable TV signals in every household.
However, due to the need to effectively prevent external interference, the structure of coaxial cables is very complex, including multiple layers of thick insulation wrapped around a central cylindrical copper conductor and an external braided shield. This complexity makes it difficult to achieve large-scale installation and wiring with coaxial cables.
5. Characteristics of Twisted Pair Cables:
5.1 Advantages:
Prevent external interference and noise from entering the cable.
Prevent the cable itself from causing interference and noise to the outside.
Minimize crosstalk.
Lower cost.
Easy to install and use.
5.2 Shortcomings:
Twisted pair cables' effectiveness against electromagnetic interference largely depends on the method of twisting, and it is necessary to ensure that the cable remains intact during installation and use. Therefore, twisted pair cables usually have strict requirements regarding maximum pulling force and minimum bending radius, among others.
6. Shielded and Unshielded:
Twisted pair cables are typically classified as Shielded Twisted Pair (STP) and Unshielded Twisted Pair (UTP).
To achieve better electromagnetic interference protection, twisted pair cables often use shielding layers.
The purpose of the shielding layer is to act as a conductive barrier to reduce electromagnetic waves from entering and provide a conduction path for induced currents. Through this conduction path, induced currents can flow through a grounding reference and return to the source. This shielding can be applied to individual wire pairs or a set of paired wires. Sometimes, each wire pair is individually shielded with a metal foil, and the entire cable can have an additional layer of metal foil and braided shielding.
7. Several types of Shielding:
7.1 Individual Shielding:
Each twisted pair in the cable is individually shielded with aluminum foil.
This type of shielding not only protects the wire from external electromagnetic interference but also prevents the wire from causing electromagnetic interference to surrounding circuits, protecting adjacent lines from crosstalk effects.
7.2 Overall Shielding:
All pairs of wires in the cable are wrapped with metal foil or braided shielding.
This shielding helps prevent electromagnetic interference from entering or escaping the cable.
7.3 Individual Shielding + Overall Shielding:
Each twisted pair is shielded with metal foil, and the entire outer layer also has metal foil or braided shielding.
This shielding helps prevent electromagnetic interference from entering or escaping the cable and protects adjacent lines from crosstalk effects.
Because shielding is made of metal, it can also serve as a ground. Sometimes, shielded twisted pair cables include a special core grounding wire, also known as a drain wire, which is electrically connected to the shielding layer, helping simplify the connection between connectors and ground.
8. Classification of Twisted Pair Cables:
In industrial, commercial, and everyday life, four pairs of twisted pair cables with symmetrical distribution are the most common basic form of network communication cables. Shielded Cat 5e, and Cat 6/6a cables typically use an F/UTP structure, while Shielded Cat 7/7A cables use an S/FTP structure.
CAT-5 and CAT-5e cables, which are defined and recognized by TIA/EIA-568-A and TIA/EIA-568-B, respectively, and provide bandwidths of 100MHz and 125MHz, are common in traditional LAN cables. They use unshielded twisted pair wiring (UTP) and are only suitable for 100 Mbps level Ethernet applications, with a maximum transmission distance of 100m. Due to their lack of support for emerging applications and signaling technologies, the use of CAT-5 and CAT-5e cables is declining significantly and is nearing the end of its product life cycle.
CAT-6, currently defined and recognized by TIA/EIA-568-B, provides a bandwidth of 250MHz, one and a half times higher than CAT-5 and CAT-5e. CAT-6 250 MHz UTP cables only support 10 Gbps short-distance applications of 35-55m, depending on the crosstalk environment.
Compliant with CAT-6a 500 MHz unshielded twisted pair (UTP) and shielded twisted pair (STP), which have become the most popular types as they pass the 10 Gbps and 100m certification tests. Currently, 10Gbps Ethernet ports have become the mainstream in the industrial market, so CAT-6a will be used in 10 Gigabit Ethernet (10 Gbit/s) in the future.
Meanwhile, European market leaders and standard setters advocate CAT-7 600MHz standards with double shielding (individual shielding for each twisted pair and overall cable shielding) for 10 Gbps 100-meter Ethernet LAN applications. CAT-7 is the informal name for the ISO/IEC 11801 Class F cable standard. This standard defines four pairs of individually shielded twisted pairs wrapped in an overall shield. Its design allows for signal transmission at 600MHz, with crosstalk performance superior to CAT-6a. However, the North American standards groups TIA and EIA-568 have not yet confirmed this standard.
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