Application and Performance Analysis of twisted pair cables for digital transmission

1. Overview

With the development of the information society, network technology becomes more and more complex. As the cornerstone of the network, keel-network cable is more important. Currently, three types of cables are used in the Network: Optical Cable, coaxial cable, and digital twisted pair cable. This article focuses on the analysis of the largest used digital pair cable.

Twisted pair cables for data are the most common transmission medium in integrated cabling. It consists of two copper wires with an insulating protective layer. The two insulated copper wires are twisted together at a certain density to reduce signal interference, the electric waves emitted by each wire during transmission are offset by the electric waves sent from the other wire. If one or more pairs of twisted pair wires are placed in an Insulating Sleeve tube, the pair cable is formed. Currently, Twisted Pair cables are divided into Unshielded Twisted Pair wires (UTP: Unshielded Twisted Pair) and Shielded Twisted Pair wires (STP: Shielded Twisted Pair ).

The bandwidth of the LAN Using the twisted pair Cable depends on the quality, length and transmission technology of the wire used. With careful selection and installation, you can reach a reliable transfer rate of several million bits per second within a limited distance. When the distance is short and appropriate transmission devices and protocols are used, the transmission rate can reach 100 Mbit/s ~ 155 Mbit/s, or even Gbit/s. The outer layer of the shielded twisted pair cable is wrapped in an aluminum box to reduce the radiation caused by the twisted pair cable when transmitting information. Unshielded twisted pair cables have become the mainstream at present, mainly because of their high cost performance, easy installation, independence and flexibility, and are suitable for structured integrated wiring.

2. Digital twisted pair Cable

Main Performance

In ANSI and EIA/TIA568 standards, cables are divided into several categories based on physical characteristics and application levels. In actual LAN applications, Category 5, Category 5, and category 6 are used as the data transmission pair cable. Currently, three types of cables are mainly used as voice lines.

The transmission frequency of the three types of cables is 16 MHz, which is used for voice transmission and data transmission with a maximum transmission rate of 10 Mbit/s. It is mainly used for 10 BASE-T, jacket is a high-quality insulating material with a transmission frequency of 100 MHz. It is used for voice transmission and data transmission and is mainly used for Ethernet cables of BASE-T; cat5e and cat6 cables are designed to adapt to the application of Gigabit Ethernet, and their performance indicators have not yet been improved.

From the perspective of actual network applications, as the underlying link of the network, its electrical performance indicators are particularly important. There are two ways to analyze the cable performance: one is the laboratory testing method, which tests the normal performance and environmental performance of the cable according to the international and China's corresponding standards; the second is the on-site performance test. The cabling system completed in the building is measured to assess the overall cabling performance after the cables are laid.

In practical application, we are most concerned with the following indicators that characterize its performance:

(1) Attenuation

Attenuation is a measure of signal loss along the link. Attenuation is related to the length of the cable. As the length increases, the signal attenuation also increases. The attenuation unit is db, indicating the signal strength ratio from the source transmitter to the receiver. Because the attenuation changes with the frequency, the attenuation of all frequencies within the application range should be measured.

(2) Crosstalk

Crosstalk is divided into near-end crosstalk and remote crosstalk (FEXT ). Near-end crosstalk loss is used to measure signal coupling from one line to the other in a UTP link. For UTP links, NEXT is a key performance indicator and the most difficult to accurately measure. As the signal frequency increases, the measurement difficulty will increase.

NEXT does not indicate the crosstalk value generated at the near-endpoint. It only indicates the crosstalk value measured at the near-endpoint. The value varies with the length of the cable. The longer the cable, the smaller the value. At the same time, the signal at the sending end also degrades, And the crosstalk on other line pairs also decreases.

(3) DC Resistance

DC loop resistance consumes part of the signal and converts it into heat. It refers to the sum of a pair of wire resistance. The DC resistance of the ISO11801 twisted pair cannot exceed 19.2 ohm. The difference between each pair cannot be too large (less than 0.1 ohm). Otherwise, it indicates poor contact and the connection point must be checked.

(4) Characteristic Impedance

Characteristic Impedance includes resistance and frequency of 1 ~ MHz inductance impedance and capacitance impedance, which are related to the distance between a pair of wires and the electrical performance of the insulator. Various cables have different characteristic impedance, while twisted pair cables have 100 ohm, 120 Ohm, and 150 ohm.

(5) attenuation crosstalk ratio (ACR)

In some frequency ranges, the proportional relationship between crosstalk and attenuation is another important parameter that reflects the cable performance. ACR is also sometimes expressed by Signal-to-noise Ratio (SNR: Signal-Noice Ratio), which is calculated by the difference between the worst attenuation and the NEXT value. A large value of ACR indicates stronger anti-interference capability. Generally, the system requires at least 10 decibels.

(6) cable Characteristics

The quality of a communication channel is described by its cable characteristics. SNR is a measure of the data signal strength when the interference signal is taken into account. If the SNR is too low, the receiver cannot distinguish between the data signal and the noise signal when receiving the data signal, which leads to a data error. Therefore, to limit data errors to a certain range, a minimum acceptable SNR must be defined.
There are several different ways to test these indicators, and their requirements are slightly different.

The above performance indicators are based on the physical characteristics of the cable. In actual network applications, people want to know what the transmission bandwidth should meet. During cable analysis, the physical bandwidth is 0 ~ 100 MHz) and data transmission bandwidth 0 ~ 100 Mbit/s) is two completely different concepts. The Shannon theorem shows the relationship between physical bandwidth and data transmission bandwidth:

C = Blog2 (1 + S/N)

C is the link speed that can be obtained, B is the link bandwidth, S is the average signal power, N is the average noise power, and the signal-to-noise ratio S/N) is usually expressed in dB, decibels = 10 × log10S/N ).

From this we can see that the physical bandwidth of the digital twisted pair cable is the basis of network applications. On this basis, different network devices are used to achieve different network transmission effects. Increasing the physical capabilities of cables can greatly reduce the cost of network equipment to achieve optimal network performance and price ratio. Table 4 lists the application standards of common networks.

3. Integrated Wiring Application Analysis

For current network applications, it is very important to correctly design the topology of the twisted pair cable for digital output.
From the standard Integrated Wiring Network Application diagram, we can clearly see the relationship between transmission media and network devices and node devices. For example, if the 10 Mbit/s Nic is used as an excuse for all the terminal devices, the Cat3 twisted pair cable can be used for the overall cabling to meet the design requirements. However, Integrated Wiring is a concealed project and will not be changed in a short period of time after construction. This requires the overall wiring to be advanced and scalable, select cables with advanced performance based on actual needs when selecting transmission media.

To adapt to the development of the network, multi-Membrane Optical Fiber is used as the transmission medium of the main subsystem in the building when designing a cable topology of a medium scale or above, CAT5, CAT5E, or CAT6 are used as the transmission media of the level subsystem. For a small-scale network architecture, you do not need to use the floor distribution area management mode, directly connect from the main distribution area to the terminal node, then you should use a high-grade digital pair cable.

Connecting parts of twisted pair cables for digital transmission is also a factor that must be taken into account throughout the system. Simply put, if CAT6's twisted pair Cable uses CAT5 connection accessories, its overall link performance indicators can only achieve the performance of CAT5 media.

Transmission media is the basic guarantee for excellent network performance. The twisted pair cable for digital transmission is currently the most cost-effective transmission medium. A detailed analysis of its performance will provide a basis for the design of the network and application network.