Performance and application of digital twisted-pair cable

1. Overview

With the development of information society, network technology has become more and more complex, and as the cornerstone of the network, keel-network cable appears more important. In the current network, there are three kinds of cable media: Optical cable, coaxial cable and digital twisted cable. This paper focuses on the application of the largest number of twisted cable.

The twisted cable used in the data is the most commonly used transmission medium in the integrated wiring engineering. It consists of two copper conductors with insulating protective layers. The two insulated copper conductors are twisted together in a certain density to reduce the degree of interference, each wire in the transmission of radiation will be emitted by another line of the radio offset. If a pair or more pairs of twisted pair are placed in an insulated casing, the twisted cable becomes. At present, the twisted cable is divided into two types: unshielded twisted pair (utp:unshielded Twisted Pair) and shielded twisted pair (stp:shielded Twisted Pair).

The bandwidth of the LAN using the twisted cable depends on the quality, length and transmission of the wire used. A reliable transfer rate of millions of bits per second can be achieved within a limited distance, as long as it is carefully selected and installed. When the distance is short and the appropriate transmission equipment and transmission protocols are used, the transmission rate can reach 100mbit/s~155mbit/s, even gigabit. The outer layer of the shielded twisted pair cable is wrapped by aluminum to reduce the radiation generated by the twisted pair as it transmits information. Non-shielded twisted cable has become the mainstream, mainly because it is cost-effective, easy to install, and has the independence and flexibility, suitable for structured integrated wiring.

2. The main performance of the digital twisted cable

In the ANSI and eia/tia568 standards, the cables are grouped into several categories by physical and application levels. At present, the application of local LAN mainly uses five kinds, super five classes and six kinds as data transmission twisted cable. The three types of cables are currently used mainly as voice lines. ---http://bianceng.cn (computer science)

Three types of cable transmission frequency of 16MHz, for voice transmission and the highest transmission rate of 10MBIT/S data transmission, mainly for 10base-t; five types of cable increased winding density, jacket a high-quality insulating material, transmission frequency of 100MHz, for voice transmission and data transmission, It is mainly used for 100BASE-T Ethernet cable, the Super Five class and six type cable is to adapt to the application of Gigabit Ethernet, and its performance index is not perfect at present.

From the point of view of practical network application, as the bottom link of network, its electrical performance index is especially important. There are two ways to analyze cable performance: One is the laboratory test method, which according to the international and China's corresponding standards for the normal performance test and environmental performance test, the second is the field-type performance test, the construction in the building of the wiring system to measure, check the overall wiring after the completion of cable laying performance.

In practical applications, we are most concerned with the performance of several indicators:

(1) Attenuation

Attenuation (attenuation) is a measure of signal loss along a link. Attenuation is related to the length of the cable, and as the length increases, so does the attenuation of the signal. Attenuation uses "DB" as the unit, indicating the ratio of the signal strength of the source transmitting terminal signal to the receiving end. As attenuation varies with frequency, the attenuation of all frequencies in the application range should be measured.

(2) Crosstalk

Crosstalk is near-end crosstalk and distal crosstalk (fext). Near-end crosstalk loss is the measurement of a UTP link from a pair of lines to another pair of lines in the signal coupling. For UTP links, Next is a key performance metric and one of the hardest to measure accurately. With the increase of signal frequency, the measurement difficulty will increase.

Next does not represent the Crosstalk value generated at the near endpoint, it simply represents the crosstalk measured at the near endpoint. This measure varies with the length of the cable, and the longer the cable, the smaller the value becomes. At the same time, the signal of the transmitter also decays, and the crosstalk of other line pairs becomes smaller.

(3) DC resistance

The DC Loop resistor consumes a portion of the signal and converts it to heat, which refers to the resistance of a pair of conductors. The DC resistance of the ISO11801 specification shall not be greater than 19.2 ohms. The difference between each pair should not be too large (less than 0.1 ohms), otherwise the contact is bad, you must check the connection point.

(4) Characteristic impedance

The characteristic impedance includes inductance impedance and capacitance impedance of resistor and frequency 1~100mhz, which is related to the distance between a pair of wires and the electrical properties of insulators. A variety of cables have different characteristics impedance, and twisted pair cable has 100 ohms, 120 ohms and 150 ohms several.

(5) Attenuation crosstalk ratio (ACR)

In some frequency ranges, the proportional relationship between crosstalk and attenuation is another important parameter to reflect cable performance. ACR is sometimes represented by the signal-to-noise ratio (Snr:signal-noice Ratio), which is computed by the difference between the worst attenuation and the next measure. The ACR value is larger, indicating the ability to resist interference is stronger. General system requirements are at least 10 db.

(6) Cable characteristics

The quality of the communication channel is described by its cable characteristics. Snr is a measure of the strength of a data signal, taking into account interference signals. If the SNR is too low, it will cause the data signal to be received, the receiver can not distinguish between the data signal and noise signal, resulting in data errors. Therefore, in order 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 vary slightly.

The above performance indicators are in accordance with the physical characteristics of the cable requirements, in the actual network applications people would like to know the transmission bandwidth should meet what kind of requirements. When analyzing cable, physical bandwidth (0~100mhz) and data transmission bandwidth (0~100MBIT/S) are two completely different concepts. From Shannon's theorem, we can see the relationship between physical bandwidth and data transmission bandwidth:

C=BLOG2 (1+s/n)

Where c is the available link speed, B is the bandwidth of the link, S is the average signal power, N is the average noise power, the signal-to-noise ratio (s/n) is usually expressed in decibels (db) and the decibel number =10xlog10 (s/n).

From this we can see that the physical bandwidth of the digital twisted cable is the foundation of the network application. Based on this, different network devices are applied to achieve different network transmission effects. Improving the physical ability of cable can greatly reduce the cost of network equipment, to achieve the best network performance price ratio. Table 4 is the application standard of the common network.

3. Integrated wiring Application Analysis

In view of current network application, it is very important to design the topological structure of twisted cable for digital output correctly.
From the standard cabling Network application diagram, we can clearly see the relationship between transmission medium and network equipment and node equipment. For example, terminal equipment all choose 10MBIT/S Network card as an excuse, then the whole wiring all use CAT3 twisted pair cable can meet the design requirements. But the integrated wiring is a concealed project, a construction will not be changed after a short period of time, so that the overall wiring to have a certain advanced and expansion, in the selection of transmission media to be based on the actual needs, choose to have a certain advance energy cable.

At present, in order to adapt to the development of the network, the multi-film fiber is used as the transmission medium of the backbone subsystem in the building, and the CAT5, CAT5e or CAT6 are used as the transmission medium of the horizontal subsystem. For small-scale network architecture, there is no need to use the floor wiring area management mode, directly from the main wiring area to connect to the terminal node, at this time should be used for high-grade digital twisted cable.

The connecting parts of twisted cable for digital transmission are also the factors that must be considered in the whole system. Simply put, if the CAT6 cable uses CAT5 connection fittings, its link overall performance index can only reach the performance of CAT5 media.

For the network to achieve good performance, transmission media is the basic protection. The twisted cable of digital transmission is the most cost-effective transmission medium at present, and its performance is analyzed in detail, which will provide the basis for designing network and Application network.