Analysis on cable selection of generic cabling sys

  • Detail

Analysis on cable selection of generic cabling system

1 Overview

cabling is one of the key links of the information network system. Therefore, the cabling system designer must comprehensively plan and design according to the network characteristics, cable performance and total system investment. The poor network performance caused by poor network wiring system design makes the system performance price ratio decline, which is the most common in the design of the wiring system. At the same time, the cost is also very expensive. Therefore, optimizing the network wiring design is the basis to ensure the high quality of the information network and the high performance of the product types

various factors shall be comprehensively considered in the selection of cables, but the category of cables and the structure of wiring (shielded cables, non screen cables, optical cables, or they shall be used together) shall be determined first in the wiring system. Cables usually use insulated conductors and one or more layers of plastic sheath. The cable usually consists of 2 to 1800 pairs. Large pair cables are usually used in backbone cabling systems, and they are particularly suitable for voice and low rate data applications. The maximum length of these cables in trunk and horizontal (hub to desktop) cabling system applications is detailed in the international standard iso/iec is11801. It should be noted that these maximum length limits apply to all media. They do not consider the impact of performance differences caused by different cable types and protocol types used in the network. In fact, the maximum cable length will depend on the system application, network type and cable quality. Table 1 difference between transmission frequency and transmission rate

it is also very important to check the available space for cable routing before determining the cable type. Factors such as size, weight and shielding flexibility mainly depend on whether the cable is made of metal foil or braided sheath, and how many conductors are used in the cable. These factors, together with the shielding and reflective materials used by the cable, will determine the ability of the cable to resist electromagnetic interference (EMI). Before selecting cables, it is also important to consider the shielding and reflective materials used by cables

in recent years, the breakthrough in research on unshielded twisted pair (UTP) enables them to transmit data at 622mbit/s or higher transmission rate. This makes it possible for people to use this lower price and smaller cable in applications that previously could only use shielded cables. UTP cables reduce EMI interference by matching cable pairs more closely. This cable is called a balanced circuit. In an ideal balanced circuit, the sum of the noise voltage introduced into the conductor is zero, so that there will be no interference in the signal transmission between the line pairs. However, this ideal situation cannot be fully realized. The signal-to-noise ratio (SNR) of the cable is used to measure the signal quality in the presence of noise signals in the cable. Due to the shielding in the shielded cable, its balance characteristics are poor. Therefore, good shielding integrity and good grounding are very important for the shielded cable. High quality UTP cables can achieve good balanced circuit characteristics without grounding or shielding of the whole circuit

since the optical fiber transmits signals through light waves, it is not affected by any form of electromagnetic shielding. Optical fiber is usually the best choice in applications requiring transmission rates over 155mbit/s and longer transmission distances. Optical fiber has the advantages of small size and durability, but its cost is higher than other types of cables. Most optical cables used in local areas are multi film optical fibers. It is easier to install than high-performance single film fiber. In most networks, optical cables are generally used as trunk lines, while UTP cables are used for Horizontal Cabling. However, with the increase of communication rate and the decline of equipment price, the number of networks using optical fiber directly to the desktop is also growing. Wireless local area network can be used as an alternative for those systems that are difficult to install cables due to time, space or other restrictions. In wireless local area networks, radio waves are used instead of physical connections to realize signal transmission. They are especially suitable for the installation of networks in old buildings

2. Cable selection

2.1 transmission frequency and transmission rate

transmission frequency and transmission rate are the two basic concepts most touched in the design of structured cabling system. The bandwidth of the cable (MHz) and the data rate transmitted on the cable (Mbps) are two distinct concepts. MHz represents the number of signal oscillations in the line per unit time, which is a physical quantity representing the frequency, while Mbps represents the number of binary bits transmitted in the line per unit time, which is a physical quantity representing the rate. The transmission frequency represents the basic bandwidth of information transmission provided by the transmission medium. The bandwidth depends on the quality of the wires used, the precise length of each wire and the transmission technology. The transmission rate is the ability to transmit information in a specific bandwidth. The indicators to measure the device transmission performance include attenuation and near end crosstalk, and the overall link performance is measured by the attenuation/crosstalk ratio acr. The wider the bandwidth, the smoother the transmission, and the higher the allowable transmission rate. The coding method in the network system establishes the connection between MHz and Mbps. Some special network coding schemes can transmit data at high speed over a limited frequency band width. In general, the designer is concerned about the maximum transmission rate of a specific transmission medium when it meets the transmission performance of the system. Table 1 below visually reflects the difference between the two

the type selection shall be comprehensively considered according to the technical performance, investment estimate, product engineering performance and after-sales service quality of the system. Selection of cat5+ and CAT6: the parameter limits tested by cat5+ are close to the experimental data. The super class 5 system can support the operation of Gigabit Ethernet, and the cat5+ systems of different manufacturers can be used mutually. CAT6 is more expensive than cat5+, but its bandwidth is expanded by 25%, which shows the enhancement of transmission rate. The CAT6 system is dedicated, which means that the components of various manufacturers have their own unique design and performance indicators. The possibility of component interoperability of different manufacturers is very small, and the component indicators are still under study. CAT6 has been adopted in a few projects. There are two problems worth paying attention to: when using CAT6 cable to polish every detail of the link, the connector devices of the same grade and produced by the manufacturer should be selected, because CAT6 is a special part, and the products of various manufacturers cannot be interconnected; CAT6 for fixed links and channels, in addition to the eia/tia standards, it is necessary to add: equivalent far end crosstalk (elfext), comprehensive equivalent far end crosstalk (PS elfext), return loss (return loss), comprehensive near end crosstalk (PS next), comprehensive attenuated crosstalk ratio (PS ACR), etc. At present, CAT6 only has the business enterprise standard. Generally speaking, the test indicators of the business standard are often higher than the specification standard, but whether it can be used as acceptance confirmation is still a pending issue

2.2 selection of shielded and unshielded twisted pair cables

in the cabling system, the most used transmission media are various cables. Commonly used cables include copper cables and optical cables, and copper cables become the main force of the cabling system because of their high cost performance, and the copper cables used in structured cabling are twisted pair. The twisted pair is composed of two copper conductors with insulation protection. The influence of signal interference can be reduced by twisting two insulated conductors together at a certain density. The radio waves radiated from each wire during transmission are offset by those from the other wire. The twisted pair is generally formed by winding two 22 ~ 26 insulated copper conductors. Twisted pair can be divided into unshielded twisted pair UTP (unshielded twisted pair) and shielded twisted pair STP (shielded twisted pair). The shielded twisted pair can be divided into aluminum foil shielded twisted pair (FTP), aluminum foil and copper double-layer shielded twisted pair (s-ftp) and independent shielded twisted pair (STP). The structure difference of twisted pair determines its performance and price difference, providing users with various options. However, how to select the transmission medium of the system is often a primary problem faced by designers. Below, we use table 2 to make a comprehensive comparison of the above twisted pairs. Table 2 price comparison of different copper cables

of course, the security technology in the system can be implemented not only on the link, but also on the cell. The unshielded system adopts steel pipe wiring, which greatly improves the anti-interference ability. With the development of technology, the methods and means of solving problems are constantly changing and improving

utp and FTP cable selection; The key depends on the influence of external EMC interference. When the interference field strength is lower than 3v/m, protective measures are generally not considered. According to the cable performance test results; In the 30MHz frequency band, the transmission effect and anti EMC ability of UTP and FTP are similar. If the transmission effect and anti EMC ability exceed the standard, the isolation degree of FTP is significantly higher than that of UTP by 20 ~ 30dB. According to the magnitude of the interference signal exceeding the standard, different shielded cables and shielded wiring equipment such as FTP, SFTP or STP can be selected respectively. The grounding requirements for FTP are strict, and it should be fully shielded at 360 °, otherwise the shielding layer will become a source of radiation interference. In addition, note that the shielding structure of FTP changes the capacitive coupling of the whole cable, and the attenuation will be slightly higher than that of UTP at the same level. The comprehensive cost of FTP is about: ftp=1.2 ~ 1.6utp. Therefore, FTP is applicable to EMC serious areas and places with strong confidentiality, such as party and government departments, airports, military departments and industrial enterprises

2.3 other factors

the type of equipment connected in the network and the communication load carried by the cable are the key factors for selecting the cable. At the same time, the following factors should be considered when selecting cables:

(1) the maximum distance between the network hub and the node (Information Port)

(2) available space for wiring in pipes, floors and ceilings

(3) degree of electromagnetic interference (EMI)

(4) possible changes of equipment serving the system and their usage

(5) system resilience and network expansibility

(6) life cycle of network requirements

(7) restrictions on cable routing and cable bending radius

(8) installation of existing cables with potential repeated use

2.4 regulations

in addition to the above qualitative analysis, the revised code for the design of generic cabling systems in buildings and building complexes (hereinafter referred to as the specification) also quantitatively stipulates the following:

(1) the anti-interference ability of various cables and wiring equipment. The system can reduce the noise by 20dB on average under the shielded generic cabling

(2) the selection principle of various cable distribution equipment should meet the following requirements:

① when the intensity of interference field in the surrounding environment or the noise level of generic cabling is lower than that specified in Clause 3 of article 12.2.1 of the specification. UTP cable system and unshielded wiring equipment can be used when the frequency of interference source signal or computer network signal is less than 30MHz and can meet the requirements in table 12.2.2 of the specification

② when the interference degree of the surrounding environment or the noise level of generic cabling is higher than that specified in Clause 3 of article 12.21, and the frequency of interference source signal or computer network signal is greater than or equal to 30MHz, different shielded cable systems and shielded wiring equipment such as FTP, SFTP, STP shall be selected according to the magnitude of their exceeding the standard. In addition, if the spacing required in table 12.2.2 of the specification cannot be guaranteed, appropriate protective measures shall be taken

③ when the user has confidentiality requirements for the system and does not allow signals to be transmitted outward, the passive building technology can reduce the active energy input and reduce the building use cost

Copyright © 2011 JIN SHI