Analysis on hot issues of shielded twisted-pair wires

Shielding and unshielded wiring system is superior to the inferior, has been a hot topic in the field of integrated wiring. Prior to the advent of 10G applications, European users used more shielding cabling systems, while North American users used more unshielded cabling systems.

With the advent of 10G applications, the situation has changed a lot. As recently as the author in the "2011 China Data Center Network and Wiring Peak Forum", many large manufacturers have indicated that in the data center uses 6 class, 6 a class, 7 class's proportion is similar, the choice 10G copper chain link user, has 75% plans to adopt the shielding wiring solution. As for the characteristics of unshielded twisted pair, we are all familiar with it, today we will summarize the hot issues of shielded twisted pair for your reference:

According to the different shielding mode, the shielded twisted pair is divided into two kinds, namely STP (Shieldedtwisted-pair) and FTP (Foiltwisted-pair). STP refers to each line has its own shielding layer of the shielded twisted-pair, and FTP is the use of the overall shielding of the dual-twisted cable should be noted that shielding only in the entire cable has shielding devices, and both ends of the correct grounding in the case to play a role. Therefore, the requirements of the entire system are shielding devices, including cables, sockets, crystal head and wiring racks, and the building needs to have a good ground system. From the electromagnetic compatibility point of view, s/ftp cable because of the use of metal woven nets and metal foil shielding layer to protect against external electromagnetic interference (EMI) or radio frequency interference (RFI) effect, can completely eliminate the line outside Crosstalk (AXT).

Difference of transmission performance between F/UTP cable and S/ftp cable

From the electromagnetic compatibility point of view, s/ftp cable because of the use of metal woven nets and metal foil shielding layer to protect against external electromagnetic interference (EMI) or radio frequency interference (RFI) effect, can completely eliminate the line outside Crosstalk (AXT). In addition, from the point of view of channel transmission capability, the S/FTP cable can provide higher signal-to-noise ratio (SNR) in the same bandwidth, so the s/ftp cable is an ideal solution for high-speed network applications.

Anti-jamming capability of shielded cable

From the index analysis of the coupling attenuation of cabling system, the ability of the twisted pair cable to resist electromagnetic interference with various kinds of cable shielding mode is increased by 20~30db. Among them: aluminum foil shielding Cable (F/UTP) for 85dB, screen/aluminum foil dual shielded pair cable (SF/UTP) for 90dB, screen total shielding/aluminum foil line pair of shielded cable (S/FTP) for 95dB.

Error rate information of shielded cable

According to the relevant data, the foreign has done the aluminum foil shielded cable (F/UTP) BER test. If the shielding layer does not ground or do not do a good grounding, under certain conditions will increase the network transmission of information generated probability of error rate. The results will reach about 30%.

When the wiring system is shielded 6 a class system, the scene does not need to test the line outside Crosstalk

Shielding system using metal shielding layer can completely eliminate line outside crosstalk (AXT), that is, the provision of high enough line outside Crosstalk (AXT) allowance, if the use of shielding 6 a (CAT.6A) wiring system, on-site construction does not need to test the line outside Crosstalk (AXT), only need to test the electrical parameters in the channel.

Because of the complexity of the testing process, long testing time, and expensive instrumentation, it is recommended that you only test for troubleshooting or when it is necessary.

The manufacturing process of unshielded twisted-pair cable can be realized by balanced transmission, but it is not enough to restrain external interference.

When the transmission bandwidth of the twisted cable is over 30MHZ, the unshielded pair cable is susceptible to external electromagnetic interference and the leakage of information. If the cable in the installation process is not standardized construction, cable to the physical structure of the changes, such as elongated or compressed, the above phenomenon will be more serious.

Information leaks still need to be considered in commercial buildings

The integrated wiring system is used for high-speed transmission. Because of the balance tolerance and other hardware reasons, it may also cause the transmission signal to space radiation, there is information in the unconscious of the possibility of leakage. In the same building, there are likely to be different units or departments. Therefore, in the design should be based on user requirements, in addition to considering electromagnetic interference, should also consider the requirements of electromagnetic radiation. This is a problem of two aspects, the adoption of shielding measures, both can be resolved.

The shielding wiring system grounding only needs to be in the wiring frame end ground

Should be in the telecommunications between the wiring box or wiring cabinets for equipotential connection, the work area shielding information socket itself is not grounded. However, in order to achieve shielding both ends of the grounding, in the work area through the shielding information outlets and shielding jumper shielding layer and network equipment of the shielding layer interoperability. When a device plugs a power cord into a power outlet, it is automatically grounded by a protective conductor (PE).

Shielding wiring system without grounding, anti-interference ability is still better than unshielded wiring system

According to the test data of the third party test lab, it shows: in the normal good grounding situation, shielding system to resist the external coupling noise 100-1000 times, even in the shielding layer without grounding or poor grounding, shielding cabling system can still be shielded wiring system for more than 10 times times.

When the wiring system has been grounded, still need to reach the equipotential connection requirements

Only grounding, not to achieve equipotential connection is not safe, grounding system must have equipotential connection. Equipotential bonding is to reduce the voltage difference between different grounding systems. Especially for the electronic information network, it can improve the performance of electromagnetic compatibility. Equipotential bonding is suitable for all kinds of wiring systems.

The "grounding" in the shielding wiring project is not specifically to release the jamming signal to the "Earth"

Shielding wiring works in the "grounding", if the purpose is to improve electromagnetic compatibility, should be understood as the shielding layer into the equipotential connection system; If it involves lightning protection, it should be understood as access to the Earth itself. Imagine, flying aircraft, rockets, satellites, space station, and its internal wiring system is unable to access the Earth, but can have very good electromagnetic compatibility, is because of its internal reliable equipotential bonding.

Channel test of shielding cabling system through connectivity inspection, does not mean that the shielding layer is not faulty

After connecting the shielding layer of the wiring frame through the frame, the "channel" shielding layer connectivity test can not be found in the project using UTP Jumper or jumper shield layer damaged or open circuit. Therefore, the shielding cabling system more emphasis on testing for the "permanent link" connection model test. The project should use qualified shielding jumper, if there is a question on the Shield jumper, you can independently verify the shield jumper.

Check there are 2 methods: one is to buy qualified jumpers, product quality by the manufacturer to ensure that this is the most commonly used methods in the project, the second is in the project for each jumper shielding layer test, which will greatly increase the construction team's workload and project costs. From the engineering point of view, the former has a contract guarantee, the latter will only be used if the jumper has doubts. Therefore, in the product quality is guaranteed, there is no need for the channel as a shielding layer of the conduction test.