Test Integrated Wiring Optical Fiber Transmission Channel

The test of the optical cable cabling system is a necessary step for project acceptance. It is also the final process for the project contractor to honor the contract with the real estate owner. Only after the system test is passed can the cabling system be completed.

The connectivity of devices is the most basic requirement for wiring system testing. The effectiveness of jumper systems can be easily tested. It is relatively difficult to test the indicator data of communication lines, generally, professional tools are used. In the TSB-76, the test of twisted pair wire is clearly defined, and the wiring system test should be carried out according to this standard.

1. Four Methods for optical fiber testing

The following methods are usually used to test optical cables in specific projects: connectivity test, end-end loss test, transceiver power test, and reflection loss test.

(1) connectivity test. Connectivity testing is the simplest test method. You only need to import light (such as flashlight) at one end of the optical fiber and check whether there is any flashing light at the other end of the optical fiber. The purpose of the connectivity test is to determine whether a breakpoint exists in the optical fiber. This method is used when purchasing optical cables.

(2) end-end loss test. End-to-End loss test adopts the plug-in test method. Using a power measuring instrument and a light source, a certain position of the tested optical fiber is used as a reference point to test the power value of the test, then, the client end is tested and the signal increase value is recorded. The difference between the two is the actual end-to-end loss value. This value can be used to determine whether the cordless connection is valid compared with the FDDI standard value.

The operation steps are divided into two steps: the first step is to refer to the measurement (P1) test to measure the loss value p1 between the known light source and the directly connected power meter; the second step is to implement the measurement (P2) test to measure the loss value P2 from the transmitter to the receiver. End-to-End power loss A is the difference between the reference measurement and the actual measurement: A = P1-P2.

(3) send and receive power test. The transceiver power test is an effective method for measuring the optical fiber link of the cabling system. The main devices used are the Optical Fiber Power tester and a jumper cable. In practical application, the two ends of the link may be far apart. However, as long as the optical power of the sending end and the receiving end is measured, the status of the optical fiber link can be determined. The procedure is as follows.

Remove the test optical fiber from the sending end and replace it with a patch cable. one end of the patch cable is the original transmitter, and the other end is the optical power tester to enable the optical transmitter to work, the optical power value of the sending end can be measured on the optical power tester.

At the receiving end, replace the original jumper with jumper cables, connect the optical power tester, and measure the optical power value of the receiving end when the optical transmitter at the sending end is working.

The optical power difference between the sending end and the receiving end is the loss produced by the optical fiber link.

(4) reflection loss test. Reflection loss testing is an effective method for optical fiber line maintenance. The optical fiber time zone reflector (OTDR) is used for testing. The basic principle is to determine the distance by the time difference between the imported light and the reflected light, so that the fault location can be accurately determined. Although the FDDI system acceptance test does not require measuring the length of the optical cable and component loss, it is also very useful data. OTDR injects the probe pulse into the optical fiber and estimates the length of the optical fiber based on the reflected light. OTDR testing is suitable for fault locating, especially for determining the locations where optical cables are disconnected or damaged. The OTDR test documentation provides important data for network diagnostics and network expansion.

2. Estimation of optical fiber connection and Link Loss

Any device connected to the optical fiber can cause different degrees of loss of the optical power. When the optical fiber is transmitted, it will also produce a certain loss. HDDI requires that the total connection loss between any two ends should be controlled within a certain range. For example, the connection loss of Multimode Optical Fiber should not exceed 11dB. It can be seen that the effective Calculation of Optical Fiber Connection loss is a very important issue for FDDI network cabling.

In general, the connection loss between end-to-end includes the following aspects.

(1) connection loss between nodes and the distribution frame, such as various connectors.

(2) attenuation of the optical fiber itself.

(3) loss arising from fiber-optic interconnection, such as fiber fusion or mechanical connection.

(4) the amount of loss and wealth reserved for the future, including overhaul connections, thermal deviations, safety considerations, and the impact of aging of the transmission device.

The optical fiber link has two basic parameters: bandwidth and power loss. Fddi pmd standard: the optical fiber distance is 2 km, the modal bandwidth is at least 500 MHz/l300pm. Select the appropriate optical fiber that meets the standards during planning and construction.

Link Loss refers to the attenuation of optical power between the port and the port, including the loss of all devices on the link. FDDI links may cause losses on optical signal transmitters, receivers, optical bypass switches, connectors, terminals, and optical fibers. The fddi pmd standard provides the maximum allowable loss value between two nodes. The maximum loss value of multi-mode optical fiber is 11dB, while that of single-mode optical fiber is divided into two types of transceiver. the maximum loss value of type I transceiver is 11dB, and that of type II transceiver is less than 33dB and greater than 14dB, the link loss value is the sum of the loss values of all components between two nodes, including the following main factors.

(L) connection from FDDI nodes to optical fiber cables (such as ST and MIC connectors ).

(2) optical fiber loss.

(3) passive components, such as optical bypass switches.

(4) safety, temperature change, aging of the transceiver, and planned Rebuild of connectors.

In the design and planning of the FDDI network, it is necessary to estimate the link loss value and check whether it complies with the fddi pmd standard. If it does not comply with the fddi pmd standard, it is necessary to reconsider the wiring scheme. For example, to use a single-mode optical fiber type II transceiver, add the source component to the connection and remove the optical bypass switch, even change the physical topology of the network, and re-calculate the link loss value until the standard is met. When calculating the link loss value, you do not need to calculate the loss value of each link. You only need to calculate the Link Loss in the worst case. In the worst case, the link is the link with the maximum optical fiber length, the maximum number of connectors and connectors, and the maximum number of optical bypass switches. Of course, if the loss value of all links is calculated and recorded, it will be very useful for future Fault Diagnosis and troubleshooting. It is necessary to calculate the link loss value in the network design. If an error occurs only after the installation is complete, it may be costly to add or replace the device, or even re-design and install the device. Because the estimation value is used during calculation, and there are many factors that affect network operation, even if the link loss calculation value meets the requirements, it cannot completely ensure that the installed network is successful.

As long as this value is less than the maximum loss value of MMF 11dB, it indicates that the design scheme is acceptable from the perspective of Link Loss.

The power loss of an optical bypass switch is 25 dB. FDDI standard recommendation: On a link with an optical bypass switch, the length of the optical fiber between any adjacent two communication sites should not exceed 400 m (dB/km ). Within this limit, even if four consecutive sites are in the bypass status, the two nodes of the four sites can still communicate, because the connection loss between any two nodes can still meet the boundary condition with the loss not greater than 11dB (4 × 2.5 + 0.4 × 2.5 = 11dB. Of course, this calculation is performed on the assumption that there is no other loss source.