Fiber optic cable manufacturers through the attenuation and bandwidth of the two parameters to illustrate the characteristics of the product. Although these parameters are the basis of good performance, but they still can not fully reflect the performance of cable.
Because these cables are mainly used for local area networks, it is necessary to examine these cables in a LAN environment. For this purpose, the Nexans Data Communications Certification Center is testing multimode fiber from a system perspective.
In the evaluation of the use of different manufacturers (due to confidentiality agreement to fade the manufacturer’s name) to determine whether they are different in the optical transmission distance. The results presented in this paper are measured in G bits and 10 Gbit Ethernet. When the test is completed, the evaluation will also include the frame rate and the transmission distance on the multimode fiber when connecting the 1G, 2G, 4G and 10G Fiber Channel transceivers, respectively.
Test the configuration of the system
In order to accurately simulate the enterprise network, the use of commercial switches in the test. Figure 1 is a schematic diagram of a test setup using several different network configurations and the following devices: Spirent or IXIA bitstream generator, IXIA switch with 10GXENPAK interface, and switches from Cisco, Extreme, SMC and Asante.
IXIA or Spirent devices generate Ethernet frames and record the number of frames received. This can calculate the frame error rate (FER). In order to get a frame error rate of less than 10-13, at least 1012 packets should be transmitted. Each test takes a long time (about eight days for a 1G data stream and approximately one day for a 10G data stream).
The test system is configured with long and short wavelength GBIC transceivers and SFP transceivers operating at 1G. There are 10G optical modules for XENPAK and XFP. The trial used six different vendors’ commercial transceivers. In addition, the use of Berk-TekGIGAlite various grades of fiber. The schedule details the type and parameters of the fiber.
Test results and discussion
Test data show that the use of different fiber, different manufacturers of transceiver performance is very different. While all products meet or exceed the optical transmission distance specified by the industry standard, some products still have relatively good performance. For example, when using standard 700MHz · km multimode fiber (LB) and GBIC transceivers, vendors B and C transceivers can transmit 1000m, while vendor D can pass 2400m. Also, the 2000MHz · km fiber (EB) greatly improves the transmission distance of all measured transceivers.
Test results show that some transceivers and transponders performance is much higher than the provisions of the standardization organization. It also shows that even if the worst-case transceiver is used, the transmission distance on the existing fiber also exceeds the maximum value of the standard. Extending the transmission length of multimode fiber will reduce the total cost of gigabit and 10 Gigabit LAN cabling. Short wavelength (SX) transceivers can be used for longer distances of communication, the cost of these devices is almost one-third of long wavelength devices. Such a large cost advantage leads to the current use of multichode fiber (about 85%) and short wavelength devices. According to the results of Nexans, multimode fiber in the LAN can transmit 600 meters at 10G, and so far, theoretically, the maximum transmission distance of multimode fiber is 300 meters.
Why the theoretical value and the actual value of such a big difference. The reason is as follows: IE EE standard recommended transmission distance is calculated in the worst environment, and the actual fiber bandwidth may be better than the standard minimum. In the cable being tested, the bandwidth is greater than the minimum allowed. At the same time, the use of different light sources, fiber bandwidth will be greatly changed. In general, multimode fibers are initially designed to match light-emitting diodes, thus defining full injection bandwidth (OFL). The full injection bandwidth corresponds to the way in which the light-emitting diodes stimulate multiple modes in multimode fibers. For modern light sources, such as vertical cavity surface emitting lasers (VCSEL), with OFL is not accurate. Over the past decade, two technological advances have extended the transmission distance of multimode fiber:
● Since there is a serious chromatic dispersion and can not be modulated, the light emitting diode can not work at a rate of 622Mbit / s and above, and therefore must use the laser. For traditional networks, it is important to understand the transmission characteristics of multimode fibers connected to light-emitting diodes. It is also important to understand the transmission characteristics of multimode fibers connected to lasers for the current G-bit and even 10G-bit networks. The light-emitting diode excites all modes in the fiber, while the VCSEL only provokes a limited mode. So use a different bandwidth measurement method.
● Developed new methods for measuring the fiber bandwidth connected to the laser: finite mode injection (RML) and differential mode delay (DMD). Finite-mode injection refers to the fact that it is only a part of the end face of the core by limiting the optical signal, which is similar to the connection of VCSEL and multimode fiber. However, this measurement is not accurate because the intensity distribution of each VCSEL output is inconsistent and the different modes are excited in the fiber, which makes it difficult to measure the actual bandwidth of the fiber. In order to solve this problem, a differential mode delay measurement method is proposed. It requires the evaluation of all the patterns excited in the fiber and the calculation of the effective mode bandwidth (EMB), keeping the differential mode delay at a minimum to achieve greater fiber bandwidth, so the effective mode bandwidth is a more accurate way to measure the bandwidth.
As with fiber, XFP transceivers rarely work in the worst possible environment. Many manufacturers try to provide the remaining transmitters and receivers. The test results show that all manufacturers of XFP transceiver transmission distance exceeds the minimum standard.
Although most of the current Ethernet transmission rate is 10M or 100M per second, but the change is rapid. A large number of computers with 10M / 100M / 1000M network cards are being connected to the network, even if there is little need for such applications, G-bit Ethernet to the desktop will also become a business reality from the technology. Once the terminal connection rate reaches G bits per second, it is probably only a matter of time to upgrade to 10G bits per second to avoid the backbone becoming a bottleneck. In the choice of fiber optic cable and transceiver for the error rate and transmission distance left a larger margin, you can reduce the initial cost of the initial installation and use fees, but also make full use of bandwidth and coverage of the target users.