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| Levels Lab Report #6 Contemporary Fiber Issues
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| Among network managers, there are two perceptions regarding fiber. The first is that multimode fiber may need to be replaced by single-mode fiber for Gigabit Ethernet links. This perception is based upon fiber characteristics established by the IEEE 802.3z
Secondly, while many would consider optical fiber to be an unlimited bandwidth media, with one fiber pretty much equal to another, this is not the case. As the demand for more bandwidth increases, the differentiation of one fiber type becomes more critical. This is similar to the situation with copper cabling networks a decade ago. At that time, the perception existed that “wire was wire” and any copper media was suitable and equal to any other.
Developing A New Test Procedure
To address these issues, the Anixter Levels Lab® performed laboratory tests to determine the transmission limits of the current generation 62.5-micron multimode fiber, compared to the test results of 50-micron multimode fiber. A dynamic test procedure was devised, along the same lines as Anixter ADHRSM (Active Data Head Room) testing for copper channels. A Netcom SmartBits® 2000 Performance Analysis System transmits a proprietary waveform developed to provide complexity and difficulty. This signal is passed through a Hewlett-Packard ProCurve® Switch equipped with Gigabit Ethernet fiber optic network cards.
The signal then passes through the channel under test, and back to another SmartBits card. This SmartBits-to-HP test is used to gauge the performance in a real-world environment with a commercial switch. While SmartBits-to-SmartBits testing (as used by others) can be performed, this is considered less of a real-world environment because it involves one high-quality piece of laboratory equipment virtually talking to itself.
The tests were conducted with Gigabit Ethernet cards using VCSEL (Vertical-Cavity Surface-Emitting Laser) transmitters and receivers, operating at the 850-nm window.
The MII transceiver was set to 1000BASE-SX transmission (auto-negotiation disabled) and configured to transmit an IEEE-compliant Ethernet frame. Frame size is varied from 64 to 1,518 bytes.
Pictorial Diagram of Test Configuration
The diagram to the right illustrates the test configuration that was used. | |
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Results
The table above, while preliminary in nature, provides several insights as a result of live data testing using difficult waveforms sent through a real-world switch card.
Both cable types are capable of supporting Gigabit Ethernet, error-free, over distances far greater than those recommended in the IEEE standards or manufacturers' conservative warranties. Economical VCSEL transmitters operating at 850 nm may be used to accomplish this performance.
Both cable types can perform with as many as a dozen dual-mated connectors along their length, indicating that the loss budgets estimated in the standards may be considered conservative.
The higher grade 62.5/125-micron multimode (InfiniCor® I-300) can support Gigabit Ethernet signals without error for distances 150 to 200 percent of what was expected, even with a large number of mated connectors along the fiber length. (The 62.5-micron example, tested with 12 connector pairs, could ostensibly provide the backbone riser in a 123-story building-taller than the Sears Tower, World Trade Center or the Petronas Towers in Malaysia.)
Laser-optimized 62.5/125-micron enhanced multimode (InfiniCor CL1000) can support Gigabit Ethernet signals without error for distances over a kilometer (over 200 percent of what was expected), even with eight “field-installed” Unicam MT-RJ connectors. (MT-RJ connectors were used here on the basis of availability and convenience. It is expected that other low-loss connectors would perform equivalently.)
Summary
LAN network managers can feel comfortable using multimode fiber in their infrastructures. In fact, laser-optimized enhanced 62.5-micron fiber is capable of handling Gigabit Ethernet traffic over much longer distances than the standards would indicate-even with numerous mated connectors along the route.
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