Feature - End Return Loss Concerns with Quabbin
Patch Cords
Explanation - What is Return Loss and Why Is It
Important?
Solutions - USB (Universal Serial Bus)
New Products - LC Connector
Expert Profile - Frank Pantalena
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End Return Loss Concerns with Patch Cords from Quabbin |
In many ways, Return Loss (RL) is the most complex noise element in a network. It is created by discontinuities and impedance mismatches within a network. Minimizing these RL echoes requires a tuned or impedance balanced network, with each component carefully selected. RL performance is usually an end-effect and, therefore, is very dependent upon patch cord and connector quality.
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Manufacturers must optimize all the electrical, dimensional, and mechanical requirements and produce a very consistent 100 ohm cable that has very little variation along the length of each pair.
Quabbin has mastered the cable design and also the techniques for high-speed manufacture and control of all critical variables. The result is a DataMax® cable that contributes very little RL to a channel, yet can easily terminate to a modular plug. High-quality DataMax 6 patch cords, built with the best plugs and flexible stranded cable, actually improve channel RL by four to six decibels. This, in turn, reduces bit errors, signal distortion, and retransmissions.
Only DataMax 6 patch cords are lot-tested to assure guaranteed RL performance. In addition, DataMax patch cable is easy to jacket-strip, identify color codes, and terminate to standard plugs. It also provides outstanding run-to-run consistency.
With higher speeds and new bidirectional signaling for 100Base-T — and, soon to come, Gigabit Ethernet — top-quality patch cords are now critical to LAN performance. You, therefore, cannot afford not to use DataMax 6 patch cords.
Please contact EKRIS Cable for a complete technical report on what these Quabbin patch cords did for a typical system.
What is Return Loss and Why Is It Important?
Return Loss (RL) is a new and very important noise measurement now being defined for Local Area Networks (LANs) and LAN components. Engineers designing LAN systems and components have known for some time that RL was present; but up until now it did not matter because most LANs have operated using Token Ring or 10Base-T protocols. Both of these signaling schemes use two pairs of conductors —one pair to send and the other to receive data. The signal energy travels unidirectionally from a transmitter at one end of each pair to a receiver at the other end. Thus, only two pairs of the commonly-used four pair cables are actually energized. However, with new, faster, bidirectional signaling being implemented, RL now matters. Following is an explanation of why.
Unidirectional and Bidirectional Networks
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Figure 1 illustrates data energy traveling unidirectionally through a two pair network. The signals lose energy (attenuate) as they travel through the cable and connectors, and there is also some undesirable signal induced from one pair to another, which is called crosstalk. Crosstalk is the major noise source in most 10Base-T Ethernet networks.
Unwanted crosstalk noise propagates in both directions through the other pair, but noise is only a problem at a receiver because it interferes with the desired signal. Noise at a transmitter can be ignored. This explains why, in unidirectional 10Base-T systems, near end crosstalk noise (NEXT) must be recognized and quantified, and far end crosstalk noise (FEXT) can be ignored. This is also why the attenuation to crosstalk (ACR) for a unidirectional network is so important. ACR closely approximates the signal-to-noise ratio that electronics engineers measure as a critical system performance indicator.
Today, many LAN owners are beginning to implement faster signaling protocols. They are considering ATM, 100Base-T or future 1000Base-T signaling. These protocols are far more complex, have much lower signaling energies, and are more susceptible to noise.
Most of the newer, faster methods also use bidirectional signaling, which changes all the conventional measurements for a LAN. As Figure 2 shows, there is now a combination transmitter/receiver (transceiver) chip at each end of each pair. Crosstalk noise still propagates in both directions in each pair, but now the FEXT can no longer be ignored because it affects a receiver.
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To further complicate matters, most of these new and faster protocols use all four pairs in the cable, with each pair bidirectionally signaling. For simplification, Figure 2 only shows a two pair network, but in a four pair network, the crosstalk noise affecting any pair must be added from all three other pairs. This measurement is termed "power sum crosstalk" and is measured both near end and far end (PSNEXT and PSFEXT). Simple two pair unidirectional networks, which could be quantified by measuring just signal attenuation and NEXT, are being replaced by more complex bidirectional networks, for which attenuation, PSNEXT, and PSFEXT must now be measured and addressed.
Not only does bidirectional signaling complicate the traditional signaling measurements, but it also adds new ones. Return Loss (RL) is one of these new, very important measurements. RL is a summation of all the reflected signal energy coming backward toward the end where it originated. It is like an echo and is not to be confused with crosstalk. Since each end of each pair has a transceiver chip, the same data stream sent out by the transmitter gets echoed back to the receiver, which is "listening" for data from the transmitter at the other end. The echo is real data, but, since it interferes with the desired signal, it must be treated as noise.
What Causes Return Loss?
Discontinuities and impedance mismatches are the two major causes of RL in a LAN. Discontinuities occur at connections where cable is terminated to plugs or jacks and within the plug/jack connection itself. A discontinuity can also occur if a cable is bent too much, kinked, or otherwise damaged. When a transmitted signal pulse hits one of these structural discontinuities, echo, or RL, occurs.
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Impedance mismatches can be macro or micro in scale, and both are important. Macro mismatches are usually component-to-component-related. If the horizontal cable in the network averages 100 ohm impedance, and the flexible patch cords are 106 ohm, each 6 ohm mismatch also causes an RL reflection.
Micro impedance variations along a cable's length are subtler but also important contributors to RL. Manufacturing tolerances during the construction of a twisted pair causes these variations. The precise impedance of a pair is determined by the insulation material used; the diameter of the copper; the diameter of the insulation; the centering of the copper within the insulation and the precision with which the two insulated wires are twisted together. If any or all of these factors vary during the manufacture of a twisted pair, the impedance will vary along its length. Thus, the horizontal cable with an average impedance of 100 ohms, mentioned previously, may have a given three-foot section at 95 ohms, the next three-foot section at 97 ohms, the next at 100 ohms, etc. These micro variations in impedance along a pair are small, but they affect high frequency signals, adding a cumulative signal echo. Figure 3 illustrates the combination of these various RL effects within one pair.
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A perfect transmission line would have no imperfections, connections, or impedance variations along its length and would, therefore, have no RL confusing the receiver. However, this is the "real world", and our networks are built using twisted pair, with plugs and jacks as connections. 10Base-T Ethernet with unidirectional signaling is not affected by RL, but the newer and higher speed protocols are, so now RL must be understood and quantified. Too much RL noise added to the PSNEXT and PSFEXT further stretches an already thin noise budget, resulting in increased bit error rate, lower signal-to-noise ratio, less network operating margin, and more downtime. In fact, patch cord RL is proving to be the most important measurement that affects the performance of any channel operating with 100Base-T or any bidirectional protocol.
Fortunately, RL can be improved by using high quality patch cords. This graph illustrates improved performance by using such cords. For more information on improving RL, please see the article on Quabbin's DataMax® 6 patch cords.
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USB (Universal Serial Bus) |
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| USB offers a hassle-free means of connecting PC peripherals. |
If you are a PC user who wants a no-hassle way to connect peripheral devices to your computer, USB is the solution. With USB-compliant PCs and peripherals, you just plug the peripherals into the PC and turn them on! The whole process is automatic. Thanks to a USB feature known as "hot-swapping," you don't even need to shut down and restart your PC to attach or remove a peripheral device.
USB also lets you connect many peripherals at one time. Many USB PCs come with two USB ports, but special USB peripherals —called USB hubs —have additional ports that let you "daisy chain" multiple devices together. The USB specification limits the lengths of a cable between full-speed devices to five meters (16', 5"). For a low-speed device, the limit is three meters (9', 10"). USB's electrical design doesn't allow for longer cables, due to the propagation of electromagnetic fields on USB data lines. Since USB is intended for a desktop environment, the range limitations were deemed acceptable.
A full-speed USB device can be up to 30 meters away from the PC, with a maximum of five hubs connected by four 5m cables and one 5m cable going to the device. With low-speed devices, you'll be able to achieve a range of up to 26 meters. USB Connections use "A-to-B"-style cables. When connecting two PCs together, you will need a specialized USB peripheral known as a USB bridge. Trying to connect them together (null modem-style) using some "A-to-A" cable, as you may find on the market, will result in shorting the two PCs' power supplies together, possibly destroying both machines. To put a USB device, like a printer, on a network, you would need something like a USB-to-Ethernet bridge that was capable of acting as a USB host. Unfortunately, no one currently makes one of these.
USB was not designed to be a LAN (Local Area Network). If you need a LAN, use a technology intended to be used as a LAN, such as Ethernet..
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LC Connector |
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The LC Connector uses the familiar insertion release mechanism, similar to an ordinary telephone plug. Half the size of ST and SC Connectors, this one is the perfect choice for applications where space is limited. The LC line is available in singlemode and multimode options, in either simplex or duplex configuration, to meet a wide range of requirements. Hybrid assemblies with custom lengths or pigtails are all made to your specifications. The LC Connector is easy to engage and disengage. It plugs into the device in one orientation, enhancing optical performance by allowing over 500 repeatable reconnects. Here at EKRIS Cable, we are making all varieties of the LC line, using either 900 micron buffered fiber, 1.6mm cordage, or 3.0mm jacketed zipcord, depending on the application. With lower insertion loss performance than some other Small Form Factor (SFF) connectors on the market, the LC Connector will become more common with time. |
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Frank Pantalena |
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Frank Pantalena joined EKRIS Cable's sales force in 1998. He previously worked in the distribution management field, where he learned the importance of telecommunications in the ever-changing business world. As part of our Cable Assembly division, Frank has gained extensive knowledge of computer technology and data communications. He has increased his expertise by completing Siecor, Molex and Berk-Tek Warranty Training. As one of our inside sales representatives, Frank is always ready to meet your connectivity needs. |
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