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| Levels Lab Report #4 Return Loss and its Effect on Network Performance
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| Return Loss and its Effect on Network Performance
Network performance is impacted by many different factors. Between the file server and the PC lay several possible obstacles that can prevent efficient data transmission. Cable, patch cords, information outlets, hub/switch ports and network interface cards—all can prevent the network from operating at peak efficiency. The Anixter Levels Lab is examining how cabling systems, transmitters, receivers and everyday workplace environments interact in transferring data from the server to the PC. The Anixter Levels Lab has found that there is a strong relationship between return loss and CRC or frame errors, and ultimately, network performance/efficiency.
Return Loss Explained
Return loss is signal that is reflected back to the originating signal source. This reflection occurs when a signal encounters a change in impedance. Impedance changes are most pronounced at transition points—where a patch cord plugs into information outlets/panels or where a horizontal cable is terminated. Return loss creates two significant problems that inhibit effective network operation. First, the signal that is reflected back toward the signal source imposes additional noise onto the cabling system, making it more difficult for the receiver to distinguish noise from signal. Second, because the impedance mismatch causes signal to be reflected, the strength of the signal is compromised, weakening the signal so that it cannot be distinguished from other noise sources on the system. The end result is diminished network efficiency due to a high level of retransmissions. |  |
Category 5
TIA/EIA 568A Category 5 specifies characteristic impedance of +/- 15 ohms for horizontal cable. This means that within the cable itself it's possible to exhibit impedance values from 85 ohms to 115 ohms. Category 5 does not specify impedance values for connecting devices, which can exhibit impedance values from 70 ohms to 120 ohms—conceivably a communication channel could exhibit a differential impedance value of over 50 ohms. These substantial impedance differences between channel devices result in significant levels of return loss.
The above graph shows a typical Category 5 system, specifically, the return loss performance. This system is in compliance with the TIA/EIA 568A specification for all specified electrical attributes. It should support 100BASE-T traffic reliably and relatively error-free. However, this cabling system experienced considerable frame error rates.
The above graph shows that this cabling system's ability to carry live data is severely hampered by its poor return loss performance. As a result, the throughput of this cabling system is extremely poor.
- Impedance mismatches
- Reflected signal
- Increased noise
- Reduced signal
- Increased frame errors
- Increased retransmissions
- Decreased network efficiency
Green Light Factor—There is link between the transmitter and receiver, however the effective data transfer rate can be significantly less than the expected 100 Mbps.
Level 6XP
Level 6XP requires better electrical performance than TIA/EIA A Category 5 and 5e. Specifically, Level 6XP requires a greater degree of impedance "matching" among channel components, which improves return loss performance. The above graph shows the return loss performance of a Level 6XP cabling system. Because the components in the channel have a greater degree of impedance "matching" over the Category 5 system, the return loss characteristics are much better. This substantially enhances the throughput performance of this system.
The above graph shows that with a Level 6XP"matched" system, efficiency is improved and frame errors are significantly reduced over Category 5. This means that this cabling system will be better able to deliver closer to the expected 100 Mbps.
- Input impedance not characteristic impedance
- Channel components are "matched"
- Improved return loss performance over Category 5 and 5e systems
- Significantly fewer errors over Category 5 and 5e systems
- Fewer retransmissions
- Greater network efficiency
- Maximized for 100BASE-T operation
Green Light Factor—With a matched Level XP system, green does mean go. Expected throughput is closer to 100 percent than with Category 5 and 5e systems.
Level 7XP
Level 7XP has the most stringent electrical requirements of any industry standard or specification. In fact, it provides twice the bandwidth of Category 5 and 50 percent more than Category 5e. These Level 7XP systems are perfectly matched—the patch cord, horizontal cable and connectors exhibit virtually the same impedance values. Anixter Level 7XP channels are required to display impedance values of 100 ohms +/- 3 ohms. This translates into only a 6-ohm impedance differential in a Level 7XP system versus more than a 50-ohm impedance differential in Category 5 systems. This allows for very low return loss in Level 7XP cabling systems. The following graphs show the return loss performance and data transmission performance. Very good return loss performance translates into few, if any, frame or bit errors. This equates to very efficient network operation.
- Input impedance not characteristic impedance
- Channel components are perfectly "matched"
- Improved return loss performance over Category 5 and 5e and Level 6XPsystems
- Significantly fewer errors over Category 5 and 5e and Level 6XP systems
- Fewer retransmissions
- Superior network efficiency
- Maximized for Gigabit Ethernet transmission.
Green Light Factor—With a matched Level 7XP system, green does mean go and fast—errors are minimized more than with any other cabling system.
The Bottom Line
Attenuation, crosstalk and ACR are good indicators of how well cabling products are made. However, return loss, which has a significant impact on how well a cabling system transports data, should be considered when assessing products for applications. Level 6XP and Level 7XP provide the greatest degree of impedance "matching" which in turn provides good return loss performance.
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