26 Oct The Impact of Cabling Parameters on Network Performance
A system’s connecting hardware plays a key role in characteristics like ILD and return loss.
Your cabling system transports information between different parts of your network, from a source to a receiver. The foundation of your roadway should be solid, using high-performance matched components with enough capacity to support today’s and tomorrow’s high-speed data applications without loss of information.
This article discusses the performance of Category 6 cabling compared to Category 5 and Category 5e systems. Understanding insertion loss (IL), insertion loss deviation (ILD), return loss, and noise from various sources is essential to unraveling the mystery of cabling and its impact on performance.
Hardware connections are an essential part of the Category 6 system. Most of the Category 6 standards-development work carried out so far has focused on connecting and associated modular-cord requirements to achieve the specified performance level, backward compatibility with Category 5 and 5e systems, and interoperability among different vendors’ Category 6 systems. Connecting-hardware performance can be described as a delicate balancing act. Signal-to-noise ratio (SNR) can help you understand how these parameters affect your network performance.
A channel’s IL (often called “attenuation”) measures the loss of signal power between its inputs and outputs. Signals with a low insertion loss, measured in decibels, are stronger. Signals that are stronger are less susceptible to environmental noise and can carry more information. IL is measured using field test equipment based on frequency. The frequency range for Category 5 and 5e cabling systems is 1 to 100 MHz. The frequency range for Category 6 is 1 to 250 MHz.
Return loss measures the mismatch between components in a channel. All components should have a nominal impedance of 100 watts. There are variances in design, manufacturing, and installation of components in practice. High return loss, also measured in dB, indicates fewer reflections and well-matched components. Errors are caused by signal reflections, which are a source of noise. Since 1000Base-T, Gigabit Ethernet, is a bi-directional system, a signal transmits in both directions on each pair, high return loss is important.
All other cabling parameters are related to either internal noise, such as near-end and far-end crosstalk, or external noise, such as alien crosstalk. A high crosstalk loss, measured in dB, means less noise and a clearer signal, which results in fewer errors. A signal that is sent on one pair couples onto an adjacent pair within the same cable, causing internal noise. It is also possible for noise to be introduced from external sources, such as other cables, induced power-line transients, or ground return currents along shielded cables.
Additional loss sources
A channel’s insertion loss cannot be calculated by adding together the specified insertion losses of its components.
An additional loss results from insertion loss deviation (ILD), which is caused by component mismatches. The TIA/EIA-568B and ISO-11801 standards specify component losses at 20° Celsius. This specification must be considered by users of cabling systems in higher-temperature environments. For every 10° Celsius increase in temperature above 20° Celsius, unshielded twisted-pair (UTP) cables lose approximately 4% of their efficiency. There can be a significant increase in loss as a result of temperature changes.
Signal reflection, measured in return loss, is also caused by differences in impedance. A mismatch loss occurs when some of the transmitted signal is reflected back to the source. It is possible for some of this reflected signal to be re-reflected back, and combine with the original signal. In addition to being delayed in time, this re-reflected signal can add to or subtract from the main signal. Re-reflected signals are called insertion loss deviations (ILDs).
High frequencies are the most susceptible to ILD, because the signal is weaker and a noise source superimposed upon the receive signal has a greater impact. At high frequencies, connector impedance mismatch is the primary cause of ILD. As a result of the blade spacing at the plug termination, the 3-6 split-pair pin assignment tends to have a higher impedance mismatch than other pin assignments.
Another cause of ILD is cable-cord impedance mismatch. For Category 5 channels, impedance mismatch can be as high as 15Ω, whereas for Category 5e channels, cable-cord mismatch is usually less than 5Ω, and for Category 6 channels it typically is 3Ω. For Fast Ethernet and Gigabit Ethernet networks, cord-cable impedance mismatch is by far the most significant contributor to noise and bit errors. At maximum power frequencies (15-30 MHz), it exceeds the noise contribution due to near-end crosstalk. Gigabit Ethernet 1000Base-T is recommended as a minimum infrastructure because of this ILD performance in Category 5 systems.
Interestingly, modeling shows that the magnitude of ILD due to cord-cable mismatch is approximately constant over the whole frequency range from 1 to 250 MHz. Also, the ILD for Category 5e and Category 6 cables and cords is small, due to the tighter control of impedance mismatch in the specifications for these systems.
As a noise source, ILD contributes to bit errors even when crosstalk is not present-for example, with shielded twisted-pair (STP) cables. Due to the geometry of the foil tape, it is much more difficult to maintain tight control of impedance in STP cable.
To achieve a cumulative data rate of 1 Gbit/sec, 1000Base-T utilizes a hybrid transformer circuit within the transceiver at each end to separate and combine the transmit and receive signals over a single wire pair. Using this scenario, each receiver receives the power-sum near-end crosstalk from three near-end transmitters, as well as the power-sum far-end crosstalk from three far-end transmitters. The IL of the cable pair attenuates the far-end noise in power-sum NEXT compared to power-sum FEXT.
Dealing with more noise
Network performance is also affected by external noise sources, such as alien crosstalk and power-line transients. Alien crosstalk occurs when cables are laid in trays, pulled through conduits, or otherwise located close together. According to our measurements for Category 6 channels, the combined alien power-sum NEXT is approximately the same as the power-sum NEXT within the cable. As a result of extensive testing at their IBDN laboratories, NORDX/CDT recommend a minimum 50-mm separation between branch power (20A) circuits and telecommunications in non-metallic raceways. For power conductors loosely laid in a partitioned furniture raceway, the testing simulated worst-case transients of up to 500 V. Twisted cables have much lower power-line transients.
In the TIA TR-42.7 Subcommittee, testing procedures and requirements for Category 6 connecting hardware have been developed to ensure very high transmission performance for 8-pin modular connectors. As mentioned earlier, Category 6 connecting hardware must be backward-compatible with existing Category 5 and Category 5e hardware, as well as interoperable with other Category 6 products.
Plug termination determines the performance of a Category 6 mated connection. A significant amount of crosstalk occurs between the blades of the plug; the highest level is between the 4-5/3-6 split pairs, where the 4-5 pins are completely contained within the magnetic field of the 3-6 pins. The jack can compensate for this configuration, so it is not the main problem. There is more of a problem with the method of terminating the wires to the back of the plug, which depends a great deal on the quality of workmanship.
As Category 6 crosstalk requirements are so strict, Category 6 plugs incorporate management bars to ensure consistent and repeatable performance. In order to ensure Category 6 performance and interoperability, terminations of patch cords and equipment cords must be performed in the factory.
Category 6 plug qualification specifications are quite detailed. To ensure consistency and repeatability between labs, it contains “test plug” magnitude and phase requirements. This is why these specifications have taken so long to develop.
Signal-to-noise ratio
The signal-to-noise ratio (SNR) is the difference between the received signal strength and the combined noise from all sources. Combination noise sources often include alien crosstalk, ILD noise, power-sum FEXT, and power-sum NEXT. Category 6 channels at 100 MHz have an SNR of 14.2 dB, while Category 5e channels have an SNR of 2.8 dB.
A cable with headroom over the standards is obviously advantageous to network owners, since it can withstand worse-than-average noise sources and still meet Category 6 specifications. In your particular environment, alien crosstalk may be several decibels louder than what is assumed in the standard. This scenario still allows a cable with a sufficient power-sum NEXT margin to meet the Category 6 specification.
The cable length is also de-rated for high-temperature environments. When the temperature is 40° Celsius, the cable length is de-rated by 6 meters, resulting in 84 meters instead of 90 meters.
Consider a connector with 180 MHz bandwidth whose performance has been degraded by 2 dB (realistic) or 4 dB (pessimistic) due to poor installation practices. As a result, the usable bandwidth is reduced below 200 MHz. An extra power-sum NEXT headroom can again compensate for a 2- or 4-dB loss due to installation practices.