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The gain of RETURN LOSS

What makes Category 5e better than Category 5? The answer lies in Return Loss performance.


May 1, 2000  


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As you are likely aware, the TIA standard for Category 5e cabling was published in January as ANSI/TIA/EIA 568-A-5 (Addendum No. 5). The Category 5e Addendum is a comprehensive document that contains many new requirements, including ten Annexes with specific measurement procedures and modeling information.

Perhaps it is timely then to examine what makes Category 5e so much better than Category 5. It is still a 100 MHz cabling system, but it ties together all the loose ends and fills in the gaps that are present within the existing Category 5 standard. The biggest difference is the Return Loss performance for a channel and for individual components. What is it and why is it so important for digital networks?

RETURN LOSS DEFINED

Return Loss is a new transmission parameter for Category 5e. It is a measure of signal reflections. When a signal is launched at one end of a channel, it propagates along the cabling system. Whenever it reaches a boundary between different components, some of the signal energy is reflected back to the source. The level of signal reflections depends on the mismatch in impedance between the components that make up a channel, including cords, cables and connectors.

When all of the signal reflections are added up, the total reflected signal power, as measured at the source, is called Return Loss. A lower value of Return Loss in dB means that a higher percentage of the transmitted signal is reflected back to the source. In an ideal channel, all of the components will be designed to have the same impedance. In practice, the impedance will vary depending on design and manufacturing tolerances.

The current Category 5 standard has a fairly loose requirement for the impedance tolerance of cables and cords, namely 100 Ohms +/- 15 Ohms. As a result, significant reflections can occur that can interfere with the operation of digital systems.

Below, is a simple table that relates the impedance mismatch at the boundary between components to the magnitude of the reflected signal power (%), the Return Loss in (dB) and the mismatch loss (dB).

Although the reflected signal power and the mismatch loss may appear to be small in Table 1, it should be noted that the signal reflections and losses accumulate as the components are added in series. But that is only part of the story. These signal reflections are also re-reflected at boundaries between different components and are added back in (superimposed) on top of the primary signal. This superimposed signal appears as a noise source at the receiver and can contribute to bit errors in digital systems. This type of noise is attributed to insertion loss deviation (ILD), which is a parameter that is specified in the TP-PMD physical layer standard that is referenced by IEEE for fast Ethernet (100BASE-TX) applications.

The Return Loss requirements for Category 5e components were developed in the TIA Copper Systems subcommittee (TR 42.7) and were based on a worst case channel model with four connectors. In order to meet these requirements, it was determined that the impedance mismatch difference between cables and cords had to be maintained to within approximately 10 Ohms. From the values shown in Table 1, the total reflected signal power for a Category 5e channel can be up to six times less than a channel that uses Category 5 components at the extreme end of the tolerance range.

NETWORK PERFORMANCE ISSUES

Network managers are concerned about the reliability and performance of their data networks, and the loss of a network or a slowdown in performance can be very costly. Network performance depends on the ability of the network cabling to support data rates of 100 Mb/s today and 1000 Mb/s or higher tomorrow.

An important question to ask is: “Does my cabling system have the built-in performance margin to deliver this information without stumbling?” One measure of network performance is the bit error rate (BER). Component impedance mismatch can be a major contributor to bit errors and can cause some Category 5 channels to exceed the BER objective for 100BASE-TX applications.

Some recent work confirms that the new TIA standard for Category 5e satisfactorily addresses the issue of component mismatch that can lead to bit errors.

FUTURE STANDARDS

It is intended that Category 5e will become the base performance level for all new data cabling installations in the future. The next revision of the TIA/EIA 568-B.1 standard, which is scheduled to be published at the end of this year, will only recognize minimum Category 5e performance for the second outlet (intended for data).CS

TABLE 1 – REFLECTED SIGNAL POWER, RETURN LOSS AND MISMATCH LOSS AS A FUNCTION OF IMPEDANCE MISMATCH
Impedance Reflected Return Mismatch
Difference Signal Power Loss Loss
(Ohms) (%) (dB) (dB)
25 1.56% 18.1 0.068
20 1.00% 20.0 0.044
15 0.56% 22.5 0.024
10 0.25% 26.0 0.011
5 0.06% 32.0 0.003
0 0.00% >50 <0.001

Paul Kish is a senior Product Manager, IBDN Systems & Standards at NORDX/CDT, Pointe Claire, PQ. He is also Chair of the TR-42 engineering committee.

Disclaimer: The information presented is the author’s view and is not official TIA correspondence.


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