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The Bottom Line on Performance

A key factor that determines if a network is running the way it should do is the Signal-to-Noise Ratio at either end of a cabling system.

April 1, 2004  

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It’s time to shed light on an extremely confusing topic, namely how best to understand cabling performance.

I recently attend-ed a meeting where the performance of different systems was being compared on a bar chart. The height of each bar represented how much better the performance was compared to the TIA standard.

All the parameters were lined up like different size candles for NEXT, PSNEXT, ELFEXT, PSELFEXT, ACR, PSACR, Return Loss, and Insertion Loss. But, how do you differentiate one system from another?

If System A has 2 dB better PSNEXT, but 5% worse Insertion Loss than System B, which is better? Is one parameter more significant than the other?

Fortunately, there is a way of looking at cabling performance to make sense of this alphabet soup. The bottom line is that it needs to be related to the performance of your network.

Ongoing discussions on the re-quirements for the next generation of 10 Gig Ethernet brings this to the forefront. Some parameters have a greater impact than others on network performance.

A key factor is the Signal-to-Noise Ratio at the receiver (the network interface) at either end of the cabling system. It is desirable to have a strong signal and a weak noise at the receiver.

Each of the cabling parameters affects either the signal strength or the noise measured at the receiver.

To establish the bottom line per-formance we first look at the signal. The signal strength is reduced by the Insertion Loss of a channel. A low Insertion Loss (measured in dB) yields a stronger signal at the receiver. As an example, a 10 dB Insertion Loss reduces the signal power at the receiver by a factor of 10.

Think balance sheet

As you can see, it is an important parameter. The signal power after accounting for Insertion Loss is the equivalent of the net revenue line on a balance sheet.

On the other side of the ledger we have all the different noise sources. These correspond to the expense lines.

We can understand the different noise sources as follows: Near End Crosstalk (NEXT) is the noise generated within the cabling system due to the near end transmitters.

Far End Crosstalk (FEXT) is the noise generated within the cabling system due to the far end transmitters. Return Loss is the noise generated due to signal reflections within the cabling system.

All these noise sources can be added using a channel model on a Voltage Sum or a Power Sum basis.

It we subtract the total noise Power from the total Signal Power we get the bottom line performance, which is the Signal-to-Noise Ratio. One such measure of bottom line performance is the Power Sum Attenuation-to-Crosstalk Ratio (PSACR).

The PSACR only accounts for the total of the near end crosstalk noise, which is usually the dominant noise source at the receiver.

The frequency range where the PSACR is greater than zero, has been used to define the available Bandwidth of the cabling system. The above is a good way to look at cabling performance and to make sense of all the different numbers. Another point to keep in mind, especially for the noise side of the ledger, is that some of these noise sources need to be discounted.

For 10 Gigabit Ethernet the following amount of noise cancellation is proposed: echo cancellation (55 dB), NEXT cancellation (40 dB) and FEXT cancellation (25 dB).

With all of the sophisticated electronics there should be nothing to worry about, right? Not quite. If we cancel out the noise sources within the cabling system, the noise that remains is external.

One of these external noise sources is the Alien Crosstalk between adjacent cables that are in close proximity.

What is new and exciting in the industry is that 10 Gig Ethernet has spurred the development of an augmented Category 6 cabling system with an improved Alien Crosstalk performance and an extended frequency range.

The specification for augmented Category 6 cabling is under development as a project in the TIA TR 42.7 subcommittee. The objective is to support a 100 meter channel distance in a worst case 4-connector channel configuration.

Paul Kish is Director, IBDN Systems & Standards at NORDX/CDT. He is also vice 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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