Connections +
Feature

‘Copper is still going strong’

The pending ratification of 40 Gbps and 100 Gbps Ethernet over fiber will not spell the end of the line for copper cabling. Not by a long shot.


March 1, 2010  


Print this page

For this issue’s article I wanted to look at the capability of balanced twisted pair cabling to support the next generation of Ethernet.

The IEEE 802.3ba standard for 40 Gbps and 100 Gbps Ethernet over fiber is expected to be ratified in June of this year. Is there a place for copper to support these higher data rates? Is 10Gbps over Category 6A cabling the end of the road for copper?

I remember in 1995 when the copper skeptics predicted that Category 5 and 100 Mbps was the limit of what copper can do and that the future was to be dominated by fiber. Here we are 15 years later and copper is still going strong supporting data rates 100 times faster than originally envisaged.

In this timeframe, we have seen the evolution of copper from Category 5 to Category 5e to Category 6 to Category 6A.

In addition, multimode optical fiber has also evolved from 62.5/125 µm OM1 to 50/125 µm OM2 to laser optimized 50/125 µm OM3 and OM4 fibers.

In order to answer these questions we need to look at the fundamentals. Let’s start by examining the technology and channel requirements for different generations of Ethernet and from these project what is needed to support 40 Gbps over copper.

Table 1 shows the data rate, the encoding technology, the symbol rate, the channel bandwidth and the channel signal-to-noise ratio (SNR) for each generation of Ethernet. Also shown (highlighted in green) are two possible scenarios for a 40 Gbps Ethernet implementation.

The first scenario uses the same encoding scheme as 10GBASE-T Ethernet using PAM 16/DSQ 128 encoding. The minimum bandwidth to support this data rate is 1600 MHz with a SNR of 26 dB.

The second scenario uses PAM 32/DSQ 512 encoding, which reduces the bandwidth requirements to 1200 MHz. However, the SNR requirements are increased by 6 dB.

See Table 1.

Note: The SNR values shown in Table 1 include a 3 dB safety margin.

What is the performance of a balance twisted-pair channel for a bandwidth of up to 1600 MHz? The key performance parameters are shown in Table 2.

The most significant parameter is the channel Insertion Loss. In developing this Table, I have used the equations for the Insertion Loss of a Category 6A channel extrapolated up to a frequency of 1600 MHz.

The Insertion Loss for a 100-meter channel at 1600 MHz is 94.9 dB. This is a very weak signal indeed and is close to the measurement noise floor at these high frequencies. If we reduce the channel distance to 50 metres, The Insertion Loss is comparable to the result obtained for a 100 metre channel at 400 MHz.

In Table 2, I also calculated the total noise taking into account some cabling performance enhancements and internal noise cancellation, including 60 dB of echo cancellation, 40 dB of NEXT cancellation and 20 dB of FEXT cancellation. The net result is a positive signal-to-noise ratio extending up to 1600 MHz for a channel of 50 meters.

See Table 2.

The Shannon capacity for the 50-metre channel shown in Table 2 is between 50 Gbps to 56 Gbps for different scenarios

So in conclusion, yes, Enhanced Category 6A copper cabling can support 40 Gbps data rates for a channel distance of up to 50 metres. It is not the end of the road yet. It does require better performing cabling that is specified up to a frequency of 1600 MHz.

This is a challenge that cabling vendors will meet. Since 40 Gbps data rates would be primarily targeted for switch to server connections in a data centre, the 50 metre distance is not a limitation for most data centre topologies. CNS

Paul Kish is Director, Systems and Standards at Belden. The information presented is the author’s view and is not official TIA correspondence.