By Dr. Eric Bogatin
www.BeTheSignal.com
December 2007
“The reports of my death are greatly exaggerated,” Mark Twain once replied to a reporter’s question. Many of us involved in technology development have borrowed this phrase more than once, especially as it applies to the use of copper-based interconnects.
The attenuation of copper cables increases with the square root of frequency and with cable length. As signal bandwidths increase, the maximum usable cable length for copper interconnects will only shrink. It would seem to be just a matter of time before the usable length of copper interconnects is too short to be useful.
Optical interconnects have been beating on the door of copper for the last ten years. While optical has clearly won for long haul interconnects, and copper is the interconnect of choice for on-chip, system-in-package, and daughtercards, the battle rages on for backplanes and short-haul cables in the 2 to 30 meter range.
In server farms, there are hundreds of individual server boxes on top of each other or server blades plugged into backplanes. Their combined performance is no longer limited by the computation speed of the processors. Rather, it is limited by either getting the heat out of the room or by the wiring density of the cables interconnecting the servers and backplanes.
Cables in wiring harnesses are not only heavy and take up a lot of room, but also restrict airflow in the overhead or under-floor plenums that share space with the cooling system. One approach to reducing the size and weight of the cable bundles is to use smaller diameter wire. However, this only increases the copper losses and decreases the maximum allowable run length.
Copper losses increase with the square root of frequency due to the current re-distribution from skin depth effects. Though a 24-gauge signal cable has a physical diameter of 500 microns, frequency components of the signal above 1 GHz only travel in the very outer 2 microns or less of the wire’s circumference. Higher frequency components see a higher resistance and more attenuation.
The removal of higher frequency components by the losses in the cable increases the rise-time degradation of the signal and causes inter-symbol interference (ISI), which results in collapse of the eye and deterministic jitter. At 5 GHz, the attenuation in 24 gauge cables is about 2 dB/m. This limits the 10 Gbps usable length of cables to about 5 meters before the bit error rate is too high.
Using larger diameter wire reduces the losses and enables higher bandwidth signals, but increases the strain on the physical plant. It would seem that copper cables might loose the battle against thin, light-weight optical cables in the server farms.
Of course, this is the same problem backplane engineers have faced. In a backplane, the effective wire diameter is ten times less than in the server farm, but the backplane run lengths are ten times shorter. The popular solution to overcome interconnect limitations in backplanes is to use the signal processing techniques of pre-emphasis or de-emphasis in the transmitter (TX) and equalization in the receiver (RX). These techniques have enabled 10-Gbps copper backplanes.
A similar approach is now available to increase the bandwidth of copper cables. While it’s difficult to retrofit new TX or RX technology in the servers or blades installed in existing facilities, it’s easy to just replace the cables with smart cables.
The introduction of new, active connector technologies from companies such as Quellan, Inc. may breathe new life back into copper for high bandwidth, short-haul applications.
The form factor for this new generation of smart connector is the same as passive cables, but embedded silicon chips process the signal for higher performance. Figure 1 shows a MicroGiga smart connector from Quellan, Inc. with embedded analog circuitry for active equalization.
Figure 1. MicroGiga Smart Connector (Photo Courtesy of Quellan, Inc.)
“Our connector is protocol-agnostic because we use analog circuitry,” Gourgen Oganessyan, strategic marketing manager with Quellan says. The analog circuits can equalize signals up to 10 Gbps and recover usable signals when the attenuation is as high as -30 dB. The goal of a smart connector, according to Oganessyan, is to make active cabling behave like passive cabling, but with better transmission quality.
In a typical server farm, rack-to-rack cables have a distribution in run lengths, which, according to Oganessyan, peaks at about 2-3 meters and still has significant numbers as long as 10-20 meters.
“As a rough rule of thumb, our smart connectors can increase the usable length by 3x.” Oganessyan said. “This means being able to use 5-20 meter long cables up to 10 Gbps or the same length cables but thinner gauge for a size and weight reduction in the wiring harnesses.”
There are fundamental limits to the attenuation of high bandwidth signals in copper conductors. It is inevitable that as signal bit rates move up in their treadmill-like advance, we will approach the boundary of copper’s limits. With the introduction of smart connectors with embedded silicon signal processing, the battle for the server farm between copper and optical might be pushed out for at least another generation.
This and other signal integrity topics are covered in Eric’s public classes and online lectures, available from his website, www.BeTheSignal.com. Send your signal integrity technical questions to DoctorIsIn@BeTheSignal.com.
Bio: Eric is president of Bogatin Enterprises, whose mission is to set the standard for signal integrity training. He is the author of Signal Integrity - Simplified, published by Prentice Hall. Check out his public signal integrity classes posted on www.BeTheSignal.com. He can be reached at eric@BeTheSignal.com.
