Dependence on optical cabling for high data rate interconnects in data centers has been borne of existing electrical (copper) cabling's inability to provide a truly viable and interchangeable alternative.

The uptake of 10GBASE-CX4 has been low, mainly due to its cumbersome and proprietary connector sitting proud of the equipment face plate, but also a result of CX4's use of 8 pairs of twin-ax copper cable creating a cable with highly limited bend radius.

10GBASE-T, using CAT5 copper cable, employs the ubiquitous but non-modular RJ45 type connector. The connector is also mounted on the face of the equipment, requiring all cable driver and interface electronics to be placed on the line card. This inflexibility together with the high power consumption of 10GBASE-T electronics (currently 4W or more), make this option unattractive for high-density switches and routers.

Two technology trends in particular, however, are challenging the interconnect status quo and are set to make optical and electrical cabling perfectly interchangeable at data rates up to10G and beyond.

SFP+
The introduction of the SFP+ pluggable module standard has opened the way to more flexible and modular interconnect options. Small and generally low in complexity, SFP+ modules are already supporting a wealth of different datacom protocols: 1, 2, 4 and 8G Fibre Channel, 100M, 1G and 10G Ethernet.

Creating versatile ports for both optical and electrical interconnects, SFP+ modules slide into the equipment chassis, locate flush with the faceplate and make for a solution that is both elegant and robust. In the optical domain in particular, SFP+ has played an important role in optimising system design. 10GBASE-SR SFP+ modules for example are ably supporting multimode fibre in fibre-to-fibre patch cords (up to a few meters) or structured cabling (up to 300m.)

While adoption of the SFP+ form factor does help reduce module cost, the cost of the high performance electrical-optical / optical-electrical conversion remains however and serves to make the optical module comparatively expensive. In addition, fibre remains relatively easy to damage (compared to copper), either at the connector or within the link.

What SFP+ also does today though, is to support passive electrical interconnects, based on thin (30AWG or less) twin-ax copper cable, that are largely exempt from the problems of bend radius, and the termination and signal conversion challenges associated with optical fibre. While such passive copper cables have been shipped for sometime, their usage has been limited to lower data rates (up to 4Gbps) and at link distances that are intra-rack or between adjacent racks (a few metres).

To make SFP+ twin-ax copper interconnects function at 10G speed and over far greater distances however, industry needs to more fully embrace a second enabling technology trend:

Equalization and retiming
While twin-ax copper cable is good for data rates up to 4Gbps, at higher date rates such as 10Gbps, its frequency dependent losses actually restrict its use to less than a meter. Over longer distances significant distortion of the received signal occurs leading to high bit error rates. Note also that the loss per meter also depends on the thickness of the copper. Thinner cable incurs higher losses.

Enter equalization technology, which has the aim to simply compensate for the frequency dependent losses in order to recover an error-free data signal. It has two flavours: transmit equalization or pre-emphasis--this pre-distorts the transmit signal before it enters the cable in order to cancel out the cable losses; and receive equalization--to further compensate for cable loss.

At the same time, retiming of the signal is also paramount on both transmit and receive sides to ensure high quality equalization can be achieved. On the transmit side, retiming helps to deliver controllable and accurate pre-emphasis, while on the receive side retiming ensures the output signal of the equalizer is low in jitter.

Now embodied in miniature CMOS-based ICs in low power analog (rather than digital) implementations, modern equalization and retiming technology can today enable 10Gb Ethernet transmission over 10 meters of 30AWG twin-ax copper cable and more than 20 meters over the thicker 24AWG variety.

10G twin-ax copper cable assemblies
The marriage of SFP+ module, equalizer, retimer technology and twin-ax copper cable then presents a choice of cable assembly: passive or active. The former relies on the system (switch, router, NIC, HBA) to provide the cable drive capability, equalization and retiming. These functions are then located on the system PCB and are used even when an optical module is inserted in the SFP+ port. While the equalization function enables longer reaches with passive twin-ax cable assemblies, exact performance and maximum reach is limited by the need to guarantee system interoperability. As the performance of the link is dependent on both end-systems as well as the cable, a certain amount of margin has to be built in, i.e. when generally 10m links would be possible, network architects will only assume 5m capability to "stay on the safe side."

Active cable assemblies incorporate transmit and receive equalization functions in the SFP+ modules at each end of the cable. Factory setup will guarantee error-free operation of the assembly as provided. Also, since the equalizers will be located closer to the cable termination inside the module, active cable assemblies generally enable longer distances over thinner cable. While active cable assemblies will be slightly higher cost than passive ones, the link will be guaranteed and will remove any interoperability issues.

Conclusion
Standardization on SFP+ modules and the coming of age of equalizer and retimer technology means that system designers have a real opportunity to mix and match 10G optical and 10G electrical cables (passive or active) in the same system. Their complete interchangeability will mean cost, power and performance can be more readily balanced and optimized to suit the specific needs of datacenter design.

About the Author
Allard Van der Horst is Chief Technologist at Phyworks, a designer and manufacturer of high performance communication ICs. He previously held the positions of Director of Applications and Product Line Manager at the company and was Manager of IP Development at Fujitsu Microelectronics. He has an MSc in Electrical Engineering and Computer Science from the University of Twente (Netherlands).