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Game-Changers: Advanced Copper Cabling and 25Gb/s Speed

By Contributed Article | June 10, 2013

Many firms consider copper cable assemblies, as recent advances make them viable options for high-speed applications.

Data center managers face the constant challenges of maintaining cabling and system power, as well as keeping data centers from overheating. Simultaneously, rising bandwidth demands force the deployment of solutions to continuously increase transmission rates.

To address all the challenges, many firms are considering copper cable assemblies, as recent advances make them viable options for high-speed applications. Many designers prefer copper interconnects to develop their channel design, simulation, and board layout expertise.

How far the bandwidth can be pushed is unknown, but 25 Gb/s channels offer potential game-changing solutions. High-speed copper products also deliver higher bandwidth compared to traditional copper at a lower cost compared to optical connectivity.

The Copper Connectivity-Optical Fiber Combination

While optical fiber is popular for long-distance network communications, copper connectivity offers cost-effective solutions for short-to-midrange applications. For designers, optical media also poses intrinsic challenges, including thermal and power management.

In contrast, passive copper cables fill the demand for low-cost, low-power, short-range (1-7m) applications and are often the best solution for stand-alone systems. Approximately 90% of applications are ≤5 meters, which includes most data connections in high-performance computing systems.

Network system data centers frequently blend optical modules and copper cable assemblies; savvy designers can readily integrate and optimize systems to leverage the strengths of each. High-speed passive 25 Gb/s copper channels also bring a competitive cost and performance solution to data center cabling — including top-of-rack, middle-of-rack, and connections to storage servers.

Increasing Data Center Capacity within the Same Footprint

High-density cables must retrieve and transmit data from storage rapidly to meet variable and peak demand. But current systems are limited by the front-panel I/O bandwidth density. One way to provide greater data rates is to increase the number of channels; however, front panels are already crowded, so adding additional ports is difficult.

Another way to expand bandwidth is increasing the data-rate of each channel, which enables data centers to grow capacity without additional floor and rack space. Increasing the data rate of each channel also increases port density at the panel.

A passive copper data rate for a single high-speed link of 10Gb/s requires 10 lanes to achieve an effective 100Gb/s data rate. Increasing the rate of each lane to 25Gb/s thus reduces the lane requirement from 10 to four lanes, saving valuable real estate and creating a higher-density front panel I/O.

All aspects of the channel should be assessed for variation across time, process, and temperature. The challenge of interoperable, high-speed electrical performance thus increases exponentially with a linear increase in speed. While 10Gb/s channels allow for margins of error in specifications, interoperability at 25Gb/s does not afford the same buffer, taking the challenge of universal interoperability to new levels.

Increasing Bandwidth to New Levels

In recent years, significant improvements have occurred in connectors, paddle card designs, connector launches, conductor terminations, and raw cables. 100Gb/s copper assemblies from many leading manufacturers integrate design improvements in almost every critical area. For example, some small form-factor, pluggable I/O systems now offer eight lanes at 25Gb/s.

As Figure 1 shows, the differential insertion loss of next-generation 25 Gb/s copper 5m cable assemblies is better than current 10 Gb/s copper 3m cable. Next-generation cable also offers improvements in crosstalk, with approximately 50% less noise than current 3m cable assemblies.

Reducing Crosstalk Noise

Integrated Crosstalk Noise (ICN) shows noise levels in high-speed 5m next-generation cables are approximately 50% less than 3m current-generation counterparts (see Figure 2). Improved ICN leads to improved signal-to-noise ratio, which improves interconnect signal integrity. This gives designers flexibility that is unavailable in current systems.

Passive copper cables use considerably less power than optical modules. At

Passive copper may also reduce the price-per-link by a factor of >3 compared to optical modules and fiber optic cables at ≤7m lengths. In terms of reliability, passive copper cable contains only one electronic component (a single EEPROM) and hence can provide MTBFs 10-50 times longer than an optical cable assembly.

Design Considerations for 25 Gb/s Copper Channel Connections

Tradeoffs are inherent in connectivity selection and design, whether optical fiber, copper transmission, or a combination. Although fiber optics is favored for long runs, copper remains the preferred solution for shorter cable-length applications. High-speed 100Gb bandwidth copper makes it an even more attractive alternative.

Key considerations for designers include high-frequency insertion loss, noise, and the SNR margin. Some component vendors provide designers with accurate and predictive component-level models and end-to-end channel models that balance between price and performance while advanced products reduce or eliminate noise as well as fluctuation even further.

Manufacturers recognize the important role of copper and how advanced signal conditioning extends the serviceable life of components while also stimulating the development of next-generation copper interfaces for large-volume production. Copper cables are thus expected to reach the broader market over the next three to five years.

Total interoperability will require proper component specification, clarity of defined and accepted compliance methods, and continued collaboration across all aspects of implementations. Reaching these goals is important since the proliferation of cloud, high-performance computing applications, server virtualization, and converged networking — along with improvements in processing — continue to drive demand for higher-bandwidth server connections. 25 Gb/s copper channels are primed to meet these cost- and energy-driven demands.

By Mark Bugg, Project Engineer, Molex Incorporated

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