The engineering community has been researching serial 10 Gb/s transmission across a backplane environment for the past ten years. One of the most important standards regarding high-speed backplane design has been recently ratified by key industry experts employing many engineering disciplines. Specifically, the IEEE 802.3ap Backplane Ethernet Task Force has spent three and a half years defining Ethernet operation over electrical backplanes up to 1 m in length. The Backplane Ethernet standard supports Gigabit Ethernet (1000BASE-KX) rates and 10 Gigabit Ethernet (10GBASE-KX4 and 10GBASE-KR) rates. Backplane designers are particularly interested in the 10GBASE-KR specification, which defines 10 Gb/s operation over a single differential pair.
The engineering effort to achieve ratification included efforts from each part of the backplane eco-system. Printed circuit board (PCB) material experts worked on introducing new lower loss materials, while connector experts introduced new connectors aimed at minimizing signal distortion due to loss, reflections, and crosstalk. PCB board manufacturers explored the use of these materials and connectors with various board design techniques aimed at minimizing plated through hole stub effects. Meanwhile, the test and measurement community focused on exploring frequency domain characterization and making accurate measurements.
Channel experts combined these efforts to improve the basic understanding of channel behavior and worked with the serial interface (serdes) design community to explore various equalization and signal processing schemes that might enable serial 10 Gb/s transmission. Prior to this collaboration, it was often necessary to make assumptions regarding various aspects of the channel or signaling conditions. The development of the 10GBASE-KR specification brought together all of these various parties, and allowed the industry as a whole to reach consensus on channel feasibility and system conditions for analysis. This provided serdes designers a defined problem with a given set of conditions. This allowed the development of an equalization strategy, and, ultimately, the signaling specification for 10GBASE-KR.
From a very simple perspective, the ability to effectively transmit a signal is related to the ratio of signal to interference that is presented to the receiver. During the development of the Backplane Ethernet specification, it became apparent that various assumptions being made were having an influence on the results of the analysis that the Task Force was using as the basis for the specification.
During the development of the 10GBASE-KR specification, it was demonstrated through analysis by various members of the Task Force that many challenges must be addressed in developing a system where any backplane channel can support transmitting a signal serially at 10 Gb/s. This article will review these scenarios in an attempt to familiarize the reader with the dangers of assumptions regarding system conditions. For system designers, it is imperative to understand the importance of clearly defining the system environment conditions. Otherwise, any incorrect assumptions made regarding the targeted system environment might invalidate the entire analysis and reduce the ability of the system to support serial 10 Gb/s across all targeted channels.
Characterizing Channel Conditions
Figure 1 provides a simplistic overview of the problem and solution space. A data stream, s (t), may be conditioned at the transmitter and / or equalized at the receiver to help overcome any limitations of the channel response Hc(f). These limitations, i.e. attenuation, reflections, and crosstalk, can be thought of as interference, as they essentially interfere with the detection of the output signal, r(t). Therefore, it is crucial to analyze the signal and all sources of interference.
