Using electronic dispersion compensation to equalize reflections in 8 Gb/s Fibre Channel design

High-performance communications for server-to-storage and server-to-server networking, video streaming, and high performance computing applications are propagating at an unprecedented rate creating a demand for more speed and higher bandwidth.

These requirements are driving blade and legacy board equipment vendors to constantly either design or upgrade products with higher bandwidths, as exhibited in the migration from 2G to 8G Fibre Channel (FC) solutions.

With the demand and density for FC increasing, a number of technical challenges arise in designing, prototyping, manufacturing, and maintenance of these systems. The need to upgrade existing legacy backplanes originally designed for lower data rates presents additional challenges in successfully attaining the performance margins that are required to deploy 8G FC.

Several vendors [1-3] have announced the availability of 8G Fibre Channel (FC) switches and host bus adaptors (HBA). The SERDES of 8G FC switches and HBA’s generally consist of a transmitter with a T spaced 3 tap finite impulse response (FIR) filter and a receiver with a variable gain input receiver and decision feedback equalizer (DFE) receiver. In general, the second tap of the transmit FIR filter is used as the main tap while the first and third taps are used to achieve pre- and post-cursor pre-emphasis, respectively.

The DFE based nonlinear receiver could be either fixed or adaptive and is effective in removing the intersymbol interference along with any signal degradations due to reflections. One of the other advantages of the DFE-based equalizer is its ability to converge quickly.

The typical convergence time can be on the order of mili-seconds (ms), which makes it an ideal solution to meet FC auto speed negotiation requirements. However, a combination of 3 tap FIR filter and DFE-based receiver may not have sufficient margin to equalize legacy backplanes at 8G data rates.

The "auto-negotiation" feature of Fibre Channel enables 8G FC switches and HBA’s to automatically sense and adapt to the data rate capability of connected legacy lower speed Fibre Channel link partner without user intervention. For example, when attaching to a 4G FC switch, an 8G FC HBA will automatically run at 4G FC. Per FC requirements, link partners need to achieve transmission word synchronization and stay error-free for 1000 words within 154 ms to pass auto speed negotiation and continue to normal operation.

This article describes the technical challenges and considerations in upgrading the Blade Center midplane to enable 8G FC solutions. The main steps taken to ensure a successful upgrade and system implementation of the electronic dispersion compensation (EDC) based Intergraded Circuits (IC’s) are discussed to ensure practical and successful system deployment.

Blade Center Chassis

The Blade Center backplane was originally designed for sub 2Gbps data rates [4]. The backplane consists of 14 blade slots, 4 switches, and other slots for supporting hot-swap capability. The Blade Center chassis has multiple versions such as E, H, S, and HT series.

Figure 1. Block diagram of a typical Blade Center Chassis channel.

The transmission line length from any switch transmit output to any receive blade adaptor input can greatly vary. The shortest transmission line on the backplane can be shorter than 1 inch while the longest channel could exceed 15 inches.

The short channels exhibit multi bit reflections and the longest transmission lines exhibit high frequency attenuations at 8Gbps data rate. A typical block diagram of the channel is provided in Figure 1 above. It should be noted that the EDC is not required at legacy FC rates such as 2G and 4G.