Introduction To High Speed PCB Backplane Design Considerations

Designers of high-speed backplane face several major challenges, such as signal attenuation, inter-symbol interference (ISI) and crosstalk. Chip products with innovative signal tuning technologies, such as high-speed backplane interface solutions, effectively address these system-level challenges, enabling system vendors to provide their customers with high-performance and scalable systems that reduce development time and cost.

In systems based on modular chassis, such as routers, Ethernet switches and storage subsystems, high-speed backplanes require high levels of signal integrity and higher system throughput. System vendors for these applications are facing a number of challenges in designing these high-speed backplanes in an economical and timely manner. They must also protect their customers’ investments in existing cable CARDS, cases and power supplies, while also supporting higher performance and providing newer services.

Today, backplanes in some systems are running on 5Gbps or faster serial link technology. To design highly reliable systems that can work at this rate, chip manufacturers are required to provide solutions that ensure error-free transmission in the backplane. This article describes the high-speed backplane based modular chassis system and its design challenges and discusses chip solutions that address these challenges.

An example of a system based on modular chassis

Modular chassis systems such as core routers, enterprise-class switches and storage subsystems all have high-speed backboards and multiple cable CARDS. System performance and capacity can be improved by adding more line CARDS and increasing the line card port density. These systems are modular and can be independently scaled. They are also designed to be highly available to ensure continuous operation.

These systems consist of slots containing redundant CARDS, line CARDS, and power modules.

They can be equipped with redundant components to increase the reliability and availability of the installation. Backplane interface solutions (also known as high-speed serial connections) provide full-duplex communication between high-speed backplanes. The speed of serial connected devices depends on the system throughput requirements. Serial connections transmit data via high-speed differential signals. This differential signal is then routed through the wire card and connector, through the backplane and through another set of high-density connectors. The channel characteristics depend on the backplane material, connector density, line width/coupling, etc.

In a typical router, these wires can range in length from 1 inch to 48 inches depending on where the wire card is inserted. The backplane interface devices in these modular chassis systems have the following key requirements:

1. Increase speed: Interface devices should be able to meet the increasing bandwidth requirements of system designers.Chip makers are currently selling 3.125-5Gbps rates and are offering samples of 6.25GBps solutions to upgrade existing backboards. Through simple switch card upgrade, system manufacturers can reuse existing cases and line CARDS, and provide a way to upgrade to higher bandwidth line CARDS, so as to provide customers with more services at a low cost;
2. Backward compatibility: The backplane interface device is required to work at the speed of the original line card in order to be compatible with the original line card;
3. High density and low power consumption: To cope with increasing network traffic, these systems require smaller footprint and higher performance and density without additional power consumption. Therefore, there is always a need for lower power consumption and faster backplane devices.
4. Manufacturability and testability: Backplane interface devices need to integrate functions such as JTAG and BIST to achieve chip level and system level testing during prototyping and manufacturing.