By Jim Gula, Product Manager, TE Connectivity Data & Devices
Cabled Backplane Systems: The
High-Speed Alternative to PCBs
Cabled backplane systems offer many benefits including high speeds, but this technology also comes with trade-offs.
As data center equipment speeds continue to increase, traditional FR4 printed circuit board
(PCB) substrates may no longer provide acceptable
transmission performance, especially at 25 Gbps and
beyond. Core switches and routers need maximum
throughput with superior signal integrity, and as these
systems achieve greater computational power, the
backplanes and number of daughter cards they support
become greater in size, number, and complexity. Many
equipment manufacturers are looking for connectivity
alternatives to PCB substrates, and high-speed cabled
backplane technology has emerged as a primary option.
In this article, we will examine the need for high-speed cabled backplane connectivity, its advantages
over PCB-based alternatives, potential drawbacks, and
key requirements for cabled backplane products that
can provide maximum flexibility and throughput with
exceptional signal integrity.
Why Cabled Backplane?
Cabled backplane technology has been in existence
for more than 10 years. Recent migration from the 10
Gbps backplane ecosystem to the 25 Gbps and beyond
backplane ecosystem is making cabled backplane
technology a more attractive solution for today’s system
architects. A cabled backplane can solve three issues: it
increases performance, enables low-loss communications
across long channels, and allows routing flexibility.
• Higher performance: Using high-speed cabled
backplanes significantly improves electrical
performance at 25 Gbps and beyond. Other than using
fiber-optic technology, the cabled approach is one of
the few alternatives for larger computing and switching
systems. Leading PCB fabrication suppliers have
developed HDI (high density interconnect) structures
to help mitigate trace routing challenges, but it takes
20-30 fabrication process steps to build these very high
layer count backplanes, and they are 5-10 times more
expensive than traditional PCB substrates.
• Low-loss communications: Loss budgets for an
entire channel are becoming tighter so designers need
to reduce the insertion loss as much as possible in
the physical connection. PCBs have insertion loss
associated with them. For example, typical Meg
6 PCBs have a loss of 0.75 dB/in at 12. 5 GHz. In
contrast, cabled backplane solutions can have a loss
of 0.11 dB/in. High-speed backplane cable allows
designers to maintain insertion loss, return loss, skew,
crosstalk, and other signal integrity attributes within
OEM performance specifications at 25 Gbps and
above. The reduced insertion loss achieved by the
high-speed cable also can allow signal integrity to be
maintained at distances two to four times greater than
a conventional PCB backplane design. This technology
is critically important as it can enable data channels of
three feet or more in full rack systems.
• Routing flexibility: With a cabled system,
interconnect manufacturers have the design flexibility
to offer the OEM a variety of system configurations.
The backplane configuration with daughter cards
mounted parallel to each other is one popular
approach. In addition, a midplane/orthogonal
configuration mounts cards in a 90-degree orientation.