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Advanced Interconnect Standards
Blend Serial and Parallel Techniques
for Best Performance and Scalability

Mario Cavalli
General Manager
HyperTransport Consortium

Executive Summary
As modern computer architectures have pushed processor clock frequencies to 4 GHz, advanced
I/O link technologies have embraced a number of new technologies and approaches to enable I/O
data streams to keep up with processor speeds. It has become commonplace in the industry to think
that parallel buses are old, slow and limited in bandwidth, and serial buses are newer, faster and
enable higher bandwidth capabilities. But a new breed of interconnect technologies featuring the
best of both serial and parallel bus features have emerged. This paper compares and contrasts
serial and parallel bus characteristics and describes how these characteristics have come together
in the HyperTransport and PCI Express interconnects.

Comparing Parallel and Serial Bus Characteristics
In the past, common on-board computer bus architectures were purely parallel and off-board
communication links were more often serial, better suited for signal shielding and long haul data
transmission. The PCI (Peripheral Component Interconnect) bus, for example