
D. A. B. Miller and H. M. Özaktas, "Limit
to the BitRate Capacity of Electrical Interconnects from the Aspect Ratio of the System
Architecture," Special Issue on Parallel Computing with Optical Interconnects, J.
Parallel and Distributed Computing 41, 4252 (1997). We show that there is
a limit to the total number of bits per second, B, of information that can flow in a
simple digital electrical interconnection that is set only by the ratio of the length l of
the interconnection to the total crosssectional dimension square root A of the
interconnect wiringthe "aspect ratio" of the interconnection. This limit is
largely independent of the details of the design of the electrical lines. The limit is
approximately B\B/sub 0/A/l/sup 2/ bits/s, with B/sub 0/\10/sup 15/ (bit/s) for
highperformance strip lines and cables, \10/sup 16/ for small onchip lines, and \10/sup
17/10/sup 18/ for equalized lines. Because the limit is scaleinvariant, neither growing
nor shrinking the system substantially changes the limit. Exceeding this limit requires
techniques such as repeatering, coding, and multilevel modulation. Such a limit will
become a problem as machines approach Tb/s information bandwidths. The limit will
particularly affect architectures in which one processor must talk reasonably directly
with many others. We argue that optical interconnects can solve this problem since they
avoid the resistive loss physics that gives this limit
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