Cryptonomicon

by Neal Stephenson

Phone company people tend to think (and do business) in terms of circuits. Hacker tourists, by contrast, tend to think in terms of bits per second. Converting between these two units of measurements is simple: on any modern phone system, the conversations are transmitted digitally, and the standard bit rate that is used for this purpose is 64 kbps. A circuit, then, in telephony jargon, amounts to a datastream of 64 kbps. Copper submarine cables of only a few decades ago could carry only a few dozen circuits — say, about 2,500 kbps total. The first generation of optical-fiber cables, by contrast, carries more than 1,000 times as much data — 280 Mbps of data per fiber pair. (Fibers always come in pairs. This practice seems obvious to a telephony person, who is in the business of setting up symmetrical two-way circuits, but makes no particular sense to a hacker tourist who tends to think in terms of one-way packet transmission. The split between these two ways of thinking runs very deep and accounts for much tumult in the telecom world, as will be explained later.) The second generation of optical-fiber cables carries 560 Mbps per fiber pair. FLAG and other third-generation systems will carry 5.3 Gbps per pair. Or, in the system of units typically used by phone company people, they will carry 60,000 circuits on each fiber pair. If you multiply 60,000 circuits times 64 kbps per circuit, you get a bit rate of only 3.84 Gbps, which leaves 1.46 Gbps unaccounted for. This bandwidt...