crosstalk, not wire length
Posted Oct 2, 2009 0:21 UTC (Fri) by gus3
In reply to: Log-structured file systems: There's one in every SSD
Parent article: Log-structured file systems: There's one in every SSD
> if 1 wire lets you transmit data at speed H, N wires will let you transmit data at a speed of NxH.
That is true, when the bus clock speed is slow enough to allow voltages and crosstalk between the wires to settle. However, as clock speeds approach 1GHz, crosstalk becomes a big problem.
> the problem with parallel buses at high speeds is that we have gotten fast enough that the timing has gotten short enough that the variation in the length of the wires ... and the speed of individual transistors varies enough to run up against the timing limits.
Wire length on a matched set of wires (whether it's cat5 with RJ-45 plugs, or a USB cable, IDE, SCSI, or even a VGA cable) has nothing to do with it. The switching speed on the transmission end can accomplish only so much, but there has to be some delay to allow the signal to settle onto the line. The culprit is the impedance present in even a single wire, that resists changes in current. The more wires there are in a bundle, the longer it takes the transmitted signal to settle across all the wires. By reducing the number of wires, the settling time goes down as well.
Related anecdote/urban legend: On the first day of a new incoming class, RAdm Grace Hopper would hold up a length of wire and ask how long it was. Most of the students would say "one foot", but the correct answer was "one nanosecond."
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