FireWire Is a Form of SCSIThis article was posted to Quadlist by Alex Timbol. It is
reprinted here with his permission. FireWire is a version of the SCSI-3 spec, which defines
both serial and parallel implementations. While it is true that the SCSI versions that run on
parallel busses transfer 8-bits (SCSI-1, SCSI-2), 16-bits
(SCSI-2 Wide), or 32-bits (SCSI-2 Ultrawide) per cycle, it
does not necessarily follow that it is faster than a high
performance serial implementation, such as FireWire or
FibreChannel. The main problem with parallel data bus implementations
is crosstalk among multiple data lines (8,16, or 32
lines). Crosstalk effectively limits the clock rate of the SCSI
bus as higher clock frequencies result in tighter coupling
among the signal lines, and consequently higher
crosstalk. Crosstalk is responsible for the bus length restrictions
and cable shielding requirements of conventional SCSI cable.
It is the brick wall that limits performance on existing
parallel data bus implementations. A serial bus does not have a problem with crosstalk
because there is only one data line. Without crosstalk as an issue, a serial bus can run at a
higher clock rate and take advantage of complex waveform
encoding -- allowing each wave cycle to carry more than one
bit of data by altering not only each pulse's amplitude, but
also its phase, frequency and zero crossover points -- over
relatively simpler and cheaper cabling. Another advantage of the serial bus is that interrupting
a single data stream is easier to do, allowing less costly
implementations of hot swap functionality. On parallel busses, hot swapping requires complex
buffering and expensive connectors such as SCA (used in high
end hard disks), which are designed to simultaneously break
all electrical connections to avoid damaging bus
components. In the early days of computing, parallel data busses,
which could move entire words (as opposed to bits) of data,
were preferred over serial busses to compensate for the slow
data rates of older bus controllers. With today's high speed
serial bus controllers, controller speed is no longer a
speed limiting issue. In the future it may be necessary to combine the benefits
of high speed bus controllers with the advantages of
parallel busses by using a crosstalk resistant medium such
as fiber optic cable, but at the moment, there is no reason
to do so, given the gigabyte data rates that have been
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