This 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 achieved by high speed serial implementations.
