Parallel vs Serial speed

[mark at buttery.org (Mark J. Dulcey)]

markw at mohawksoft.com wrote:
>> Date: Tue, 06 Feb 2007 21:46:22 -0500
>> From: David Kramer <david at thekramers.net>
>> markw at mohawksoft.com wrote:
>>
>>> In the above examples, we see that the drives perform similarly, but
>>> that
>>> the SATA interface gives an advantage in transfer speeds, but not the
>>> 100mb/s vs 300mb/s advertised difference.
>> Here's something I don't undertand.  My gut tells me that a parallel
>> interface, which sends all the bits at once, should be several times
>> faster than a serial interface, which must send one bit at a time.  So
>> how is SATA faster than IDE?
> 
> Well, obviously, parallel is "faster" than serial, with the caveat that
> serial can be faster if it does something that parallel can't.
> 
> Reciting from memory, I recall that SATA uses low voltage differential to
> communicate. This is MUCH faster and more immune to noise than the simple
> TTL logic level IDE communication.
> 
> Theoretically, an 8, 16, or 32 bit cable would be faster, but also more
> expensive to produce.

There isn't much point to going to parallel bits for SATA (yet); 
300MB/second is already faster than the drive mechanisms can go.

However, the parallel principle comes into play in PCI Express. PCIe x1 
is about the same speed as PCI; it's serial, but runs at a much higher 
bit rate than PCI's 33Mbps. PCIe x16, which is typically used for video 
cards, moves 16 bits in parallel, using 16 data lanes that are the same 
speed as PCIe x1; the result is that it's twice as fast as AGP 8X. 
(There is also an intermediate level, PCIe x4, that is used for some 
high-performance peripherals like disk controllers; it moves 4 bits at a 
time.)


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