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Serial SCSI Standard Coming Soon
rchatterjee writes "SCSI is very close to joining ATA in leaving a parallel interface design behind in favor of serial one. Serial attached SCSI, as the standard will be known, is expected to be ratified sometime in the second quarter of this year according to this article at Computerworld. Hard drive manufacturers Seagate and Maxtor have already said that they will have drives conforming to the new standard shipping by the end of the year. The new standard will shatter the current SCSI throughput limit of 320 megabit/sec with a starting maximum throughput of 3 gigabit/sec. But before this thread turns into a SCSI fanboy vs. ATA fanboy flame war this other article states that Serial Attached SCSI will be compatible with SATA drives so you can have the best of both worlds."
320 megabytes is about 2.5 gigabits ... which is a lot closer to 3 gigabits than the erroneous 320 megabits figure.
For more info on Serial attached SCSI check out this page:
n d.html
http://www.lsilogic.com/products/islands/sas_isla
To reduce crosstalk between the wires so that you can run at faster speeds. Indeed, the "rounded" IDE cables often reduce performance by 5% or so. We're getting better at data throughput though, so we can use serial technologies and actually get faster transfer rates. Good riddance to ribbon cables :P
>Why not simply roll the ribbons up into cables?
Impedance, crosstalk (mentioned) and price.
It takes seconds to crimp a ribbon cable. Cheap and easy. You can even do it yourself!
Taking a bunch of twisted pair wires (which is what would be required to keep the impedance and crosstalk bearable) and soldering them onto connectors individually takes a lot more effort, and therefore costs more.
Not to mention fabbing individual strands of insulated wire and twisting them together costs more than running 5 wires parallel to each other and simply coating them all at the same time with PVC.
If you could be told what you can see or read, then it follows that you could be told what to say or think - BoC
I don't think so. The reason there is a tremendous push towards serial right now is because parallel interfaces create more interference at higher frequencies. The theory with serial is that you can push the frequency as high as you want without the interference.
--
"What do you want me to do? Whack a guy? Off a guy? Whack off a guy? Cause I'm married."
You're mistaken. It's Small Computer System Interface. See dictionary.com
SCSI: Small Computer Systems Interface
descended from
SASI: Shugart & Associates Systems Interface
STOP . AMERICA . NOW
With PCI-X 1066 8.6GB/s bus tranfers are possible so that should be too much of a problem. Also, the InfiniBand aims to solve that problem. One can see that 6GB bus' were planned even in this older dell whitepaper suggests.
Firewire is low end consumer product...even with its successor (which is taking longer than expected to ship) running at 800Mbits/s (100 Megabytes/second) it falls short of current SCSI technology running @ 320MB/s. As such there is no one who would seriously consider firewire for a large scale server handling many gigabytes/terabytes of data. Firewire is just too slow of a bus for big needs, but does fills its convenience needs in the consumer market. Everything has it's own niche... that's why heavily marked up servers/mainframes/supercomputers still exist instead of cheaper home machines which just can't fill the requirements.
There are a couple important notes about Serial-attached SCSI (SAS) that I think are important.
First, SAS uses a point-to-point topology similar to Serial-ATA instead of a shared bus like SCSI. This means each drive has access to full bandwidth, not just one (the bottleneck being the card itself).
Second, according to the SAS working group, SAS comes in three speeds; 150, 300 and 600 MB/s. I'm not sure where that 3 Gbps figure came from.
Third, unlike Serial-ATA or parallel SCSI, SAS is full duplex like fibre channel. This should have some interesting effects on latency.
Fourth, SAS uses the same physical connector as Serial-ATA and in fact can use Serial-ATA drives in legacy mode.
The world is neither black nor white nor good nor evil, only many shades of CowboyNeal.
Serial Streaming Architecture that is. It's a 40MBps serial hardware layer that runs SCSI protocal. It's configured in a loop so there's automatic redundancy in case one link gets disconnected. And a single segment can be up to 20 meters long. Anyone have a Shark (ESS)? It's all SSA inside.
coward
There is a good article here
_ 03 -03-03.asp?article_id=211
http://www.snwonline.com/whats_new/sas_and_sata
The article states that the SAS drives won't work on a SATA channel, but SATA drive will on the SAS.
I wonder if mobo makers like ASUS, ABIT, MSI and the likes will choose to have SAS ships on the mobo instead of SATA, as a performance feature?
Lets hope so it would sure open a lot of option for upgrading a PC over time.
FYI - signals do not travel at the speed of light. Somewhere around 50-60% the speed of light in most types of cable.
Overcoming the differences in arrival time of signals in a parallel cable is not significantly more difficult than handling clocking (and maybe clock recovery) and buffering and serial-to-parallel conversion on a serial interface.
The main reason that parallel interfaces were popular years ago when things like SCSI were established was the electronics at the time just weren't very fast. The 74LS00 family logic that SCSI and parallel printer ports were designed around had a maximum clock rate of about 30Mhz. Add in margin for cable noise and distortion and 5-10Mhz was absolutely the most you could manage through any distance. So, if that wasn't fast enough for what you wanted to do, you used more wires in parallel.
These days, it's relatively easy to put multi-gigahertz logic onto chips, and the fewer wires in a cable and connector, the cheaper, so serial wins.
One detail is that SAS is now point to point, just like SATA, and not a bus, but they also indicate that there would be boxes to split a single connection to a bunch of devices, sort of like network hubs. The protocol addresses 128 devices. It isn't clear whether a hub could have SATA devices hooked to it, or if that would require 1 serial channel per device from the host adapter. That is what I understood to be the case for SATA (need one port for each device, no hubs or sharing). The most important protocol difference should be that SAS is still multipoint, even if the connections are point-to-point, so both hosts and adapters need to arbitrate for the bus, while SATA hosts adapters just send out commands and data and wait for the drive to respond on the reverse channel.
It wouldn't surprise me if devices eventually just supported both protocols, and maybe even auto-sensed the type of adapter on the other end. By the time these interfaces get common, I expect the cost differences to be negligible, so It begs the question of why SATA would survive. Because the cost differences are going to be sunk into the chipset designs with almost no marginal cost differences, both system and drive makers will probably save more by reducing the size of their product lines by having one product for both.
Contrary to popular beleif the SATA cables are approx 1mm thick, 6-7.5mm wide and quite "awkward" to work with :(
:(
:(
I for one will be doing my best to hunt down a supplier which makes precise lengths so I can have mine cut to size as they aren't as easy to route as a ribbon cable (seriously!)
Plus if you have 6 devices that's SIX cables in the box instead of 3,... - one of the small shortcomings of SATa
(when I first heard about it, I was under the impression it dasiy chained with an "in" and an "out" port - boy did I think that was FANTASTIC... but I was sorely disapointed when I discovered I was incorrect)