Cut to Ribbons

What do PCs, pigtails and old oak trees have in common? All are places you’re likely to find ribbons, but an emerging technology could soon cross PCs off that list.

Parallel advanced technology attachment (ATA) has been the interface protocol of choice for desktops and laptops for about 20 years.

Also known as IDE (integrated drive electronics), the storage device interconnect owes its longevity to its simplicity, low cost and frequent speed improvements. But according to analysts, the five-centimetre-wide 40-pin ribbon’s days are numbered.

“”There have been more and more demands on the rest of the system,”” says David Freund, a server technology and storage analyst with Nashua, N.H.-based Illuminata Inc.

“”Parallel buses have kind of run out of gas. It’s harder and harder to get parallel signalling to work at higher and higher speeds as all the other parts of the computer have been speeding up.””

Freund says the challenge was how to increase the number of bits per second throughput on cables that supported only about 4 MB/s at the beginning.

Not that this was the only issue. Other problems included the five-volt signalling spec, high pin count (40), cable length limitation (40 cm) and size and data integrity issues.

Voltage requirements have been tumbling, according to Freund, to the point where most of the internal electronics now operate in the millivolt range. This disparity made supporting the higher voltage diff-icult.

As for the ribbon, Freund says it created two problems. Its limited length restricted PC design — drives had to be within 40 cm of the motherboard — and the sheer size of the cable constricted the passage of air, with an impact on internal temperatures. “”It does interfere with flow of air through a machine, which in the more powerful (machines) matters,”” he says.

Forrester Research Inc. analyst Bob Zimmerman agrees.

“”This is especially important when we’re thinking about a bladed system, as opposed to a desktop system,”” Zimmerman says. The amount of cabling and the heat generated in an environment with a number of blade servers in a 19-inch rack array is a recipe for disaster.

“”If the cabling is blocking the air flow you’re going to have one great mass of melted plastic and wire.””

Not surprisingly, the SerialATA Workgroup says serial ATA is the answer to these issues.

According to specs from the group, comprised of Dell, Maxtor Corp., Intel Corp., APT Technologies Inc. and Seagate Technology, the new standard

supports a cable up to one metre long, uses four pins, operates at 250 millivolts, has thinner, more flexible cabling, features cyclic redundancy check error-detection mechanism and has a 10-year roadmap for performance improvement.

The transfer rates for serial ATA start at 150 MB/s — not a huge improvement over parallel’s 133 MB/s — but the third generation should reach 600 MB/s.

While the benefits at the desktop level are obvious, Zimmerman and Freund say serial ATA will find its way into an enterprise’s back end in servers and storage.

“”I see it as a secondary disc. By that I mean the primary disk array maybe fibre channel-based, (or) it may be serial attached SCSI, where the bulk of those are slightly faster,”” Zimmerman says. “”I use it for data that I can usually recreate from some other source.””

Freund says IT managers know not all data has the same priority. You can probably wait for a few milliseconds for archived e-mail, he says, as opposed to online transaction processing. “”ATA is leaving the desktop — that’s a foregone conclusion,”” Freund adds. “”It’s going to creep further into the infrastructure, not unlike commodity computers have.””

Second-generation serial ATA is expected to incorporate a number of features — enclosure management and support for hot-swapping, to name two — that will drive it further into the back office. Freund says command queuing is near the top of that list.

“”You have no idea where on that physical media the data is. So instead of issuing a command, waiting, getting the data, issuing the next command, waiting,”” Freund says, “”you can issue a bunch of commands and let the drive sort out how to get them done the fastest.””

Native command queuing in Serial ATA allows a drive to take a series of commands, re-order them intelligently, then re-submit them to the host for more efficient execution. Serial ATA command queuing can handle up to 32 outstanding commands. No one views it, however, as a challenger to SCSI and fibre channel at this point.

“”A sweet spot for this technology is entry-level servers and even low-end RAID (redundant array of independent disks),”” Freund says. “”More enterprise-capable features are appearing

in serial ATA drives.”” But, he says, they’re not the equal of SCSI or fibre channel. The command queuing depth isn’t as great as SCSI’s — serial ATA can handle a 32-command queue, while SCSI allows a 256-command queue and offers more data handling options.

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Jim Love, Chief Content Officer, IT World Canada

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