Serial interfaces yield cabling benefits

Serial interfaces yield cabling benefits


For years now, ATA (also known as IDE and, in the last several years, as EIDE as well) has been the connection of choice for disk drives on the desktop. In enterprise IT rooms, SCSI devices represent the majority of high-performance disk devices in direct attach, JBODs and RAID arrays, while Fibre Channel disks account for about one-third of enterprise disks.

Within the last year serial ATA (called SATA) disks have begun to appear in very large numbers and, within a few more quarters, vendors will cease manufacturing parallel ATA devices. The way these disks are used is changing as well; SATA devices have moved to the floor of enterprise IT shops, where they provide cheaper alternatives to Fibre Channel and SCSI devices and offer IT managers an opportunity to provide an additional "tier" of storage between enterprise arrays and tape.

The same thing is happening with the interface to SCSI disks.

SCSI, historically a parallel interface, has in the last 15 years evolved to support faster devices, doubling its I/O capability every few years. What was originally a 5-megabit interface doubled to 10, then moved to 20, and so on until it hit a transfer rate of 320M bit/sec.

Why all the effort on the part of disk drive manufacturers to move us from parallel to serial drive interfaces? As you might expect, there are several reasons.

Foremost among the reasons is the fact that current (and foreseeable) technology has no practical way to make parallel interfaces go any faster. Inherent to parallel bus architectures is the need for all signals to be received at the receiving device at the same time (thus, "parallel"). As the interfaces increase in speed, the ability to send signals in parallel without introducing serious line noise at the same time diminishes.

Serial devices, on the other hand, offer opportunities to really pump up the volume. Because signals can be sent one after the other (that is, "serially"), there is much less opportunity for electrical fields to interfere with one another. This has a number of interesting results. Most obvious are the following, both of which are a sure bet to make vendors and users very happy:

  • Because fewer signals need to be sent at the same time, the need for cabling is reduced. This means vendors are saying goodbye to those wide (or when rolled up, thick) cables and will now be using much thinner ones to transfer the same amount of data.

  • Signals can be carried on the wire much further, which means that cables can now be much longer than was the case previously.
The most beneficial result of reducing cable width and enabling longer cables is that system designers now have much less to worry about when it comes to accommodating the need for airflow inside the devices they are building. Eventually, this may contribute to their ability to squeeze some cost out of their products.

Cabling advantages only scratch the surface of what serial technologies are going to provide. We'll dig in a bit deeper next time.

Mike Karp is senior analyst with Enterprise Management Associates, focusing on storage, storage management and the methodology that brings these issues into the marketplace. He has spent more than 20 years in storage, systems management and telecommunications.

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