Serial ATA Promises Big Performance
The first ATA hard drives appeared on the scene in the late 1980's, and have evolved into the latest ATA100 and ATA133 standards. Also known as IDE (for integrated drive electronics), the ATA standard has gone through a number of iterations. Today's ATA hard drives max out at 133MB/sec (Maxtor) and 100MB/sec (everyone else). The original ATA standard specified a connection speed of 3.3MB/sec. Early ATA drives offered 10-40MB of storage -- a staggering amount at the time, but completely useless for most PC applications today. Capacities have evolved along with connection speeds, and we now have 320GB ATA drives available. However, today's hard drives still use an interconnect standard that's over fifteen years old, even as capacities and drive technologies have progressed.
The ATA standard is a 16-bit, parallel connection. Parallel ATA uses source-synchronous (non-interlocked) clocking, which means that the clock signal is actually sent with the data. This can create problems as data rates -- and hence, clock rates -- increase. Because of potential signal reflection and signal skew issues, the ATA100 standard reduced the voltage for ATA100 signaling to 3.3v. The high clock rates also require 80-conductor cables, with alternating ground and signal wires. The net result is a maximum cable length of 18 inches for reliable operation in a wide variety of environments.
Serial ATA Defined: Serial ATA is, as the name implies, a serial link. A single Serial ATA (S-ATA) cable consists of a minimum of four wires, with differential pairs for transmitting and receiving data. The standard also allows for additional ground wires as deemed necessary. Maximum cable length for the S-ATA 1.0 standard is 1 meter (roughly 3.1 feet). This makes external S-ATA drives possible.
S-ATA is also point-to-point. Each S-ATA connection supports a single drive, so the days of figuring out which jumper to set for master or slave will become an historic artifact.
Making S-ATA point-to-point also makes termination much easier, as opposed to parallel ATA's requirement to have a device attached to the middle of the cable. Today's systems typically only support two S-ATA connections. This is partly because current systems still require parallel ATA connections and partly because all of today's Serial ATA implementations work through PCI host adapter cards or chips. Being bound to PCI adds additional overhead and potentially limits throughput.
S-ATA also offers "first party" DMA support, meaning that devices aren't dependent on a host controller for DMA. The standard also has hot-swapping designed in, which means you can (in theory) swap drives while the system is running.
S-ATA uses a 7-pin connector (to accommodate any additional ground wires), and is considerably more compact than the parallel ATA plug.
As you can see, four S-ATA cables and connectors take up roughly the same room as a single parallel ATA cable. In the future, when motherboard core logic directly supports S-ATA, we'll probably see as many as four S-ATA connections on a motherboard.
Unfortunately, parallel ATA won't vanish overnight. If nothing else, optical drive makers will transition to S-ATA more slowly, since they view the additional bandwidth as more a luxury than a necessity for their applications.
In the rapidly moving computer industry, there are rarely the kinds of revolutionary changes like what is about to take place in secondary storage segment. Soon the hard drives and configuration methods that have existed since the origins of the personal computer will change forever. The basic IDE technology has been around for nearly twenty years. When the lifetimes of other computer components like CPUs and video are measured in months, twenty years ago seems like prehistory. While it has been surprising to see it last for so long, it is time for the good old IDE ribbon cable to go and bring on the new generation - Serial ATA.
Even though many thought that the eventual successor of the aging IDE interface would be replaced by a completely new technology akin to SCSI, Serial ATA is more like a blast from the past. Serial technology was once considered obsolete. Look at the serial ports that are gradually being replaced on today’s PCs. Interestingly enough, those ports are not being placed by different types of data transfer systems, but by other serial devices like USB 2.0 and IEEE 1394. In fact, most of the fastest data transfer technologies around are serial based like RDRAM and one very familiar to all, Ethernet. Serial is the wave of the future, and it will give ATA hard drives the bandwidth and features to compete with SCSI.
What Exactly Is Serial ATA?:
As opposed to the old Ultra/ATA 133 and the standards that came before it, Serial ATA (SATA) is obviously based on serial signaling technology. This means that those old IDE ribbon cables were used to send a parallel signal those forty wires were used to transfer data along many parallel routes. IDE is also known in retrospect as Parallel ATA (PATA). All these wires next to each other were sensitive to interference and caused all kinds of problems. Serial ATA signals are transferred at a more efficient voltage of 250mV compared to the 5V of Ultra/ATA and interference is cancelled out by two phase-reversed signals. It is unlikely that many more advancements could be made even if Parallel ATA were to still have an extended lifetime. In addition, Serial ATA is, obviously by name, a serial technology. It can only “talk” to one device per channel. While this might seem like a step back from the two devices per channel allowed with Ultra/ATA, this is quickly offset by other plusses that will be described shortly.
Serial ATA vs. Parallel ATA Device Diagram
Hard drives are the primary bottlenecks in today’s high-speed computers, and this is partially due to the limitations of PATA. Since Serial ATA is a completely new standard, there will be a industry wide move over to new hard drives, controllers, and connectors- essentially everything will move to Serial ATA.
Advantages of Serial ATA - Dreams can come True:
Naturally Serial ATA will be faster than Ultra/ATA, but the majority of advancements at first will be most useful from a practical standpoint. Anyone who has worked with computers knows what a pain those afore mentioned IDE ribbon cables can be. They are bulky, inflexible, fragile, and too short. This caused hard disks and optical drives to be placed in strange positions and resulted in many frustrated system builders. The width of those cables also blocked airflow and generally created a much more clumsy setup than necessary. With today’s CPUs and GPUs pumping out so much heat; good airflow is essential to a stable system.
Just imagine. Soon those hated IDE cables will be banished forever. Hard to believe, but it is finally becoming a reality. A reality made possible trough Serial ATA. Serial ATA cables will be much, much thinner and infinitely more flexible. Since Serial ATA only requires seven wires per device, the new cables will only be 8mm wide. Another dimensional advantage is that while IDE cables could only be up to 40cm long, Serial ATA cables will be plenty long at 1m. Of course since the cables are smaller and contain only seven wires, the connectors will be more compact as well. This will save space on motherboards and the hard drives themselves. The latest trend in the computer industry seems to be small form factor PCs, or SFF PCs. Serial ATA will help make PCs even smaller and more efficient.
Serial ATA Cables and Connectors (Source: Molex)
The Design - A World Apart:
Parallel ATA and Serial ATA Connectors (Source: Intel)
Even though Serial ATA only supports one device per channel, this cancelled out by two of the bonuses of Serial ATA. First and foremost, the cables and connectors are smaller so they can be easily fit in even if there has to one of each per device. Secondly, Serial ATA is extremely easy to set up. It is software compatible with Parallel ATA, and although it wont officially be supported until Microsoft’s Longhorn OS, Serial ATA devices will work with all versions of Windows. Seeing that there can only be one device per channel, there is no need to worry about Master and Slave configurations any longer either, another big relief.
One of the primary factors in keeping Ultra/ATA out of the enterprise/server market in favor of SCSI was not just the additional bandwidth and lower access times of SCSI but the inflexibility of the Ultra/ATA interface. Ultra/ATA was unreliable when compared to SCSI and did not support hot swapping, power saving or any other advanced features. Serial ATA will bring a lower cost alternative to this market with support for these features and an improved data checking system.
Finally, the information everyone wants to know about Serial ATA: Just how fast is it? Serial ATA will be introduced at a bandwidth of 150MB/s. This is a seemingly disappointing 13% improvement over Ultra/ATA 133. However, today’s hard drives rarely ever use that much bandwidth, so 150MB/s is more than enough. The only real need for this bandwidth is in multi-disk RAID arrays. RAID is becoming more and more popular so the increased bandwidth will be of use to some users. Speed freaks are rest assured to know that Serial ATA plans a series of two evolutions, both doubling the available bandwidth.
The Hardware - Serial ATA Hard Drives and Controllers:
The industry support for Serial ATA is massive Serial ATA motherboards and controllers are already on the market, but the hard drives are not. Such a situation might seem pointless, but it assures that Serial ATA will be ready to go as soon as the hard drives hit the market and allows users to take advantage of the more practical advantages of Serial ATA in the mean time. Current controllers like Highpoint’s 1520 RocketRAID ship with Serial ATA cables and converters that allow the cables to work with current IDE hard drives. In fact, the controllers themselves are just IDE controllers with Marvell or Silicon Image Parallel to Serial ATA converter chips. Motherboards with onboard Serial ATA also use similar solutions. Since the they are based on IDE controllers, current Serial ATA devices are limited to 133MB/s of bandwidth, but that’s all the current hard drives can handle anyway. True Serial ATA controllers won’t be available until the disks are launched, and the first chipset with Serial ATA support will probably be Intel’s Granite Bay with the ICH5 Southbridge.
HighPoint RocketRAID 1520 Serial ATA RAID Controller with Serial ATA cable and SATA to IDE adapter. (Source: HighPoint)
Even when Serial ATA hard drives are available in December, there won’t be any noticeable performance gains and it is partially due to the minor increase in bandwidth. Mentioned earlier was the fact that today’s IDE hard drives can’t take advantage of the bandwidth available to them. There are many factors that must be taken into consideration when designing hard drives for the desktop market. SCSI drives are notorious for their noise, heat and vibration levels. These low points are not acceptable at the consumer level. Noise and heat don’t matter too much in huge server rooms, but they can quickly become a problem in desktops. Unfortunately, this means that the extremely low seek times and high spindle speeds that make SCSI drives so fast are not available in consumer market. Basically until cooler, quieter, smoother drives can be manufactured in high volume, desktop hard drives will continue to make baby steps in speed. This doesn’t’ mean that the next generation drives released with Serial ATA support will not more advanced than current models. Look for 8MB cache buffers and seek times less than 8.9ms to be the target.
The Industry - Where are they?:
The industry support for Serial ATA is massive. Just to show how big the launch will be, here is a list products announced in Intel’s Serial ATA paper at this Fall’s IDF:
The list contains everything here from connectors to controllers and drives. Interestingly enough there is no Western Digital, but be assured they will be there with drives in December.
A Glimpse of the Future:
Serial ATA II will debut in 2004 offering 300MB/s of bandwidth, but unless some more advanced hard drives are developed, the bandwidth might go to waste. Things get even better in 2007 with Serial ATA III and its gargantuan 600MB/s. Serial ATA II and III will be fed from new peripheral technology, most likely PCI express, to deliver all that bandwidth.
Serial ATA Roadmap Through 2004 (Source: Intel)
Serial ATA will be around for sometime to come and it really is something to get excited about. All things users have been asking from hard drives are finally here with Serial ATA. Serial ATA is called an evolution of IDE hard drive, but it is a revolution in many ways. There is even a movement to introduce Serial ATA to the notebook market. Even though it won’t make a difference in performance, again the smaller cables will help make notebooks even more compact. Everything is going Serial ATA, so it is tangible to assume that notebooks will follow suit.
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